# Use elixir_bootstrap module to be able to bootstrap Kernel.
# The bootstrap module provides simpler implementations of the
# functions removed, simple enough to bootstrap.
import Kernel, except: [@: 1, defmodule: 2, def: 1, def: 2, defp: 2,
                        defmacro: 1, defmacro: 2, defmacrop: 2]
import :elixir_bootstrap

defmodule Kernel do
  @moduledoc """
  `Kernel` provides the default macros and functions
  Elixir imports into your environment. These macros and functions
  can be skipped or cherry-picked via the `import` macro. For
  instance, if you want to tell Elixir not to import the `if`
  macro, you can do:

      import Kernel, except: [if: 2]

  Elixir also has special forms that are always imported and
  cannot be skipped. These are described in `Kernel.SpecialForms`.

  Some of the functions described in this module are inlined by
  the Elixir compiler into their Erlang counterparts in the `:erlang`
  module. Those functions are called BIFs (builtin internal functions)
  in Erlang-land and they exhibit interesting properties, as some of
  them are allowed in guards and others are used for compiler
  optimizations.

  Most of the inlined functions can be seen in effect when capturing
  the function:

      iex> &Kernel.is_atom/1
      &:erlang.is_atom/1

  Those functions will be explicitly marked in their docs as
  "inlined by the compiler".
  """

  ## Delegations to Erlang with inlining (macros)

  @doc """
  Returns an integer or float which is the arithmetical absolute value of `number`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> abs(-3.33)
      3.33

      iex> abs(-3)
      3

  """
  @spec abs(number) :: number
  def abs(number) do
    :erlang.abs(number)
  end

  @doc """
  Invokes the given `fun` with the array of arguments `args`.

  Inlined by the compiler.

  ## Examples

      iex> apply(fn x -> x * 2 end, [2])
      4

  """
  @spec apply(fun, [any]) :: any
  def apply(fun, args) do
    :erlang.apply(fun, args)
  end

  @doc """
  Invokes the given `fun` from `module` with the array of arguments `args`.

  Inlined by the compiler.

  ## Examples

      iex> apply(Enum, :reverse, [[1, 2, 3]])
      [3,2,1]

  """
  @spec apply(module, atom, [any]) :: any
  def apply(module, fun, args) do
    :erlang.apply(module, fun, args)
  end

  @doc """
  Extracts the part of the binary starting at `start` with length `length`.
  Binaries are zero-indexed.

  If start or length references in any way outside the binary, an
  `ArgumentError` exception is raised.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> binary_part("foo", 1, 2)
      "oo"

  A negative length can be used to extract bytes at the end of a binary:

      iex> binary_part("foo", 3, -1)
      "o"

  """
  @spec binary_part(binary, pos_integer, integer) :: binary
  def binary_part(binary, start, length) do
    :erlang.binary_part(binary, start, length)
  end

  @doc """
  Returns an integer which is the size in bits of `bitstring`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> bit_size(<<433::16, 3::3>>)
      19

      iex> bit_size(<<1, 2, 3>>)
      24

  """
  @spec bit_size(bitstring) :: non_neg_integer
  def bit_size(bitstring) do
    :erlang.bit_size(bitstring)
  end

  @doc """
  Returns the number of bytes needed to contain `bitstring`.

  That is, if the number of bits in `bitstring` is not divisible by 8,
  the resulting number of bytes will be rounded up. This operation
  happens in constant time.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> byte_size(<<433::16, 3::3>>)
      3

      iex> byte_size(<<1, 2, 3>>)
      3

  """
  @spec byte_size(binary) :: non_neg_integer
  def byte_size(binary) do
    :erlang.byte_size(binary)
  end

  @doc """
  Performs an integer division.

  Raises an error if one of the arguments is not an integer.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> div(5, 2)
      2

  """
  @spec div(integer, integer) :: integer
  def div(left, right) do
    :erlang.div(left, right)
  end

  @doc """
  Stops the execution of the calling process with the given reason.

  Since evaluating this function causes the process to terminate,
  it has no return value.

  Inlined by the compiler.

  ## Examples

  When a process reaches its end, by default it exits with
  reason `:normal`. You can also call it explicitly if you
  want to terminate a process but not signal any failure:

      exit(:normal)

  In case something goes wrong, you can also use `exit/1` with
  a different reason:

      exit(:seems_bad)

  If the reason is not `:normal`, all linked process to the
  exited process will crash (unless they are trapping exits).

  ## OTP exits

  Exits are used by OTP to determine if a process exited abnormally
  or not. The following exits are considered "normal":

    * `exit(:normal)`
    * `exit(:shutdown)`
    * `exit({:shutdown, term})`

  Exiting with any other reason is considered abnormal and treated
  as a crash. This means the default supervisor behaviour kicks in,
  error reports are emitted, etc.

  This behaviour is relied on in many different places. For example,
  `ExUnit` uses `exit(:shutdown)` when exiting the test process to
  signal linked processes, supervision trees and so on to politely
  shutdown too.

  ## CLI exits

  Building on top of the exit signals mentioned above, if the
  process started by the command line exits with any of the three
  reasons above, its exit is considered normal and the Operating
  System process will exit with status 0.

  It is, however, possible to customize the Operating System exit
  signal by invoking:

      exit({:shutdown, integer})

  This will cause the OS process to exit with the status given by
  `integer` while signaling all linked OTP processes to politely
  shutdown.

  Any other exit reason will cause the OS process to exit with
  status `1` and linked OTP processes to crash.
  """
  @spec exit(term) :: no_return
  def exit(reason) do
    :erlang.exit(reason)
  end

  @doc """
  Returns the head of a list, raises `badarg` if the list is empty.

  Inlined by the compiler.
  """
  @spec hd(list) :: term
  def hd(list) do
    :erlang.hd(list)
  end

  @doc """
  Returns `true` if `term` is an atom; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_atom(term) :: boolean
  def is_atom(term) do
    :erlang.is_atom(term)
  end

  @doc """
  Returns `true` if `term` is a binary; otherwise returns `false`.

  A binary always contains a complete number of bytes.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_binary(term) :: boolean
  def is_binary(term) do
    :erlang.is_binary(term)
  end

  @doc """
  Returns `true` if `term` is a bitstring (including a binary); otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_bitstring(term) :: boolean
  def is_bitstring(term) do
    :erlang.is_bitstring(term)
  end

  @doc """
  Returns `true` if `term` is either the atom `true` or the atom `false` (i.e. a boolean);
  otherwise returns false.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_boolean(term) :: boolean
  def is_boolean(term) do
    :erlang.is_boolean(term)
  end

  @doc """
  Returns `true` if `term` is a floating point number; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_float(term) :: boolean
  def is_float(term) do
    :erlang.is_float(term)
  end

  @doc """
  Returns `true` if `term` is a function; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_function(term) :: boolean
  def is_function(term) do
    :erlang.is_function(term)
  end

  @doc """
  Returns `true` if `term` is a function that can be applied with `arity` number of arguments;
  otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_function(term, non_neg_integer) :: boolean
  def is_function(term, arity) do
    :erlang.is_function(term, arity)
  end

  @doc """
  Returns `true` if `term` is an integer; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_integer(term) :: boolean
  def is_integer(term) do
    :erlang.is_integer(term)
  end

  @doc """
  Returns `true` if `term` is a list with zero or more elements; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_list(term) :: boolean
  def is_list(term) do
    :erlang.is_list(term)
  end

  @doc """
  Returns `true` if `term` is either an integer or a floating point number;
  otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_number(term) :: boolean
  def is_number(term) do
    :erlang.is_number(term)
  end

  @doc """
  Returns `true` if `term` is a pid (process identifier); otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_pid(term) :: boolean
  def is_pid(term) do
    :erlang.is_pid(term)
  end

  @doc """
  Returns `true` if `term` is a port identifier; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_port(term) :: boolean
  def is_port(term) do
    :erlang.is_port(term)
  end

  @doc """
  Returns `true` if `term` is a reference; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_reference(term) :: boolean
  def is_reference(term) do
    :erlang.is_reference(term)
  end

  @doc """
  Returns `true` if `term` is a tuple; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_tuple(term) :: boolean
  def is_tuple(term) do
    :erlang.is_tuple(term)
  end

  @doc """
  Returns `true` if `term` is a map; otherwise returns `false`.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec is_map(term) :: boolean
  def is_map(term) do
    :erlang.is_map(term)
  end

  @doc """
  Returns the length of `list`.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> length([1, 2, 3, 4, 5, 6, 7, 8, 9])
      9

  """
  @spec length(list) :: non_neg_integer
  def length(list) do
    :erlang.length(list)
  end

  @doc """
  Returns an almost unique reference.

  The returned reference will re-occur after approximately 2^82 calls;
  therefore it is unique enough for practical purposes.

  Inlined by the compiler.

  ## Examples

      make_ref() #=> #Reference<0.0.0.135>

  """
  @spec make_ref() :: reference
  def make_ref() do
    :erlang.make_ref()
  end

  @doc """
  Returns the size of a map.

  This operation happens in constant time.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec map_size(map) :: non_neg_integer
  def map_size(map) do
    :erlang.map_size(map)
  end

  @doc """
  Return the biggest of the two given terms according to
  Erlang's term ordering. If the terms compare equal, the
  first one is returned.

  Inlined by the compiler.

  ## Examples

      iex> max(1, 2)
      2

  """
  @spec max(term, term) :: term
  def max(first, second) do
    :erlang.max(first, second)
  end

  @doc """
  Return the smallest of the two given terms according to
  Erlang's term ordering. If the terms compare equal, the
  first one is returned.

  Inlined by the compiler.

  ## Examples

      iex> min(1, 2)
      1

  """
  @spec min(term, term) :: term
  def min(first, second) do
    :erlang.min(first, second)
  end

  @doc """
  Returns an atom representing the name of the local node.
  If the node is not alive, `:nonode@nohost` is returned instead.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec node() :: node
  def node do
    :erlang.node
  end

  @doc """
  Returns the node where the given argument is located.
  The argument can be a pid, a reference, or a port.
  If the local node is not alive, `nonode@nohost` is returned.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec node(pid|reference|port) :: node
  def node(arg) do
    :erlang.node(arg)
  end

  @doc """
  Calculates the remainder of an integer division.

