spf13 · songzhibin97 · Jan 10, 2025 · Aug 20, 2025 · Aug 20, 2025 · Aug 20, 2025
diff --git a/basic.go b/basic.go
@@ -62,6 +62,13 @@ func ToBoolE(i any) (bool, error) {
 			return ToBoolE(i)
 		}
 
+		// Try custom conversion interface
+		if setter, ok := i.(ValueSetter); ok {
+			var result bool
+			err := setter.SetValue(&result)
+			return result, err
+		}
+
 		return false, fmt.Errorf(errorMsg, i, i, false)
 	}
 }
@@ -126,6 +133,13 @@ func ToStringE(i any) (string, error) {
 			return ToStringE(i)
 		}
 
+		// Try custom conversion interface
+		if setter, ok := i.(ValueSetter); ok {
+			var result string
+			err := setter.SetValue(&result)
+			return result, err
+		}
+
 		return "", fmt.Errorf(errorMsg, i, i, "")
 	}
 }
diff --git a/cast.go b/cast.go
@@ -6,7 +6,10 @@
 // Package cast provides easy and safe casting in Go.
 package cast
 
-import "time"
+import (
+	"fmt"
+	"time"
+)
 
 const errorMsg = "unable to cast %#v of type %T to %T"
 const errorMsgWith = "unable to cast %#v of type %T to %T: %w"
@@ -58,6 +61,8 @@ func ToE[T Basic](i any) (T, error) {
 		v, err = ToTimeE(i)
 	case time.Duration:
 		v, err = ToDurationE(i)
+	default:
+		return t, fmt.Errorf("unknown basic type: %T", t)
 	}
 
 	if err != nil {
@@ -67,18 +72,24 @@ func ToE[T Basic](i any) (T, error) {
 	return v.(T), nil
 }
 
-// Must is a helper that wraps a call to a cast function and panics if the error is non-nil.
-func Must[T any](i any, err error) T {
-	if err != nil {
-		panic(err)
-	}
-
-	return i.(T)
-}
-
 // To casts any value to a [Basic] type.
 func To[T Basic](i any) T {
 	v, _ := ToE[T](i)
 
 	return v
 }
+
+// Must panics if there is an error, otherwise returns the value.
+func Must[T any](v T, err error) T {
+	if err != nil {
+		panic(err)
+	}
+	return v
+}
+
+// ValueSetter is an interface for types that can provide custom conversion logic.
+// When a conversion function encounters a type it cannot handle in its default case,
+// it will check if the type implements ValueSetter and use it for custom conversion.
+type ValueSetter interface {
+	SetValue(any) error
+}
diff --git a/example_valuesetter_test.go b/example_valuesetter_test.go
@@ -0,0 +1,120 @@
+package cast
+
+import (
+	"fmt"
+	"time"
+)
+
+// CustomBool demonstrates a custom type that implements ValueSetter
+type CustomBool struct {
+	value string
+}
+
+func (c CustomBool) SetValue(target any) error {
+	switch t := target.(type) {
+	case *bool:
+		*t = c.value == "yes" || c.value == "true" || c.value == "1"
+		return nil
+	case *string:
+		if c.value == "yes" || c.value == "true" || c.value == "1" {
+			*t = "true"
+		} else {
+			*t = "false"
+		}
+		return nil
+	default:
+		return fmt.Errorf("unsupported target type: %T", target)
+	}
+}
+
+// CustomTime demonstrates a custom time type
+type CustomTime struct {
+	timestamp int64
+}
+
+func (c CustomTime) SetValue(target any) error {
+	switch t := target.(type) {
+	case *time.Time:
+		*t = time.Unix(c.timestamp, 0)
+		return nil
+	case *string:
+		*t = time.Unix(c.timestamp, 0).Format(time.RFC3339)
+		return nil
+	default:
+		return fmt.Errorf("unsupported target type: %T", target)
+	}
+}
+
+// CustomInt demonstrates a custom integer type
+type CustomInt struct {
+	hexValue string
+}
+
+func (c CustomInt) SetValue(target any) error {
+	switch t := target.(type) {
+	case *int:
+		if c.hexValue == "0xFF" {
+			*t = 255
+		} else {
+			*t = 0
+		}
+		return nil
+	case *string:
+		*t = c.hexValue
+		return nil
+	default:
+		return fmt.Errorf("unsupported target type: %T", target)
+	}
+}
+
+func ExampleValueSetter_bool() {
+	custom := CustomBool{value: "yes"}
+
+	result, err := ToBoolE(custom)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+
+	fmt.Printf("Bool result: %v\n", result)
+	// Output: Bool result: true
+}
+
+func ExampleValueSetter_string() {
+	custom := CustomBool{value: "yes"}
+
+	result, err := ToStringE(custom)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+
+	fmt.Printf("String result: %s\n", result)
+	// Output: String result: true
+}
+
+func ExampleValueSetter_time() {
+	custom := CustomTime{timestamp: 1609459200} // 2021-01-01 00:00:00 UTC
+
+	result, err := ToTimeE(custom)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+
+	fmt.Printf("Time result: %s\n", result.Format("2006-01-02"))
+	// Output: Time result: 2021-01-01
+}
+
+func ExampleValueSetter_int() {
+	custom := CustomInt{hexValue: "0xFF"}
+
+	result, err := ToIntE(custom)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+
+	fmt.Printf("Int result: %d\n", result)
+	// Output: Int result: 255
+}
diff --git a/number.go b/number.go
@@ -188,6 +188,13 @@ func toNumberE[T Number](i any, parseFn func(string) (T, error)) (T, error) {
 			return toNumberE(i, parseFn)
 		}
 
