utop is an improved toplevel for OCaml. It can run in a terminal or in Emacs. It supports line edition, history, real-time and context sensitive completion, colors, and more.

It integrates with the tuareg and typerex modes in Emacs.

The easiest and recommended way of installing utop is via opam:

$ opam install utop

If you want to build it manually, you should install all the dependencies listed in the next section.

• OCaml (>= 3.12)
• findlib
• react
• lwt (>= 2.4.0) built with react support
• Camomile (>= 0.8)
• zed (>= 1.2)
• lambda-term (>= 1.2)

For building the development version, you also need to install oasis (>= 0.3.0).

utop also requires OCaml compiler libraries. Since OCaml 4.00 they are already installed, for previous versions:

• if you are using debian, they are available as the package ocaml-compiler-libs,
• if you are using godi, they are installed by default,
• if you installed ocaml by hand, you can run the script utils/install-compiler-libs.sh.

To build and install utop:

$ ./configure
$ make
$ make install

To build the documentation:

$ make doc

It will then be installed by make install.

To build and execute tests:

$ ./configure --enable-tests
$ make test

To use utop, simply run:

$ utop

utop display a bar after the prompt which is used to show possible completions in real-time. You can navigate in it using M-left and M-right, and select one completion using M-tab. The M denotes the meta key, which is Alt most of the time.

To add colors to utop, copy one of the files utoprc-dark or utoprc-light to ~/.utoprc. utoprc-dark is for terminals with dark colors (such as white on black) and utoprc-light is for terminals with light colors (such as black on white).

You can customize the prompt of utop by setting the reference UTop.prompt.

Key bindings in the terminal can be changed by writing a ~/.lambda-term-inputrc file. For example:

[read-line]
C-left: complete-bar-prev
C-right: complete-bar-next
C-down: complete-bar

If manual pages are correctly installed you can see a description of this file by executing:

$ man 5 lambda-term-inputrc

To use utop in emacs, first you need to make sure emacs can find the command utop and the file utop.el. If you installed utop via opam you can copy-paste this code into you ~/.emacs file:

;; Setup environment variables using opam
(dolist (var (car (read-from-string (shell-command-to-string "opam config env --sexp"))))
  (setenv (car var) (cadr var)))

;; Update the emacs path
(setq exec-path (split-string (getenv "PATH") path-separator))

;; Update the emacs load path
(push (concat (getenv "OCAML_TOPLEVEL_PATH") "/../../share/emacs/site-lisp") load-path)

;; Automatically load utop.el
(autoload 'utop "utop" "Toplevel for OCaml" t)

Then you can execute utop inside emacs with: M-x utop.

You can replace the default toplevel used by the tuareg or typerex mode by utop, for that add the following lines to your ~/.emacs file:

(autoload 'utop-setup-ocaml-buffer "utop" "Toplevel for OCaml" t)
(add-hook 'tuareg-mode-hook 'utop-setup-ocaml-buffer)
(add-hook 'typerex-mode-hook 'utop-setup-ocaml-buffer)

You can also complete text in a tuareg or typerex buffer using the environment of the toplevel. For that bind the function utop-edit-complete to the key you want.

If you get this error when running utop in a terminal or in emacs this means that the environment variable CAML_LD_LIBRARY_PATH is not set correctly:

Fatal error: cannot load shared library dlllwt-unix_stubs
Reason: dlopen(dlllwt-unix_stubs.so, 138): image not found

It shall point to the directory stublibs inside your ocaml installation.

If you want to create a custom toplevel with utop instead of the classic one you need to link it with utop and its dependencies and call UTop_main.main in the last linked unit. You also need to pass the -thread switch when linking the toplevel.

The easiest way to do that is by using ocamlfind:

$ ocamlfind ocamlmktop -o myutop -thread -linkpkg -package utop myutop_main.cmo

Where myutop_main.ml contains:

let () = UTop_main.main ()

You can also use the ocamlc sub-command instead of ocamlmktop, in this case you need to pass these thee extra arguments:

• -linkall to be sure all units are linked into the produced toplevel
• -package compiler-libs.toplevel
• -predicates create_toploop

With the last option ocamlfind will generate a small ocaml unit, linked just before myutop_main.cmo, which will register at startup packages already linked in the toplevel so they are not loaded again by the #require directive. It does the same with the ocamlmktop sub-command.

For example:

$ camlfind ocamlc -o myutop -thread -linkpkg -linkall -predicates create_toploop \
    -package compiler-libs.toplevel,utop myutop.cmo

Note that if you are not using ocamlfind, you will need to do that yourself. You have to call Topfind.don't_load with the list of all packages linked with the toplevel.

A full example using ocamlbuild is provided in the examples/custom-utop directory.