  Raises an error if one of the arguments is not an integer.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> rem(5, 2)
      1

  """
  @spec rem(integer, integer) :: integer
  def rem(left, right) do
    :erlang.rem(left, right)
  end

  @doc """
  Returns an integer by rounding the given number.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> round(5.5)
      6

  """
  @spec round(number) :: integer
  def round(number) do
    :erlang.round(number)
  end

  @doc """
  Sends a message to the given `dest` and returns the message.

  `dest` may be a remote or local pid, a (local) port, a locally
  registered name, or a tuple `{registered_name, node}` for a registered
  name at another node.

  Inlined by the compiler.

  ## Examples

      iex> send self(), :hello
      :hello

  """
  @spec send(dest :: pid | port | atom | {atom, node}, msg) :: msg when msg: any
  def send(dest, msg) do
    :erlang.send(dest, msg)
  end

  @doc """
  Returns the pid (process identifier) of the calling process.

  Allowed in guard clauses. Inlined by the compiler.
  """
  @spec self() :: pid
  def self() do
    :erlang.self()
  end

  @doc """
  Spawns the given function and returns its pid.

  Check the modules `Process` and `Node` for other functions
  to handle processes, including spawning functions in nodes.

  Inlined by the compiler.

  ## Examples

      current = Kernel.self
      child   = spawn(fn -> send current, {Kernel.self, 1 + 2} end)

      receive do
        {^child, 3} -> IO.puts "Received 3 back"
      end

  """
  @spec spawn((() -> any)) :: pid
  def spawn(fun) do
    :erlang.spawn(fun)
  end

  @doc """
  Spawns the given module and function passing the given args
  and returns its pid.

  Check the modules `Process` and `Node` for other functions
  to handle processes, including spawning functions in nodes.

  Inlined by the compiler.

  ## Examples

      spawn(SomeModule, :function, [1, 2, 3])

  """
  @spec spawn(module, atom, list) :: pid
  def spawn(module, fun, args) do
    :erlang.spawn(module, fun, args)
  end

  @doc """
  Spawns the given function, links it to the current process and returns its pid.

  Check the modules `Process` and `Node` for other functions
  to handle processes, including spawning functions in nodes.

  Inlined by the compiler.

  ## Examples

      current = Kernel.self
      child   = spawn_link(fn -> send current, {Kernel.self, 1 + 2} end)

      receive do
        {^child, 3} -> IO.puts "Received 3 back"
      end

  """
  @spec spawn_link((() -> any)) :: pid
  def spawn_link(fun) do
    :erlang.spawn_link(fun)
  end

  @doc """
  Spawns the given module and function passing the given args,
  links it to the current process and returns its pid.

  Check the modules `Process` and `Node` for other functions
  to handle processes, including spawning functions in nodes.

  Inlined by the compiler.

  ## Examples

      spawn_link(SomeModule, :function, [1, 2, 3])

  """
  @spec spawn_link(module, atom, list) :: pid
  def spawn_link(module, fun, args) do
    :erlang.spawn_link(module, fun, args)
  end

  @doc """
  Spawns the given function, monitors it and returns its pid
  and monitoring reference.

  Check the modules `Process` and `Node` for other functions
  to handle processes, including spawning functions in nodes.

  Inlined by the compiler.

  ## Examples

      current = Kernel.self
      spawn_monitor(fn -> send current, {Kernel.self, 1 + 2} end)

  """
  @spec spawn_monitor((() -> any)) :: {pid, reference}
  def spawn_monitor(fun) do
    :erlang.spawn_monitor(fun)
  end

  @doc """
  Spawns the given module and function passing the given args,
  monitors it and returns its pid and monitoring reference.

  Check the modules `Process` and `Node` for other functions
  to handle processes, including spawning functions in nodes.

  Inlined by the compiler.

  ## Examples

      spawn_monitor(SomeModule, :function, [1, 2, 3])

  """
  @spec spawn_monitor(module, atom, list) :: {pid, reference}
  def spawn_monitor(module, fun, args) do
    :erlang.spawn_monitor(module, fun, args)
  end

  @doc """
  A non-local return from a function. Check `Kernel.SpecialForms.try/1` for more information.

  Inlined by the compiler.
  """
  @spec throw(term) :: no_return
  def throw(term) do
    :erlang.throw(term)
  end

  @doc """
  Returns the tail of a list. Raises `ArgumentError` if the list is empty.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec tl(maybe_improper_list) :: maybe_improper_list
  def tl(list) do
    :erlang.tl(list)
  end

  @doc """
  Returns an integer by truncating the given number.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> trunc(5.5)
      5

  """
  @spec trunc(number) :: integer
  def trunc(number) do
    :erlang.trunc(number)
  end

  @doc """
  Returns the size of a tuple.

  This operation happens in constant time.

  Allowed in guard tests. Inlined by the compiler.
  """
  @spec tuple_size(tuple) :: non_neg_integer
  def tuple_size(tuple) do
    :erlang.tuple_size(tuple)
  end

  @doc """
  Arithmetic plus.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 + 2
      3

  """
  @spec (number + number) :: number
  def left + right do
    :erlang.+(left, right)
  end

  @doc """
  Arithmetic minus.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 - 2
      -1

  """
  @spec (number - number) :: number
  def left - right do
    :erlang.-(left, right)
  end

  @doc """
  Arithmetic unary plus.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> +1
      1

  """
  @spec (+number) :: number
  def (+value) do
    :erlang.+(value)
  end

  @doc """
  Arithmetic unary minus.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> -2
      -2

  """
  @spec (-number) :: number
  def (-value) do
    :erlang.-(value)
  end

  @doc """
  Arithmetic multiplication.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 * 2
      2

  """
  @spec (number * number) :: number
  def left * right do
    :erlang.*(left, right)
  end

  @doc """
  Arithmetic division.

  The result is always a float. Use `div` and `rem` if you want
  a natural division or the remainder.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 / 2
      0.5

      iex> 2 / 1
      2.0

  """
  @spec (number / number) :: float
  def left / right do
    :erlang./(left, right)
  end

  @doc """
  Concatenates two lists.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> [1] ++ [2, 3]
      [1,2,3]

      iex> 'foo' ++ 'bar'
      'foobar'

  """
  @spec (list ++ term) :: maybe_improper_list
  def left ++ right do
    :erlang.++(left, right)
  end

  @doc """
  Removes the first occurrence of an item on the left
  for each item on the right.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> [1, 2, 3] -- [1, 2]
      [3]

      iex> [1, 2, 3, 2, 1] -- [1, 2, 2]
      [3,1]

  """
  @spec (list -- list) :: list
  def left -- right do
    :erlang.--(left, right)
  end

  @doc """
  Boolean not. Argument must be a boolean.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> not false
      true

  """
  @spec not(boolean) :: boolean
  def not(arg) do
    :erlang.not(arg)
  end

  @doc """
  Returns `true` if left is less than right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 < 2
      true

  """
  @spec (term < term) :: boolean
  def left < right do
    :erlang.<(left, right)
  end

  @doc """
  Returns `true` if left is more than right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 > 2
      false

  """
  @spec (term > term) :: boolean
  def left > right do
    :erlang.>(left, right)
  end

  @doc """
  Returns `true` if left is less than or equal to right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 <= 2
      true

  """
  @spec (term <= term) :: boolean
  def left <= right do
    :erlang."=<"(left, right)
  end

  @doc """
  Returns `true` if left is more than or equal to right.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 >= 2
      false

  """
  @spec (term >= term) :: boolean
  def left >= right do
    :erlang.>=(left, right)
  end

  @doc """
  Returns `true` if the two items are equal.

  This operator considers 1 and 1.0 to be equal. For match
  semantics, use `===` instead.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 == 2
      false

      iex> 1 == 1.0
      true

  """
  @spec (term == term) :: boolean
  def left == right do
    :erlang.==(left, right)
  end

  @doc """
  Returns `true` if the two items are not equal.

  This operator considers 1 and 1.0 to be equal. For match
  comparison, use `!==` instead.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 != 2
      true

      iex> 1 != 1.0
      false

  """
  @spec (term != term) :: boolean
  def left != right do
    :erlang."/="(left, right)
  end

  @doc """
  Returns `true` if the two items are match.

  This operator gives the same semantics as the one existing in
  pattern matching, i.e., `1` and `1.0` are equal, but they do
  not match.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 === 2
      false

      iex> 1 === 1.0
      false

  """
  @spec (term === term) :: boolean
  def left === right do
    :erlang."=:="(left, right)
  end

  @doc """
  Returns `true` if the two items do not match.

  All terms in Elixir can be compared with each other.

  Allowed in guard tests. Inlined by the compiler.

  ## Examples

      iex> 1 !== 2
      true

      iex> 1 !== 1.0
      true

  """
  @spec (term !== term) :: boolean
  def left !== right do
    :erlang."=/="(left, right)
  end

  @doc """
  Get the element at the zero-based `index` in `tuple`.

  Allowed in guard tests. Inlined by the compiler.

  ## Example

      iex> tuple = {:foo, :bar, 3}
      iex> elem(tuple, 1)
      :bar

  """
  @spec elem(tuple, non_neg_integer) :: term
  def elem(tuple, index) do
    :erlang.element(index + 1, tuple)
  end

  @doc """
  Puts the element in `tuple` at the zero-based `index` to the given `value`.

  Inlined by the compiler.

  ## Example

      iex> tuple = {:foo, :bar, 3}
      iex> put_elem(tuple, 0, :baz)
      {:baz, :bar, 3}

  """
  @spec put_elem(tuple, non_neg_integer, term) :: tuple
  def put_elem(tuple, index, value) do
    :erlang.setelement(index + 1, tuple, value)
  end

  ## Implemented in Elixir

  @doc """
  Boolean or. Requires only the first argument to be a
  boolean since it short-circuits.

  Allowed in guard tests.

  ## Examples

      iex> true or false
      true

  """
  defmacro left or right do
    quote do: __op__(:orelse, unquote(left), unquote(right))
  end

  @doc """
  Boolean and. Requires only the first argument to be a
  boolean since it short-circuits.

  Allowed in guard tests.

  ## Examples

      iex> true and false
      false

  """
  defmacro left and right do
    quote do: __op__(:andalso, unquote(left), unquote(right))
  end

  @doc """
  Receives any argument and returns `true` if it is `false`
  or `nil`. Returns `false` otherwise. Not allowed in guard
  clauses.

