add all validator, simplify error messages

in.go

@@ -2,24 +2,29 @@ package validate
 
 import "errors"
 
+// Validation error.
+var (
+	ErrValueNotAllowed = errors.New("not allowed")
+)
+
 // In validates whether a value is found in a slice of allowed values.
-func In[T comparable](allow []T) func(T) error {
+func In[T comparable](allow ...T) func(T) error {
 	return func(value T) error {
 		for _, cmp := range allow {
 			if cmp == value {
 				return nil
 			}
 		}
-		return errors.New("Not an allowed value")
+		return ErrValueNotAllowed
 	}
 }
 
 // NotIn validates whether a value is not found in a slice of disallowed values.
-func NotIn[T comparable](allow []T) func(T) error {
+func NotIn[T comparable](allow ...T) func(T) error {
 	return func(value T) error {
 		for _, cmp := range allow {
 			if cmp == value {
-				return errors.New("Not an allowed value")
+				return ErrValueNotAllowed
 			}
 		}
 		return nil