+		// Try custom conversion interface
+		if setter, ok := i.(ValueSetter); ok {
+			var result T
+			err := setter.SetValue(&result)
+			return result, err
+		}
+
 		return 0, fmt.Errorf(errorMsg, i, i, n)
 	}
 }

diff --git a/slice.go b/slice.go
@@ -28,6 +28,13 @@ func ToSliceE(i any) ([]any, error) {
 
 		return s, nil
 	default:
+		// Try custom conversion interface
+		if setter, ok := i.(ValueSetter); ok {
+			var result []any
+			err := setter.SetValue(&result)
+			return result, err
+		}
+
 		return s, fmt.Errorf(errorMsg, i, i, s)
 	}
 }

diff --git a/time.go b/time.go
@@ -56,6 +56,13 @@ func ToTimeInDefaultLocationE(i any, location *time.Location) (tim time.Time, er
 	case nil:
 		return time.Time{}, nil
 	default:
+		// Try custom conversion interface
+		if setter, ok := i.(ValueSetter); ok {
+			var result time.Time
+			err := setter.SetValue(&result)
+			return result, err
+		}
+
 		return time.Time{}, fmt.Errorf(errorMsg, i, i, time.Time{})
 	}
 }
@@ -96,6 +103,13 @@ func ToDurationE(i any) (time.Duration, error) {
 			return ToDurationE(i)
 		}
 
+		// Try custom conversion interface
+		if setter, ok := i.(ValueSetter); ok {
+			var result time.Duration
+			err := setter.SetValue(&result)
+			return result, err
+		}
+
 		return 0, fmt.Errorf(errorMsg, i, i, time.Duration(0))
 	}
 }

diff --git a/valuesetter_test.go b/valuesetter_test.go
@@ -0,0 +1,85 @@
+package cast
+
+import (
+	"testing"
+	"time"
+)
+
+// TestValueSetterIntegration tests that ValueSetter interface works correctly
+// with all conversion functions in their default branches
+func TestValueSetterIntegration(t *testing.T) {
+	// Test with CustomBool
+	customBool := CustomBool{value: "yes"}
+
+	// Test ToBoolE
+	result, err := ToBoolE(customBool)
+	if err != nil {
+		t.Errorf("ToBoolE failed: %v", err)
+	}
+	if !result {
+		t.Errorf("Expected true, got %v", result)
+	}
+
+	// Test ToStringE
+	strResult, err := ToStringE(customBool)
+	if err != nil {
+		t.Errorf("ToStringE failed: %v", err)
+	}
+	if strResult != "true" {
+		t.Errorf("Expected 'true', got %v", strResult)
+	}
+
+	// Test with CustomTime
+	customTime := CustomTime{timestamp: 1609459200} // 2021-01-01 00:00:00 UTC
+
+	timeResult, err := ToTimeE(customTime)
+	if err != nil {
+		t.Errorf("ToTimeE failed: %v", err)
+	}
+	expected := time.Unix(1609459200, 0)
+	if !timeResult.Equal(expected) {
+		t.Errorf("Expected %v, got %v", expected, timeResult)
+	}
+
+	// Test with CustomInt
+	customInt := CustomInt{hexValue: "0xFF"}
+
+	intResult, err := ToIntE(customInt)
+	if err != nil {
+		t.Errorf("ToIntE failed: %v", err)
+	}
+	if intResult != 255 {
+		t.Errorf("Expected 255, got %v", intResult)
+	}
+}
+
+// TestStandardConversionPriority ensures standard conversions work normally
+// and ValueSetter is only used as fallback
+func TestStandardConversionPriority(t *testing.T) {
+	// Test that standard string conversion works normally
+	result, err := ToStringE("hello")
+	if err != nil {
+		t.Errorf("ToStringE failed: %v", err)
+	}
+	if result != "hello" {
+		t.Errorf("Expected 'hello', got %v", result)
+	}
+
+	// Test that standard bool conversion works normally
+	boolResult, err := ToBoolE(true)
+	if err != nil {
+		t.Errorf("ToBoolE failed: %v", err)
+	}
+	if !boolResult {
+		t.Errorf("Expected true, got %v", boolResult)
+	}
+
+	// Test that standard int conversion works normally
+	intResult, err := ToIntE(42)
+	if err != nil {
+		t.Errorf("ToIntE failed: %v", err)
+	}
+	if intResult != 42 {
+		t.Errorf("Expected 42, got %v", intResult)
+	}
+}