  ## Examples

      iex> !Enum.empty?([])
      false

      iex> !List.first([])
      true

  """
  defmacro !(arg)

  defmacro !({:!, _, [arg]}) do
    optimize_boolean(quote do
      case unquote(arg) do
        x when x in [false, nil] -> false
        _ -> true
      end
    end)
  end

  defmacro !(arg) do
    optimize_boolean(quote do
      case unquote(arg) do
        x when x in [false, nil] -> true
        _ -> false
      end
    end)
  end

  @doc """
  Concatenates two binaries.

  ## Examples

      iex> "foo" <> "bar"
      "foobar"

  The `<>` operator can also be used in guard clauses as
  long as the first part is a literal binary:

      iex> "foo" <> x = "foobar"
      iex> x
      "bar"

  """
  defmacro left <> right do
    concats = extract_concatenations({:<>, [], [left, right]})
    quote do: << unquote_splicing(concats) >>
  end

  # Extracts concatenations in order to optimize many
  # concatenations into one single clause.
  defp extract_concatenations({:<>, _, [left, right]}) do
    [wrap_concatenation(left)|extract_concatenations(right)]
  end

  defp extract_concatenations(other) do
    [wrap_concatenation(other)]
  end

  defp wrap_concatenation(binary) when is_binary(binary) do
    binary
  end

  defp wrap_concatenation(other) do
    {:::, [], [other, {:binary, [], nil}]}
  end

  @doc """
  Raises an exception.

  If the argument is a binary, it raises `RuntimeError`
  using the given argument as message.

  If an atom, it will become a call to `raise(atom, [])`.

  If anything else, it will just raise the given exception.

  ## Examples

      raise "Given values do not match"

      try do
        1 + :foo
      rescue
        x in [ArithmeticError] ->
          IO.puts "that was expected"
          raise x
      end

  """
  defmacro raise(msg) do
    # Try to figure out the type at compilation time
    # to avoid dead code and make dialyzer happy.
    msg = case not is_binary(msg) and bootstraped?(Macro) do
      true  -> Macro.expand(msg, __CALLER__)
      false -> msg
    end

    case msg do
      msg when is_binary(msg) ->
        quote do
          :erlang.error RuntimeError.exception(unquote(msg))
        end
      {:<<>>, _, _} = msg ->
        quote do
          :erlang.error RuntimeError.exception(unquote(msg))
        end
      alias when is_atom(alias) ->
        quote do
          :erlang.error unquote(alias).exception([])
        end
      _ ->
        quote do
          case unquote(msg) do
            msg when is_binary(msg) ->
              :erlang.error RuntimeError.exception(msg)
            atom when is_atom(atom) ->
              :erlang.error atom.exception([])
            %{__struct__: struct, __exception__: true} = other when is_atom(struct) ->
              :erlang.error other
          end
        end
    end
  end

  @doc """
  Raises an exception.

  Calls `.exception` on the given argument passing
  the attributes in order to retrieve the appropriate exception
  structure.

  Any module defined via `defexception/1` automatically
  implements `exception(attrs)` callback expected by `raise/2`.

  ## Examples

      iex> raise(ArgumentError, message: "Sample")
      ** (ArgumentError) Sample

  """
  defmacro raise(exception, attrs) do
    quote do
      :erlang.error unquote(exception).exception(unquote(attrs))
    end
  end

  @doc """
  Raises an exception preserving a previous stacktrace.

  Works like `raise/1` but does not generate a new stacktrace.

  Notice that `System.stacktrace` returns the stacktrace
  of the last exception. That said, it is common to assign
  the stacktrace as the first expression inside a `rescue`
  clause as any other exception potentially raised (and
  rescued) in between the rescue clause and the raise call
  may change the `System.stacktrace` value.

  ## Examples

      try do
        raise "Oops"
      rescue
        exception ->
          stacktrace = System.stacktrace
          if Exception.message(exception) == "Oops" do
            reraise exception, stacktrace
          end
      end
  """
  defmacro reraise(msg, stacktrace) do
    # Try to figure out the type at compilation time
    # to avoid dead code and make dialyzer happy.

    case Macro.expand(msg, __CALLER__) do
      msg when is_binary(msg) ->
        quote do
          :erlang.raise :error, RuntimeError.exception(unquote(msg)), unquote(stacktrace)
        end
      {:<<>>, _, _} = msg ->
        quote do
          :erlang.raise :error, RuntimeError.exception(unquote(msg)), unquote(stacktrace)
        end
      alias when is_atom(alias) ->
        quote do
          :erlang.raise :error, unquote(alias).exception([]), unquote(stacktrace)
        end
      msg ->
        quote do
          stacktrace = unquote(stacktrace)
          case unquote(msg) do
            msg when is_binary(msg) ->
              :erlang.raise :error, RuntimeError.exception(msg), stacktrace
            atom when is_atom(atom) ->
              :erlang.raise :error, atom.exception([]), stacktrace
            %{__struct__: struct, __exception__: true} = other when is_atom(struct) ->
              :erlang.raise :error, other, stacktrace
          end
        end
    end
  end

  @doc """
  Raises an exception preserving a previous stacktrace.

  Works like `raise/2` but does not generate a new stacktrace.

  See `reraise/2` for more details.

  ## Examples

      try do
        raise "Oops"
      rescue
        exception ->
          stacktrace = System.stacktrace
          reraise WrapperError, [exception: exception], stacktrace
      end
  """
  defmacro reraise(exception, attrs, stacktrace) do
    quote do
      :erlang.raise :error, unquote(exception).exception(unquote(attrs)), unquote(stacktrace)
    end
  end

  @doc """
  Matches the term on the left against the regular expression or string on the
  right. Returns true if `left` matches `right` (if it's a regular expression)
  or contains `right` (if it's a string).

  ## Examples

      iex> "abcd" =~ ~r/c(d)/
      true

      iex> "abcd" =~ ~r/e/
      false

      iex> "abcd" =~ "bc"
      true

      iex> "abcd" =~ "ad"
      false

  """
  def left =~ right when is_binary(left) and is_binary(right) do
    :binary.match(left, right) != :nomatch
  end

  def left =~ right when is_binary(left) do
    Regex.match?(right, left)
  end

  @doc ~S"""
  Inspect the given argument according to the `Inspect` protocol.
  The second argument is a keywords list with options to control
  inspection.

  ## Options

  `inspect/2` accepts a list of options that are internally
  translated to an `Inspect.Opts` struct. Check the docs for
  `Inspect.Opts` to see the supported options.

  ## Examples

      iex> inspect(:foo)
      ":foo"

      iex> inspect [1, 2, 3, 4, 5], limit: 3
      "[1, 2, 3, ...]"

      iex> inspect("olá" <> <<0>>)
      "<<111, 108, 195, 161, 0>>"

      iex> inspect("olá" <> <<0>>, binaries: :as_strings)
      "\"olá\\0\""

      iex> inspect("olá", binaries: :as_binaries)
      "<<111, 108, 195, 161>>"

  Note that the inspect protocol does not necessarily return a valid
  representation of an Elixir term. In such cases, the inspected result
  must start with `#`. For example, inspecting a function will return:

      inspect fn a, b -> a + b end
      #=> #Function<...>

  """
  @spec inspect(Inspect.t, Keyword.t) :: String.t
  def inspect(arg, opts \\ []) when is_list(opts) do
    opts  = struct(Inspect.Opts, opts)
    limit = case opts.pretty do
      true  -> opts.width
      false -> :infinity
    end
    IO.iodata_to_binary(
      Inspect.Algebra.format(Inspect.Algebra.to_doc(arg, opts), limit)
    )
  end

  @doc """
  Creates and updates structs.

  The struct argument may be an atom (which defines `defstruct`)
  or a struct itself. The second argument is any Enumerable that
  emits two-item tuples (key-value) during enumeration.

  If one of the keys in the Enumerable does not exist in the struct,
  they are automatically discarded.

  This function is useful for dynamically creating and updating
  structs.

  ## Example

      defmodule User do
        defstruct name: "john"
      end

      struct(User)
      #=> %User{name: "john"}

      opts = [name: "meg"]
      user = struct(User, opts)
      #=> %User{name: "meg"}

      struct(user, unknown: "value")
      #=> %User{name: "meg"}

  """
  @spec struct(module | map, Enum.t) :: map
  def struct(struct, kv \\ [])

  def struct(struct, []) when is_atom(struct) or is_tuple(struct) do
    apply(struct, :__struct__, [])
  end

  def struct(struct, kv) when is_atom(struct) or is_tuple(struct) do
    struct(apply(struct, :__struct__, []), kv)
  end

  def struct(%{__struct__: _} = struct, kv) do
    Enum.reduce(kv, struct, fn {k, v}, acc ->
      case :maps.is_key(k, acc) and k != :__struct__ do
        true  -> :maps.put(k, v, acc)
        false -> acc
      end
    end)
  end

  @doc """
  Gets a value from a nested structure.

  Uses the `Access` protocol to traverse the structures
  according to the given `keys`, unless the `key` is a
  function.

  If a key is a function, the function will be invoked
  passing three arguments, the operation (`:get`), the
  data to be accessed, and a function to be invoked next.

  This means `get_in/2` can be extended to provide
  custom lookups. The downside is that functions cannot be
  stored as keys in the accessed data structures.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_in(users, ["john", :age])
      27

  In case any of entries in the middle returns `nil`, `nil` will be returned
  as per the Access protocol:

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_in(users, ["unknown", :age])
      nil

  When one of the keys is a function, the function is invoked.
  In the example below, we use a function to get all the maps
  inside a list:

      iex> users = [%{name: "john", age: 27}, %{name: "meg", age: 23}]
      iex> all = fn :get, data, next -> Enum.map(data, next) end
      iex> get_in(users, [all, :age])
      [27, 23]

  If the previous value before invoking the function is nil,
  the function *will* receive nil as a value and must handle it
  accordingly.
  """
  @spec get_in(Access.t, nonempty_list(term)) :: term
  def get_in(data, keys)

  def get_in(data, [h]) when is_function(h),
    do: h.(:get, data, &(&1))
  def get_in(data, [h|t]) when is_function(h),
    do: h.(:get, data, &get_in(&1, t))

  def get_in(nil, [_]),
    do: nil
  def get_in(nil, [_|t]),
    do: get_in(nil, t)

  def get_in(data, [h]),
    do: Access.get(data, h)
  def get_in(data, [h|t]),
    do: get_in(Access.get(data, h), t)

  @doc """
  Puts a value in a nested structure.

  Uses the `Access` protocol to traverse the structures
  according to the given `keys`, unless the `key` is a
  function. If the key is a function, it will be invoked
  as specified in `get_and_update_in/3`.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> put_in(users, ["john", :age], 28)
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  In case any of entries in the middle returns `nil`,
  an error will be raised when trying to access it next.
  """
  @spec put_in(Access.t, nonempty_list(term), term) :: Access.t
  def put_in(data, keys, value) do
    elem(get_and_update_in(data, keys, fn _ -> {nil, value} end), 1)
  end

  @doc """
  Updates a key in a nested structure.

  Uses the `Access` protocol to traverse the structures
  according to the given `keys`, unless the `key` is a
  function. If the key is a function, it will be invoked
  as specified in `get_and_update_in/3`.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> update_in(users, ["john", :age], &(&1 + 1))
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  In case any of entries in the middle returns `nil`,
  an error will be raised when trying to access it next.
  """
  @spec update_in(Access.t, nonempty_list(term), (term -> term)) :: Access.t
  def update_in(data, keys, fun) do
    elem(get_and_update_in(data, keys, fn x -> {nil, fun.(x)} end), 1)
  end

  @doc """
  Gets a value and updates a nested structure.

  It expects a tuple to be returned, containing the value
  retrieved and the update one.

  Uses the `Access` protocol to traverse the structures
  according to the given `keys`, unless the `key` is a
  function.

  If a key is a function, the function will be invoked
  passing three arguments, the operation (`:get_and_update`),
  the data to be accessed, and a function to be invoked next.

  This means `get_and_update_in/3` can be extended to provide
  custom lookups. The downside is that functions cannot be stored
  as keys in the accessed data structures.

  ## Examples

  This function is useful when there is a need to retrieve the current
  value (or something calculated in function of the current value) and
  update it at the same time. For example, it could be used to increase
  the age of a user by one and return the previous age in one pass:

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_and_update_in(users, ["john", :age], &{&1, &1 + 1})
      {27, %{"john" => %{age: 28}, "meg" => %{age: 23}}}

  When one of the keys is a function, the function is invoked.
  In the example below, we use a function to get and increment all
  ages inside a list:

      iex> users = [%{name: "john", age: 27}, %{name: "meg", age: 23}]
      iex> all = fn :get_and_update, data, next ->
      ...>   Enum.map(data, next) |> List.unzip() |> List.to_tuple()
      ...> end
      iex> get_and_update_in(users, [all, :age], &{&1, &1 + 1})
      {[27, 23], [%{name: "john", age: 28}, %{name: "meg", age: 24}]}

  If the previous value before invoking the function is nil,
  the function *will* receive `nil` as a value and must handle it
  accordingly (be it by failing or providing a sane default).
  """
  @spec get_and_update_in(Access.t, nonempty_list(term),
                          (term -> {get, term})) :: {get, Access.t} when get: var
  def get_and_update_in(data, keys, fun)

  def get_and_update_in(data, [h], fun) when is_function(h),
    do: h.(:get_and_update, data, fun)
  def get_and_update_in(data, [h|t], fun) when is_function(h),
    do: h.(:get_and_update, data, &get_and_update_in(&1, t, fun))

  def get_and_update_in(data, [h], fun),
    do: Access.get_and_update(data, h, fun)
  def get_and_update_in(data, [h|t], fun),
    do: Access.get_and_update(data, h, &get_and_update_in(&1, t, fun))

  @doc """
  Puts a value in a nested structure via the given `path`.

  This is similar to `put_in/3`, except the path is extracted via
  a macro rather than passing a list. For example:

      put_in(opts[:foo][:bar], :baz)

  Is equivalent to:

      put_in(opts, [:foo, :bar], :baz)

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. For more information about the supported path
  expressions, please check `get_and_update_in/2` docs.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> put_in(users["john"][:age], 28)
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> put_in(users["john"].age, 28)
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  """
  defmacro put_in(path, value) do
    [h|t] = unnest(path, [], "put_in/2")
    expr  = nest_get_and_update_in(h, t, quote(do: fn _ -> {nil, unquote(value)} end))
    quote do: :erlang.element(2, unquote(expr))
  end

  @doc """
  Updates a nested structure via the given `path`.

  This is similar to `update_in/3`, except the path is extracted via
  a macro rather than passing a list. For example:

      update_in(opts[:foo][:bar], &(&1 + 1))

  Is equivalent to:

      update_in(opts, [:foo, :bar], &(&1 + 1))

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. For more information about the supported path
  expressions, please check `get_and_update_in/2` docs.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> update_in(users["john"][:age], &(&1 + 1))
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> update_in(users["john"].age, &(&1 + 1))
      %{"john" => %{age: 28}, "meg" => %{age: 23}}

  """
  defmacro update_in(path, fun) do
    [h|t] = unnest(path, [], "update_in/2")
    expr = nest_get_and_update_in(h, t, quote(do: fn x -> {nil, unquote(fun).(x)} end))
    quote do: :erlang.element(2, unquote(expr))
  end

  @doc """
  Gets a value and updates a nested data structure via the given `path`.

  This is similar to `get_and_update_in/3`, except the path is extracted
  via a macro rather than passing a list. For example:

      get_and_update_in(opts[:foo][:bar], &{&1, &1 + 1})

  Is equivalent to:

      get_and_update_in(opts, [:foo, :bar], &{&1, &1 + 1})

  Note that in order for this macro to work, the complete path must always
  be visible by this macro. See the Paths section below.

  ## Examples

      iex> users = %{"john" => %{age: 27}, "meg" => %{age: 23}}
      iex> get_and_update_in(users["john"].age, &{&1, &1 + 1})
      {27, %{"john" => %{age: 28}, "meg" => %{age: 23}}}

  ## Paths

  A path may start with a variable, local or remote call, and must be
  followed by one or more:

    * `foo[bar]` - access a field; in case an intermediate field is not
      present or returns nil, an empty map is used

    * `foo.bar` - access a map/struct field; in case the field is not
      present, an error is raised

  Here are some valid paths:

      users["john"][:age]
      users["john"].age
      User.all["john"].age
      all_users()["john"].age

  Here are some invalid ones:

      # Does a remote call after the initial value
      users["john"].do_something(arg1, arg2)

      # Does not access any field
      users

  """
  defmacro get_and_update_in(path, fun) do
    [h|t] = unnest(path, [], "get_and_update_in/2")
    nest_get_and_update_in(h, t, fun)
  end

  defp nest_get_and_update_in([], fun),  do: fun
  defp nest_get_and_update_in(list, fun) do
    quote do
      fn x -> unquote(nest_get_and_update_in(quote(do: x), list, fun)) end
    end
  end

  defp nest_get_and_update_in(h, [{:access, key}|t], fun) do
    quote do
      Access.get_and_update(
        unquote(h),
        unquote(key),
        unquote(nest_get_and_update_in(t, fun))
      )
    end
  end

  defp nest_get_and_update_in(h, [{:map, key}|t], fun) do
    quote do
      Access.Map.get_and_update!(unquote(h), unquote(key), unquote(nest_get_and_update_in(t, fun)))
    end
  end

  defp unnest({{:., _, [Access, :get]}, _, [expr, key]}, acc, kind) do
    unnest(expr, [{:access, key}|acc], kind)
  end

  defp unnest({{:., _, [expr, key]}, _, []}, acc, kind)
      when is_tuple(expr) and elem(expr, 0) != :__aliases__ and elem(expr, 0) != :__MODULE__ do
    unnest(expr, [{:map, key}|acc], kind)
  end

  defp unnest(other, [], kind) do
    raise ArgumentError,
      "expected expression given to #{kind} to access at least one element, got: #{Macro.to_string other}"
  end

  defp unnest(other, acc, kind) do
    case proper_start?(other) do
      true -> [other|acc]
      false ->
        raise ArgumentError,
          "expression given to #{kind} must start with a variable, local or remote call " <>
          "and be followed by an element access, got: #{Macro.to_string other}"
    end
  end

  defp proper_start?({{:., _, [expr, _]}, _, _args})
    when is_atom(expr)
    when elem(expr, 0) == :__aliases__
    when elem(expr, 0) == :__MODULE__, do: true

  defp proper_start?({atom, _, _args})
    when is_atom(atom), do: true

  defp proper_start?(other),
    do: not is_tuple(other)

  @doc """
  Converts the argument to a string according to the
  `String.Chars` protocol.

  This is the function invoked when there is string interpolation.

  ## Examples

      iex> to_string(:foo)
      "foo"

  """
  # If it is a binary at compilation time, simply return it.
  defmacro to_string(arg) when is_binary(arg), do: arg

  defmacro to_string(arg) do
    quote do: String.Chars.to_string(unquote(arg))
  end

  @doc """
  Convert the argument to a list according to the List.Chars protocol.

  ## Examples

      iex> to_char_list(:foo)
      'foo'

  """
  defmacro to_char_list(arg) do
    quote do: List.Chars.to_char_list(unquote(arg))
  end

  @doc """
  Checks if the given argument is nil or not.
  Allowed in guard clauses.

  ## Examples

      iex> nil?(1)
      false

      iex> nil?(nil)
      true

  """
  defmacro nil?(x) do
    quote do: unquote(x) == nil
  end

  @doc """
  A convenient macro that checks if the right side matches
  the left side. The left side is allowed to be a match pattern.

  ## Examples

      iex> match?(1, 1)
      true

      iex> match?(1, 2)
      false

      iex> match?({1, _}, {1, 2})
      true

  Match can also be used to filter or find a value in an enumerable:

      list = [{:a, 1}, {:b, 2}, {:a, 3}]
      Enum.filter list, &match?({:a, _}, &1)

  Guard clauses can also be given to the match:

      list = [{:a, 1}, {:b, 2}, {:a, 3}]
      Enum.filter list, &match?({:a, x} when x < 2, &1)

  However, variables assigned in the match will not be available
  outside of the function call:

      iex> match?(x, 1)
      true

      iex> binding([:x]) == []
      true

  """
  defmacro match?(pattern, expr)

  # Special case underscore since it always matches
  defmacro match?({:_, _, atom}, _right) when is_atom(atom) do
    true
  end

  defmacro match?(left, right) do
    quote do
      case unquote(right) do
        unquote(left) ->
          true
        _ ->
          false
      end
    end
  end

  @doc """
  Read and write attributes of th current module.

  The canonical example for attributes is annotating that a module
  implements the OTP behaviour called `gen_server`:

      defmodule MyServer do
        @behaviour :gen_server
        # ... callbacks ...
      end

  By default Elixir supports all Erlang module attributes, but any developer
  can also add custom attributes:

      defmodule MyServer do
        @my_data 13
        IO.inspect @my_data #=> 13
      end

  Unlike Erlang, such attributes are not stored in the module by
  default since it is common in Elixir to use such attributes to store
  temporary data. A developer can configure an attribute to behave closer
  to Erlang by calling `Module.register_attribute/3`.

  Finally, notice that attributes can also be read inside functions:

      defmodule MyServer do
        @my_data 11
        def first_data, do: @my_data
        @my_data 13
        def second_data, do: @my_data
      end

      MyServer.first_data #=> 11
      MyServer.second_data #=> 13

  It is important to note that reading an attribute takes a snapshot of
  its current value. In other words, the value is read at compilation
  time and not at runtime. Check the module `Module` for other functions
  to manipulate module attributes.
  """
  defmacro @(expr)

  # Typespecs attributes are special cased by the compiler so far
  defmacro @({name, _, args}) do
    # Check for Macro as it is compiled later than Module
    case bootstraped?(Module) do
      false -> nil
      true  ->
        assert_module_scope(__CALLER__, :@, 1)
        function? = __CALLER__.function != nil

        case is_list(args) and length(args) == 1 and typespec(name) do
          false ->
            do_at(args, name, function?, __CALLER__)
          macro ->
            case bootstraped?(Kernel.Typespec) do
              false -> nil
              true  -> quote do: Kernel.Typespec.unquote(macro)(unquote(hd(args)))
            end
        end
    end
  end

  # @attribute value
  defp do_at([arg], name, function?, env) do
    case function? do
      true ->
        raise ArgumentError, "cannot dynamically set attribute @#{name} inside function"
      false ->
        case name do
          :behavior ->
            :elixir_errors.warn warn_info(env_stacktrace(env)),
                                "@behavior attribute is not supported, please use @behaviour instead"
          _ ->
            :ok
        end

        quote do: Module.put_attribute(__MODULE__, unquote(name), unquote(arg))
    end
  end

  # @attribute or @attribute()
  defp do_at(args, name, function?, env) when is_atom(args) or args == [] do
    stack = env_stacktrace(env)

    case function? do
      true ->
        attr = Module.get_attribute(env.module, name, stack)
        :erlang.element(1, :elixir_quote.escape(attr, false))
      false ->
        escaped = case stack do
          [] -> []
          _  -> Macro.escape(stack)
        end
        quote do: Module.get_attribute(__MODULE__, unquote(name), unquote(escaped))
    end
  end

  # All other cases
  defp do_at(args, name, _function?, _env) do
    raise ArgumentError, "expected 0 or 1 argument for @#{name}, got: #{length(args)}"
  end

  defp warn_info([entry|_]) do
    opts = elem(entry, tuple_size(entry) - 1)
    Exception.format_file_line(Keyword.get(opts, :file), Keyword.get(opts, :line)) <> " "
  end

  defp warn_info([]) do
    ""
  end

  defp typespec(:type),     do: :deftype
  defp typespec(:typep),    do: :deftypep
  defp typespec(:opaque),   do: :defopaque
  defp typespec(:spec),     do: :defspec
  defp typespec(:callback), do: :defcallback
  defp typespec(_),         do: false

  @doc """
  Returns the binding for the given context as a keyword list.

  The variable name is the key and the variable value is the value.

  ## Examples

      iex> x = 1
      iex> binding()
      [x: 1]
      iex> x = 2
      iex> binding()
      [x: 2]

      iex> binding(:foo)
      []
      iex> var!(x, :foo) = 1
      1
      iex> binding(:foo)
      [x: 1]

  """
  defmacro binding(context \\ nil) do
    in_match? = Macro.Env.in_match?(__CALLER__)
    for {v, c} <- __CALLER__.vars, c == context do
      {v, wrap_binding(in_match?, {v, [], c})}
    end
  end

  defp wrap_binding(true, var) do
    quote do: ^(unquote(var))
  end

  defp wrap_binding(_, var) do
    var
  end

  @doc """
  Provides an `if` macro. This macro expects the first argument to
  be a condition and the rest are keyword arguments.

  ## One-liner examples

      if(foo, do: bar)

  In the example above, `bar` will be returned if `foo` evaluates to
  `true` (i.e. it is neither `false` nor `nil`). Otherwise, `nil` will be returned.

  An `else` option can be given to specify the opposite:

      if(foo, do: bar, else: baz)

  ## Blocks examples

  Elixir also allows you to pass a block to the `if` macro. The first
  example above would be translated to:

      if foo do
        bar
      end

  Notice that `do/end` becomes delimiters. The second example would
  then translate to:

      if foo do
        bar
      else
        baz
      end

  If you want to compare more than two clauses, you can use the `cond/1`
  macro.
  """
  defmacro if(condition, clauses) do
    do_clause = Keyword.get(clauses, :do, nil)
    else_clause = Keyword.get(clauses, :else, nil)

    optimize_boolean(quote do
      case unquote(condition) do
        x when x in [false, nil] -> unquote(else_clause)
        _ -> unquote(do_clause)
      end
    end)
  end

  @doc """
  Evaluates and returns the do-block passed in as a second argument
  unless clause evaluates to true.
  Returns nil otherwise.
  See also `if`.

  ## Examples

      iex> unless(Enum.empty?([]), do: "Hello")
      nil

      iex> unless(Enum.empty?([1,2,3]), do: "Hello")
      "Hello"

  """
  defmacro unless(clause, options) do
    do_clause   = Keyword.get(options, :do, nil)
    else_clause = Keyword.get(options, :else, nil)
    quote do
      if(unquote(clause), do: unquote(else_clause), else: unquote(do_clause))
    end
  end

  @doc """
  Allows you to destructure two lists, assigning each term in the right to the
  matching term in the left. Unlike pattern matching via `=`, if the sizes of
  the left and right lists don't match, destructuring simply stops instead of
  raising an error.

  ## Examples

      iex> destructure([x, y, z], [1, 2, 3, 4, 5])
      iex> {x, y, z}
      {1, 2, 3}

  Notice in the example above, even though the right
  size has more entries than the left, destructuring works
  fine. If the right size is smaller, the remaining items
  are simply assigned to nil:

      iex> destructure([x, y, z], [1])
      iex> {x, y, z}
      {1, nil, nil}

  The left side supports any expression you would use
  on the left side of a match:

      x = 1
      destructure([^x, y, z], [1, 2, 3])

  The example above will only work if x matches
  the first value from the right side. Otherwise,
  it will raise a CaseClauseError.
  """
  defmacro destructure(left, right) when is_list(left) do
    Enum.reduce left, right, fn item, acc ->
      {:case, meta, args} =
        quote do
          case unquote(acc) do
            [unquote(item)|t] ->
              t
            other when other == [] or other == nil ->
              unquote(item) = nil
          end
        end
      {:case, [{:export_head,true}|meta], args}
    end
  end

  @doc """
  Returns a range with the specified start and end.
  Includes both ends.

  ## Examples

      iex> 0 in 1..3
      false

      iex> 1 in 1..3
      true

      iex> 2 in 1..3
      true

      iex> 3 in 1..3
      true

  """
  defmacro first .. last do
    {:%{}, [], [__struct__: Elixir.Range, first: first, last: last]}
  end

  @doc """
  Provides a short-circuit operator that evaluates and returns
  the second expression only if the first one evaluates to true
  (i.e. it is not nil nor false). Returns the first expression
  otherwise.

  ## Examples

      iex> Enum.empty?([]) && Enum.empty?([])
      true

      iex> List.first([]) && true
      nil

      iex> Enum.empty?([]) && List.first([1])
      1

      iex> false && throw(:bad)
      false

  Notice that, unlike Erlang's `and` operator,
  this operator accepts any expression as an argument,
  not only booleans, however it is not allowed in guards.
  """
  defmacro left && right do
    quote do
      case unquote(left) do
        x when x in [false, nil] ->
          x
        _ ->
          unquote(right)
      end
    end
  end

  @doc """
  Provides a short-circuit operator that evaluates and returns the second
  expression only if the first one does not evaluate to true (i.e. it
  is either nil or false). Returns the first expression otherwise.

  ## Examples

      iex> Enum.empty?([1]) || Enum.empty?([1])
      false

      iex> List.first([]) || true
      true

      iex> Enum.empty?([1]) || 1
      1

      iex> Enum.empty?([]) || throw(:bad)
      true

  Notice that, unlike Erlang's `or` operator,
  this operator accepts any expression as an argument,
  not only booleans, however it is not allowed in guards.
  """
  defmacro left || right do
    quote do
      case unquote(left) do
        x when x in [false, nil] ->
          unquote(right)
        x ->
          x
      end
    end
  end

  @doc """
  `|>` is the pipe operator.

  This operator introduces the expression on the left as
  the first argument to the function call on the right.

  ## Examples

      iex> [1, [2], 3] |> List.flatten()
      [1, 2, 3]

  The example above is the same as calling `List.flatten([1, [2], 3])`,
  i.e. the argument on the left side of `|>` is introduced as the first
  argument of the function call on the right side.

  This pattern is mostly useful when there is a desire to execute
  a bunch of operations, resembling a pipeline:

      iex> [1, [2], 3] |> List.flatten |> Enum.map(fn x -> x * 2 end)
      [2, 4, 6]

  The example above will pass the list to `List.flatten/1`, then get
  the flattened list and pass to `Enum.map/2`, which will multiply
  each entry in the list per two.

  In other words, the expression above simply translates to:

      Enum.map(List.flatten([1, [2], 3]), fn x -> x * 2 end)

  Beware of operator precedence when using the pipe operator.
  For example, the following expression:

      String.graphemes "Hello" |> Enum.reverse

  Translates to:

      String.graphemes("Hello" |> Enum.reverse)

  Which will result in an error as Enumerable protocol is not defined
  for binaries. Adding explicit parenthesis resolves the ambiguity:

      String.graphemes("Hello") |> Enum.reverse

  Or, even better:

      "Hello" |> String.graphemes |> Enum.reverse

  """
  defmacro left |> right do
    [{h, _}|t] = Macro.unpipe({:|>, [], [left, right]})
    :lists.foldl fn {x, pos}, acc -> Macro.pipe(acc, x, pos) end, h, t
  end

  @doc """
  Returns true if the `module` is loaded and contains a
  public `function` with the given `arity`, otherwise false.

  Notice that this function does not load the module in case
  it is not loaded. Check `Code.ensure_loaded/1` for more
  information.
  """
  @spec function_exported?(atom | tuple, atom, integer) :: boolean
  def function_exported?(module, function, arity) do
    :erlang.function_exported(module, function, arity)
  end

  @doc """
  Returns true if the `module` is loaded and contains a
  public `macro` with the given `arity`, otherwise false.

  Notice that this function does not load the module in case
  it is not loaded. Check `Code.ensure_loaded/1` for more
  information.
  """
  @spec macro_exported?(atom, atom, integer) :: boolean
  def macro_exported?(module, macro, arity) do
    case :code.is_loaded(module) do
      {:file, _} -> :lists.member({macro, arity}, module.__info__(:macros))
      _ -> false
    end
  end

  @doc """
  Access the given element using the qualifier according
  to the `Access` protocol. All calls in the form `foo[bar]`
  are translated to `access(foo, bar)`.

  The usage of this protocol is to access a raw value in a
  keyword list.

      iex> sample = [a: 1, b: 2, c: 3]
      iex> sample[:b]
      2

  """

  @doc """
  Checks if the element on the left side is member of the
  collection on the right side.

  ## Examples

      iex> x = 1
      iex> x in [1, 2, 3]
      true

  This macro simply translates the expression above to:

      Enum.member?([1,2,3], x)

  ## Guards

  The `in` operator can be used on guard clauses as long as the
  right side is a range or a list. Elixir will then expand the
  operator to a valid guard expression. For example:

      when x in [1,2,3]

  Translates to:

      when x === 1 or x === 2 or x === 3

  When using ranges:

      when x in 1..3

  Translates to:

      when x >= 1 and x <= 3

  """
  defmacro left in right do
    cache = (__CALLER__.context == nil)

    right = case bootstraped?(Macro) do
      true  -> Macro.expand(right, __CALLER__)
      false -> right
    end

    case right do
      _ when cache ->
        quote do: Elixir.Enum.member?(unquote(right), unquote(left))
      [] ->
        false
      [h|t] ->
        :lists.foldr(fn x, acc ->
          quote do
            unquote(comp(left, x)) or unquote(acc)
          end
        end, comp(left, h), t)
      {:%{}, [], [__struct__: Elixir.Range, first: first, last: last]} ->
        in_range(left, Macro.expand(first, __CALLER__), Macro.expand(last, __CALLER__))
      _ ->
        raise ArgumentError, <<"invalid args for operator in, it expects a compile time list ",
                                        "or range on the right side when used in guard expressions, got: ",
                                        Macro.to_string(right) :: binary>>
    end
  end

  defp in_range(left, first, last) do
    case opt_in?(first) and opt_in?(last) do
      true  ->
        case first <= last do
          true  -> increasing_compare(left, first, last)
          false -> decreasing_compare(left, first, last)
        end
      false ->
        quote do
          (:erlang."=<"(unquote(first), unquote(last)) and
           unquote(increasing_compare(left, first, last)))
          or
          (:erlang."<"(unquote(last), unquote(first)) and
           unquote(decreasing_compare(left, first, last)))
        end
    end
  end

  defp opt_in?(x), do: is_integer(x) or is_float(x) or is_atom(x)

  defp comp(left, right) do
    quote(do: :erlang."=:="(unquote(left), unquote(right)))
  end

  defp increasing_compare(var, first, last) do
    quote do
      :erlang.">="(unquote(var), unquote(first)) and
      :erlang."=<"(unquote(var), unquote(last))
    end
  end

  defp decreasing_compare(var, first, last) do
    quote do
      :erlang."=<"(unquote(var), unquote(first)) and
      :erlang.">="(unquote(var), unquote(last))
    end
  end

  @doc """
  When used inside quoting, marks that the variable should
  not be hygienized. The argument can be either a variable
  unquoted or in standard tuple form `{name, meta, context}`.

  Check `Kernel.SpecialForms.quote/2` for more information.
  """
  defmacro var!(var, context \\ nil)

  defmacro var!({name, meta, atom}, context) when is_atom(name) and is_atom(atom) do
    do_var!(name, meta, context, __CALLER__)
  end

  defmacro var!(x, _context) do
    raise ArgumentError, "expected a var to be given to var!, got: #{Macro.to_string(x)}"
  end

  defp do_var!(name, meta, context, env) do
    # Remove counter and force them to be vars
    meta = :lists.keydelete(:counter, 1, meta)
    meta = :lists.keystore(:var, 1, meta, {:var, true})

    case Macro.expand(context, env) do
      x when is_atom(x) ->
        {name, meta, x}
      x ->
        raise ArgumentError, "expected var! context to expand to an atom, got: #{Macro.to_string(x)}"
    end
  end

  @doc """
  When used inside quoting, marks that the alias should not
  be hygienezed. This means the alias will be expanded when
  the macro is expanded.

  Check `Kernel.SpecialForms.quote/2` for more information.
  """
  defmacro alias!(alias)

  defmacro alias!(alias) when is_atom(alias) do
    alias
  end

  defmacro alias!({:__aliases__, meta, args}) do
    # Simply remove the alias metadata from the node
    # so it does not affect expansion.
    {:__aliases__, :lists.keydelete(:alias, 1, meta), args}
  end

  ## Definitions implemented in Elixir

  @doc ~S"""
  Defines a module given by name with the given contents.

  It returns the module name, the module binary and the
  block contents result.

  ## Examples

      iex> defmodule Foo do
      ...>   def bar, do: :baz
      ...> end
      iex> Foo.bar
      :baz

  ## Nesting

  Nesting a module inside another module affects its name:

      defmodule Foo do
        defmodule Bar do
        end
      end

  In the example above, two modules `Foo` and `Foo.Bar` are created.
  When nesting, Elixir automatically creates an alias, allowing the
  second module `Foo.Bar` to be accessed as `Bar` in the same lexical
  scope.

  This means that, if the module `Bar` is moved to another file,
  the references to `Bar` needs to be updated or an alias needs to
  be explicitly set with the help of `Kernel.SpecialForms.alias/2`.

  ## Dynamic names

  Elixir module names can be dynamically generated. This is very
  useful for macros. For instance, one could write:

      defmodule String.to_atom("Foo#{1}") do
        # contents ...
      end

  Elixir will accept any module name as long as the expression
  returns an atom. Note that, when a dynamic name is used, Elixir
  won't nest the name under the current module nor automatically
  set up an alias.
  """
  defmacro defmodule(alias, do: block) do
    env   = __CALLER__
    boot? = bootstraped?(Macro)

    expanded =
      case boot? do
        true  -> Macro.expand(alias, env)
        false -> alias
      end

    {expanded, with_alias} =
      case boot? and is_atom(expanded) do
        true ->
          # Expand the module considering the current environment/nesting
          full = expand_module(alias, expanded, env)

          # Generate the alias for this module definition
          {new, old} = module_nesting(env.module, full)
          meta = [defined: full, context: env.module] ++ alias_meta(alias)

          {full, {:alias, meta, [old, [as: new, warn: false]]}}
        false ->
          {expanded, nil}
      end

    {escaped, _} = :elixir_quote.escape(block, false)
    module_vars  = module_vars(env.vars, 0)

    quote do
      unquote(with_alias)
      :elixir_module.compile(unquote(expanded), unquote(escaped),
                             unquote(module_vars), __ENV__)
    end
  end

  defp alias_meta({:__aliases__, meta, _}), do: meta
  defp alias_meta(_), do: []

  # defmodule :foo
  defp expand_module(raw, _module, _env) when is_atom(raw),
    do: raw

  # defmodule Elixir.Alias
  defp expand_module({:__aliases__, _, [:Elixir|t]}, module, _env) when t != [],
    do: module

  # defmodule Alias in root
  defp expand_module({:__aliases__, _, _}, module, %{module: nil}),
    do: module

  # defmodule Alias nested
  defp expand_module({:__aliases__, _, t}, _module, env),
    do: :elixir_aliases.concat([env.module|t])

  # defmodule _
  defp expand_module(_raw, module, env),
    do: :elixir_aliases.concat([env.module, module])

  # quote vars to be injected into the module definition
  defp module_vars([{key, kind}|vars], counter) do
    var =
      case is_atom(kind) do
        true  -> {key, [], kind}
        false -> {key, [counter: kind], nil}
      end

    under = String.to_atom(<<"_@", :erlang.integer_to_binary(counter)::binary>>)
    args  = [key, kind, under, var]
    [{:{}, [], args}|module_vars(vars, counter+1)]
  end

  defp module_vars([], _counter) do
    []
  end

  # Gets two modules names and return an alias
  # which can be passed down to the alias directive
  # and it will create a proper shortcut representing
  # the given nesting.
  #
  # Examples:
  #
  #     module_nesting('Elixir.Foo.Bar', 'Elixir.Foo.Bar.Baz.Bat')
  #     {'Elixir.Baz', 'Elixir.Foo.Bar.Baz'}
  #
  # In case there is no nesting/no module:
  #
  #     module_nesting(nil, 'Elixir.Foo.Bar.Baz.Bat')
  #     {false, 'Elixir.Foo.Bar.Baz.Bat'}
  #
  defp module_nesting(nil, full),
    do: {false, full}

  defp module_nesting(prefix, full) do
    case split_module(prefix) do
      [] -> {false, full}
      prefix -> module_nesting(prefix, split_module(full), [], full)
    end
  end

  defp module_nesting([x|t1], [x|t2], acc, full),
    do: module_nesting(t1, t2, [x|acc], full)
  defp module_nesting([], [h|_], acc, _full),
    do: {String.to_atom(<<"Elixir.", h::binary>>),
          :elixir_aliases.concat(:lists.reverse([h|acc]))}
  defp module_nesting(_, _, _acc, full),
    do: {false, full}

  defp split_module(atom) do
    case :binary.split(Atom.to_string(atom), ".", [:global]) do
      ["Elixir"|t] -> t
      _ -> []
    end
  end

  @doc """
  Defines a function with the given name and contents.

  ## Examples

      defmodule Foo do
        def bar, do: :baz
      end

      Foo.bar #=> :baz

  A function that expects arguments can be defined as follow:

      defmodule Foo do
        def sum(a, b) do
          a + b
        end
      end

  In the example above, we defined a function `sum` that receives
  two arguments and sums them.

  """
  defmacro def(call, expr \\ nil) do
    define(:def, call, expr, __CALLER__)
  end

  @doc """
  Defines a function that is private. Private functions are
  only accessible from within the module in which they are defined.

  Check `def/2` for more information

  ## Examples

      defmodule Foo do
        def bar do
          sum(1, 2)
        end

        defp sum(a, b), do: a + b
      end

  In the example above, `sum` is private and accessing it
  through `Foo.sum` will raise an error.
  """
  defmacro defp(call, expr \\ nil) do
    define(:defp, call, expr, __CALLER__)
  end

  @doc """
  Defines a macro with the given name and contents.

  ## Examples

      defmodule MyLogic do
        defmacro unless(expr, opts) do
          quote do
            if !unquote(expr), unquote(opts)
          end
        end
      end

      require MyLogic
      MyLogic.unless false do
        IO.puts "It works"
      end

  """
  defmacro defmacro(call, expr \\ nil) do
    define(:defmacro, call, expr, __CALLER__)
  end

  @doc """
  Defines a macro that is private. Private macros are
  only accessible from the same module in which they are defined.

  Check `defmacro/2` for more information
  """
  defmacro defmacrop(call, expr \\ nil) do
    define(:defmacrop, call, expr, __CALLER__)
  end

  defp define(kind, call, expr, env) do
    assert_module_scope(env, kind, 2)
    assert_no_function_scope(env, kind, 2)
    line = env.line

    {call, uc} = :elixir_quote.escape(call, true)
    {expr, ue} = :elixir_quote.escape(expr, true)

    # Do not check clauses if any expression was unquoted
    check_clauses = not(ue or uc)
    pos = :elixir_locals.cache_env(env)

    quote do
      :elixir_def.store_definition(unquote(line), unquote(kind), unquote(check_clauses),
                                   unquote(call), unquote(expr), unquote(pos))
    end
  end

  @doc """
  Defines a struct for the current module.

  A struct is a tagged map that allows developers to provide
  default values for keys, tags to be used in polymorphic
  dispatches and compile time assertions.

  To define a struct, a developer needs to only define
  a function named `__struct__/0` that returns a map with the
  structs field. This macro is a convenience for defining such
  function, with the addition of a type `t` and deriving
  conveniences.

  For more information about structs, please check
  `Kernel.SpecialForms.%/2`.

  ## Examples

      defmodule User do
        defstruct name: nil, age: nil
      end

  Struct fields are evaluated at definition time, which allows
  them to be dynamic. In the example below, `10 + 11` will be
  evaluated at compilation time and the age field will be stored
  with value `21`:

      defmodule User do
        defstruct name: nil, age: 10 + 11
      end

  ## Deriving

  Although structs are maps, by default structs do not implement
  any of the protocols implemented for maps. For example, if you
  attempt to use the access protocol with the User struct, it
  will lead to an error:

      %User{}[:age]
      ** (Protocol.UndefinedError) protocol Access not implemented for %User{...}

  However, `defstruct/2` allows implementation for protocols to
  derived by defining a `@derive` attribute as a list before `defstruct/2`
  is invoked:

      defmodule User do
        @derive [Access]
        defstruct name: nil, age: 10 + 11
      end

      %User{}[:age] #=> 21

  For each protocol given to `@derive`, Elixir will assert there is an
  implementation of that protocol for maps and check if the map
  implementation defines a `__deriving__/3` callback. If so, the callback
  is invoked, otherwise an implementation that simply points to the map
  one is automatically derived.

  ## Types

  It is recommended to define types for structs, by convention this type
  is called `t`. To define a struct in a type the struct literal syntax
  is used:

      defmodule User do
        defstruct name: "john", age: 25
        @type t :: %User{name: String.t, age: integer}
      end

  It is recommended to only use the struct syntax when defining the struct's
  type. When referring to another struct use `User.t`, not `%User{}`. Fields
  in the struct not included in the type defaults to `term`.

  Private structs that are not used outside its module should use the private
  type attribute `@typep`. Public structs whose internal structure is private
  to the local module (you are not allowed to pattern match it or directly
  access fields) should use the `@opaque` attribute. Structs whose internal
  structure is public should use `@type`.
  """
  defmacro defstruct(fields) do
    fields =
      quote bind_quoted: [fields: fields] do
        fields = :lists.map(fn
          {key, _} = pair when is_atom(key) -> pair
          key when is_atom(key) -> {key, nil}
          other -> raise ArgumentError, "struct field names must be atoms, got: #{inspect other}"
        end, fields)

        @struct :maps.put(:__struct__, __MODULE__, :maps.from_list(fields))

        case Module.get_attribute(__MODULE__, :derive) do
          [] ->
            :ok
          derive ->
            Protocol.__derive__(derive, __MODULE__, __ENV__)
        end

        @spec __struct__() :: %__MODULE__{}
        def __struct__() do
          @struct
        end
      end

    quote do
      unquote(fields)
      fields
    end
  end

  @doc ~S"""
  Defines an exception.

  Exceptions are structs backed by a module that implements
  the Exception behaviour. The Exception behaviour requires
  two functions to be implemented:

    * `exception/1` - that receives the arguments given to `raise/2`
       and returns the exception struct. The default implementation
       accepts a set of keyword arguments that is merged into the
       struct.

    * `message/1` - receives the exception struct and must return its
      message. Most commonly exceptions have a message field which
      by default is accessed by this function. However, if your exception
      does not have a message field, this function must be explicitly
      implemented.

  Since exceptions are structs, all the API supported by `defstruct/1`
  is also available in `defexception/1`.

  ## Raising exceptions

  The most common way to raise an exception is via the `raise/2`
  function:

      defmodule MyAppError do
        defexception [:message]
      end

      raise MyAppError,
        message: "did not get what was expected, got: #{inspect value}"

  In many cases it is more convenient to pass the expected value to
  `raise` and generate the message in the `exception/1` callback:

      defmodule MyAppError do
        defexception [:message]

        def exception(value) do
          msg = "did not get what was expected, got: #{inspect value}"
          %MyAppError{message: msg}
        end
      end

      raise MyAppError, value

  The example above is the preferred mechanism for customizing
  exception messages.
  """
  defmacro defexception(fields) do
    fields = case is_list(fields) do
      true  -> [{:__exception__, true}|fields]
      false -> quote(do: [{:__exception__, true}] ++ unquote(fields))
    end

    quote do
      @behaviour Exception
      fields = defstruct unquote(fields)

      @spec exception(Keyword.t) :: Exception.t
      def exception(args) when is_list(args) do
        Kernel.struct(__struct__, args)
      end

      defoverridable exception: 1

      if Keyword.has_key?(fields, :message) do
        @spec message(Exception.t) :: String.t
        def message(exception) do
          exception.message
        end

        defoverridable message: 1
      end
    end
  end

  @doc """
  Defines a protocol.

  A protocol specifies an API that should be defined by its
  implementations.

  ## Examples

  In Elixir, only `false` and `nil` are considered falsy values.
  Everything else evaluates to true in `if` clauses. Depending
  on the application, it may be important to specify a `blank?`
  protocol that returns a boolean for other data types that should
  be considered `blank?`. For instance, an empty list or an empty
  binary could be considered blanks.

  We could implement this protocol as follow:

      defprotocol Blank do
        @doc "Returns true if data is considered blank/empty"
        def blank?(data)
      end

  Now that the protocol is defined, we can implement it. We need
  to implement the protocol for each Elixir type. For example:

      # Integers are never blank
      defimpl Blank, for: Integer do
        def blank?(number), do: false
      end

      # Just empty list is blank
      defimpl Blank, for: List do
        def blank?([]), do: true
        def blank?(_),  do: false
      end

      # Just the atoms false and nil are blank
      defimpl Blank, for: Atom do
        def blank?(false), do: true
        def blank?(nil),   do: true
        def blank?(_),     do: false
      end

  And we would have to define the implementation for all types.
  The supported types available are:

    * Structs (see below)
    * `Tuple`
    * `Atom`
    * `List`
    * `BitString`
    * `Integer`
    * `Float`
    * `Function`
    * `PID`
    * `Map`
    * `Port`
    * `Reference`
    * `Any` (see below)

  ## Protocols + Structs

  The real benefit of protocols comes when mixed with structs.
  For instance, Elixir ships with many data types implemented as
  structs, like `HashDict` and `HashSet`. We can implement the
  `Blank` protocol for those types as well:

      defimpl Blank, for: [HashDict, HashSet] do
        def blank?(enum_like), do: Enum.empty?(enum_like)
      end

  If a protocol is not found for a given type, it will fallback to
  `Any`.

  ## Fallback to any

  In some cases, it may be convenient to provide a default
  implementation for all types. This can be achieved by
  setting `@fallback_to_any` to `true` in the protocol
  definition:

      defprotocol Blank do
        @fallback_to_any true
        def blank?(data)
      end

  Which can now be implemented as:

      defimpl Blank, for: Any do
        def blank?(_), do: true
      end

  One may wonder why such fallback is not true by default.

  It is two-fold: first, the majority of protocols cannot
  implement an action in a generic way for all types. In fact,
  providing a default implementation may be harmful, because users
  may rely on the default implementation instead of providing a
  specialized one.

  Second, falling back to `Any` adds an extra lookup to all types,
  which is unnecessary overhead unless an implementation for Any is
  required.

  ## Types

  Defining a protocol automatically defines a type named `t`, which
  can be used as:

      @spec present?(Blank.t) :: boolean
      def present?(blank) do
        not Blank.blank?(blank)
      end

  The `@spec` above expresses that all types allowed to implement the
  given protocol are valid argument types for the given function.

  ## Reflection

  Any protocol module contains three extra functions:


    * `__protocol__/1` - returns the protocol name when `:name` is given, and a
      keyword list with the protocol functions when `:functions` is given

    * `impl_for/1` - receives a structure and returns the module that
      implements the protocol for the structure, `nil` otherwise

    * `impl_for!/1` - same as above but raises an error if an implementation is
      not found

  ## Consolidation

  In order to cope with code loading in development, protocols in
  Elixir provide a slow implementation of protocol dispatching specific
  to development.

  In order to speed up dispatching in production environments, where
  all implementations are known up-front, Elixir provides a feature
  called protocol consolidation. For this reason, all protocols are
  compiled with `debug_info` set to true, regardless of the option
  set by `elixirc` compiler. The debug info though may be removed
  after consolidation.

  For more information on how to apply protocol consolidation to
  a given project, please check the functions in the `Protocol`
  module or the `mix compile.protocols` task.
  """
  defmacro defprotocol(name, do: block) do
    Protocol.__protocol__(name, do: block)
  end

  @doc """
  Defines an implementation for the given protocol. See
  `defprotocol/2` for examples.

  Inside an implementation, the name of the protocol can be accessed
  via `@protocol` and the current target as `@for`.
  """
  defmacro defimpl(name, opts, do_block \\ []) do
    merged = Keyword.merge(opts, do_block)
    merged = Keyword.put_new(merged, :for, __CALLER__.module)
    Protocol.__impl__(name, merged)
  end

  @doc """
  Makes the given functions in the current module overridable. An overridable
  function is lazily defined, allowing a developer to customize it.

  ## Example

      defmodule DefaultMod do
        defmacro __using__(_opts) do
          quote do
            def test(x, y) do
              x + y
            end

            defoverridable [test: 2]
          end
        end
      end

      defmodule InheritMod do
        use DefaultMod

        def test(x, y) do
          x * y + super(x, y)
        end
      end

  As seen as in the example `super` can be used to call the default
  implementation.
  """
  defmacro defoverridable(tuples) do
    quote do
      Module.make_overridable(__MODULE__, unquote(tuples))
    end
  end

  @doc """
  `use` is a simple mechanism for using a given module into
  the current context.

  ## Examples

  For example, in order to write tests using the ExUnit framework,
  a developer should use the `ExUnit.Case` module:

      defmodule AssertionTest do
        use ExUnit.Case, async: true

        test "always pass" do
          assert true
        end
      end

  By calling `use`, a hook called `__using__` will be invoked in
  `ExUnit.Case` which will then do the proper setup.

  Simply put, `use` is simply a translation to:

      defmodule AssertionTest do
        require ExUnit.Case
        ExUnit.Case.__using__([async: true])

        test "always pass" do
          assert true
        end
      end

  """
  defmacro use(module, opts \\ []) do
    expanded = Macro.expand(module, __CALLER__)

    case is_atom(expanded) do
      false ->
        raise ArgumentError, "invalid arguments for use, expected an atom or alias as argument"
      true ->
        quote do
          require unquote(expanded)
          unquote(expanded).__using__(unquote(opts))
        end
    end
  end

  @doc """
  Defines the given functions in the current module that will
  delegate to the given `target`. Functions defined with
  `defdelegate` are public and are allowed to be invoked
  from external. If you find yourself wishing to define a
  delegation as private, you should likely use import
  instead.

  Delegation only works with functions, delegating to macros
  is not supported.

  ## Options

    * `:to` - the expression to delegate to. Any expression
      is allowed and its results will be calculated on runtime.

    * `:as` - the function to call on the target given in `:to`.
      This parameter is optional and defaults to the name being
      delegated.

    * `:append_first` - if true, when delegated, first argument
      passed to the delegate will be relocated to the end of the
      arguments when dispatched to the target.

      The motivation behind this is because Elixir normalizes
      the "handle" as a first argument and some Erlang modules
      expect it as last argument.

  ## Examples

      defmodule MyList do
        defdelegate reverse(list), to: :lists
        defdelegate [reverse(list), map(callback, list)], to: :lists
        defdelegate other_reverse(list), to: :lists, as: :reverse
      end

      MyList.reverse([1, 2, 3])
      #=> [3,2,1]

      MyList.other_reverse([1, 2, 3])
      #=> [3,2,1]

  """
  defmacro defdelegate(funs, opts) do
    funs = Macro.escape(funs, unquote: true)
    quote bind_quoted: [funs: funs, opts: opts] do
      target = Keyword.get(opts, :to) ||
        raise ArgumentError, "Expected to: to be given as argument"

      append_first = Keyword.get(opts, :append_first, false)

      for fun <- List.wrap(funs) do
        {name, args} =
          case Macro.decompose_call(fun) do
            {_, _} = pair -> pair
            _ -> raise ArgumentError, "invalid syntax in defdelegate #{Macro.to_string(fun)}"
          end

        actual_args =
          case append_first and args != [] do
            true  -> tl(args) ++ [hd(args)]
            false -> args
          end

        fun = Keyword.get(opts, :as, name)

        def unquote(name)(unquote_splicing(args)) do
          unquote(target).unquote(fun)(unquote_splicing(actual_args))
        end
      end
    end
  end

  ## Sigils

  @doc ~S"""
  Handles the sigil ~S. It simply returns a string
  without escaping characters and without interpolations.

  ## Examples

      iex> ~S(foo)
      "foo"

      iex> ~S(f#{o}o)
      "f\#{o}o"

  """
  defmacro sigil_S(string, []) do
    string
  end

  @doc ~S"""
  Handles the sigil ~s. It returns a string as if it was double quoted
  string, unescaping characters and replacing interpolations.

  ## Examples

      iex> ~s(foo)
      "foo"

      iex> ~s(f#{:o}o)
      "foo"

      iex> ~s(f\#{:o}o)
      "f\#{:o}o"

  """
  defmacro sigil_s({:<<>>, line, pieces}, []) do
    {:<<>>, line, Macro.unescape_tokens(pieces)}
  end

  @doc ~S"""
  Handles the sigil ~C. It simply returns a char list
  without escaping characters and without interpolations.

  ## Examples

      iex> ~C(foo)
      'foo'

      iex> ~C(f#{o}o)
      'f\#{o}o'

  """
  defmacro sigil_C({:<<>>, _line, [string]}, []) when is_binary(string) do
    String.to_char_list(string)
  end

  @doc ~S"""
  Handles the sigil ~c. It returns a char list as if it were a single
  quoted string, unescaping characters and replacing interpolations.

  ## Examples

      iex> ~c(foo)
      'foo'

      iex> ~c(f#{:o}o)
      'foo'

      iex> ~c(f\#{:o}o)
      'f\#{:o}o'

  """

  # We can skip the runtime conversion if we are
  # creating a binary made solely of series of chars.
  defmacro sigil_c({:<<>>, _line, [string]}, []) when is_binary(string) do
    String.to_char_list(Macro.unescape_string(string))
  end

  defmacro sigil_c({:<<>>, line, pieces}, []) do
    binary = {:<<>>, line, Macro.unescape_tokens(pieces)}
    quote do: String.to_char_list(unquote(binary))
  end

  @doc """
  Handles the sigil ~r. It returns a Regex pattern.

  ## Examples

      iex> Regex.match?(~r(foo), "foo")
      true

  """
  defmacro sigil_r({:<<>>, _line, [string]}, options) when is_binary(string) do
    binary = Macro.unescape_string(string, fn(x) -> Regex.unescape_map(x) end)
    regex  = Regex.compile!(binary, :binary.list_to_bin(options))
    Macro.escape(regex)
  end

  defmacro sigil_r({:<<>>, line, pieces}, options) do
    binary = {:<<>>, line, Macro.unescape_tokens(pieces, fn(x) -> Regex.unescape_map(x) end)}
    quote do: Regex.compile!(unquote(binary), unquote(:binary.list_to_bin(options)))
  end

  @doc ~S"""
  Handles the sigil ~R. It returns a Regex pattern without escaping
  nor interpreting interpolations.

  ## Examples

      iex> Regex.match?(~R(f#{1,3}o), "f#o")
      true

  """
  defmacro sigil_R({:<<>>, _line, [string]}, options) when is_binary(string) do
    regex = Regex.compile!(string, :binary.list_to_bin(options))
    Macro.escape(regex)
  end

  @doc ~S"""
  Handles the sigil ~w. It returns a list of "words" split by whitespace.

  ## Modifiers

    * `s`: strings (default)
    * `a`: atoms
    * `c`: char lists

  ## Examples

      iex> ~w(foo #{:bar} baz)
      ["foo", "bar", "baz"]

      iex> ~w(--source test/enum_test.exs)
      ["--source", "test/enum_test.exs"]

      iex> ~w(foo bar baz)a
      [:foo, :bar, :baz]

  """

  defmacro sigil_w({:<<>>, _line, [string]}, modifiers) when is_binary(string) do
    split_words(Macro.unescape_string(string), modifiers)
  end

  defmacro sigil_w({:<<>>, line, pieces}, modifiers) do
    binary = {:<<>>, line, Macro.unescape_tokens(pieces)}
    split_words(binary, modifiers)
  end

  @doc ~S"""
  Handles the sigil ~W. It returns a list of "words" split by whitespace
  without escaping nor interpreting interpolations.

  ## Modifiers

    * `s`: strings (default)
    * `a`: atoms
    * `c`: char lists

  ## Examples

      iex> ~W(foo #{bar} baz)
      ["foo", "\#{bar}", "baz"]

  """
  defmacro sigil_W({:<<>>, _line, [string]}, modifiers) when is_binary(string) do
    split_words(string, modifiers)
  end

  defp split_words("", _modifiers), do: []

  defp split_words(string, modifiers) do
    mod =
      case modifiers do
        [] -> ?s
        [mod] when mod == ?s or mod == ?a or mod == ?c -> mod
        _else -> raise ArgumentError, "modifier must be one of: s, a, c"
      end

    case is_binary(string) do
      true ->
        case mod do
          ?s -> String.split(string)
          ?a -> for p <- String.split(string), do: String.to_atom(p)
          ?c -> for p <- String.split(string), do: String.to_char_list(p)
        end
      false ->
        case mod do
          ?s -> quote do: String.split(unquote(string))
          ?a -> quote do: for(p <- String.split(unquote(string)), do: String.to_atom(p))
          ?c -> quote do: for(p <- String.split(unquote(string)), do: String.to_char_list(p))
        end
    end
  end

  ## Shared functions

  defp optimize_boolean({:case, meta, args}) do
    {:case, [{:optimize_boolean, true}|meta], args}
  end

  # We need this check only for bootstrap purposes.
  # Once Kernel is loaded and we recompile, it is a no-op.
  case :code.ensure_loaded(Kernel) do
    {:module, _} ->
      defp bootstraped?(_), do: true
    {:error, _} ->
      defp bootstraped?(module), do: :code.ensure_loaded(module) == {:module, module}
  end

  defp assert_module_scope(env, fun, arity) do
    case env.module do
      nil -> raise ArgumentError, "cannot invoke #{fun}/#{arity} outside module"
      _   -> :ok
    end
  end

  defp assert_no_function_scope(env, fun, arity) do
    case env.function do
      nil -> :ok
      _   -> raise ArgumentError, "cannot invoke #{fun}/#{arity} inside function/macro"
    end
  end

  defp env_stacktrace(env) do
    case bootstraped?(Path) do
      true  -> Macro.Env.stacktrace(env)
      false -> []
    end
  end
end
