README.md

@@ -1,7 +1,85 @@
 # Go Validate
 
-A suite of straightforward validation functions. You put something in, you get back `nil` or an error.
+Small, composable value validators for Go. A validator is just a
+`func(T) error` — it returns `nil` or an error. You compose them with
+`All` or `Collect` and drop in plain closures wherever you need something
+custom. No struct tags, no reflection, no dependencies outside the
+standard library.
+
+```go
+username := validate.All(
+	validate.MinLength(3),
+	validate.MaxLength(16),
+	validate.Chars("abcdefghijklmnopqrstuvwxyz0123456789_"),
+	func(s string) error {
+		if strings.HasPrefix(s, "_") {
+			return errors.New("must not start with an underscore")
+		}
+		return nil
+	},
+)
+
+if err := username(input); err != nil {
+	// errors.Is(err, validate.ErrDisallowedChars) still works
+}
+```
+
+## Why this and not one of the established packages
+
+| Package | Style |
+|---|---|
+| [`go-playground/validator`](https://github.com/go-playground/validator) | struct tags (`validate:"required,email"`) driven by reflection |
+| [`go-ozzo/ozzo-validation`](https://github.com/go-ozzo/ozzo-validation) | rules composed in code, through a `Rule` interface over `interface{}` |
+| [`asaskevich/govalidator`](https://github.com/asaskevich/govalidator) | a bag of `IsEmail` / `IsURL` string helpers |
+
+This package is the code-composition style (closest to ozzo-validation)
+with two constraints held on purpose:
+
+- **Validators are ordinary generic functions.** `In`, `Equal` and the
+  rest are type-checked by the compiler — pass the wrong type and it does
+  not build. The tag/reflection libraries can't do this; they predate
+  generics.
+- **Errors are plain sentinels.** `errors.Is(err, ErrInvalidEmail)`
+  composes with the standard `errors` package. There is no bespoke
+  `ValidationErrors` type to learn. `Err` matches any error from the
+  package.
+
+Everything is value-level: there is no struct walker. You wire fields
+together yourself (a few lines) and decide how to present the result.
+
+## Fail fast or collect everything
+
+```go
+// stops at the first failure
+validate.All(rules...)
+
+// runs every rule, joins the failures with errors.Join;
+// errors.Is still matches each one
+validate.Collect(rules...)
+```
+
+## Building blocks
+
+| Group | Functions |
+|---|---|
+| Compose | `All`, `Collect` |
+| Presence / equality | `Required`, `Equal`, `In`, `NotIn` |
+| String length & content | `MinLength`, `MaxLength` (runes), `MinLengthBytes`, `MaxLengthBytes`, `Chars`, `ExceptChars`, `Prefix`, `Suffix`, `Contains`, `Match` |
+| Formats | `Email`, `URL`, `UUID` |
+| Numbers | `Min`, `Max`, `MinFloat32`, `MaxFloat32`, `MinFloat64`, `MaxFloat64` |
+| Slices | `MinSize`, `MaxSize` |
+| Errors | `Error`, `NewError`, `Err` |
+
+Each returns (or is) a `func(T) error`, so anything you write with the
+same shape composes with them.
+
+## Not a fit if…
+
+You want **struct-tag validation**, **translated / i18n messages**, or a
+**large catalogue** of built-in checks (credit cards, ISO codes, CIDRs, …).
+Use [`go-playground/validator`](https://github.com/go-playground/validator)
+for that. This package deliberately stays small.
 
 ## License
 
-See [LICENSE.md](./LICENSE.md)
+MIT. See [LICENSE.md](./LICENSE.md).

all.go

@@ -1,7 +1,8 @@
 package validate
 
-// All validates a value using a sequence of validation functions.
-// If any validation function returns an error, the sequence stops and the error is returned.
+// All validates a value against a sequence of validation functions,
+// stopping and returning the first error. See Collect to run every
+// function and report all failures at once.
 func All[T any](fs ...func(T) error) func(T) error {
 	return func(value T) error {
 		for _, f := range fs {

chars.go

@@ -20,7 +20,7 @@ func Chars(allow string) func(string) error {
 	}
 }
 
-// ExceptChars validates whether a string does not contain disallowed characters.
+// ExceptChars validates that a string contains none of the given characters.
 func ExceptChars(disallow string) func(string) error {
 	return func(value string) error {
 		for _, r := range disallow {

chars_test.go

@@ -13,7 +13,7 @@ func ExampleChars() {
 }
 
 func ExampleExceptChars() {
-	testExceptChars := Chars("0123456789abcdef")
+	testExceptChars := ExceptChars("0123456789abcdef")
 	fmt.Println(testExceptChars("invalid input"))
 	// Output: contains disallowed characters
 }

doc.go

@@ -0,0 +1,41 @@
+// Package validate provides small, composable value validators.
+//
+// A validator is just a func(T) error: it returns nil when the value is
+// acceptable, or an error describing the problem. Nothing has to implement
+// an interface, so a plain closure works anywhere a validator is expected.
+// Combine validators with [All] (stop at the first failure) or [Collect]
+// (run them all and join every failure).
+//
+//	username := validate.All(
+//		validate.MinLength(3),
+//		validate.MaxLength(16),
+//		validate.Chars("abcdefghijklmnopqrstuvwxyz0123456789_"),
+//	)
+//	if err := username(input); err != nil {
+//		// errors.Is(err, validate.ErrDisallowedChars) still works here
+//	}
+//
+// Errors are plain sentinel values (see [Error]); match them with
+// errors.Is. [Err] matches any error produced by this package.
+//
+// # Comparison with other packages
+//
+//   - go-playground/validator drives validation from struct tags and
+//     reflection.
+//   - go-ozzo/ozzo-validation composes rules in code, but through a Rule
+//     interface over interface{}.
+//   - asaskevich/govalidator is a bag of IsX string helpers.
+//
+// This package is the code-composition style with two constraints kept
+// deliberately: validators are ordinary generic functions, checked by the
+// compiler with no reflection ([In], [Equal] and the rest will not accept
+// the wrong type), and there are no dependencies outside the standard
+// library.
+//
+// # When to use something else
+//
+// Reach for go-playground/validator if you want struct-tag validation,
+// translated messages, or a large catalogue of built-in checks. This
+// package stays small and leaves struct/field wiring and error
+// presentation to the caller.
+package validate

error.go

@@ -2,10 +2,9 @@ package validate
 
 import "fmt"
 
-// Validation error.
-var (
-	Err = Error{}
-)
+// Err is the zero Error. It carries no message, so errors.Is(x, Err) is
+// true for any error produced by this package.
+var Err = Error{}
 
 // Error represents a validation error.
 type Error struct {
@@ -13,8 +12,9 @@ type Error struct {
 	Data    []any
 }
 
-// Error retrieves the message of a validation Error.
-// If it has Data, the message will be formatted.
+// Error returns the error message. If Data is non-empty, Message is used
+// as an fmt.Sprintf format string and Data as its arguments (this is how
+// the sentinels with %d/%q verbs are filled in, e.g. via With).
 func (e Error) Error() string {
 	if len(e.Data) > 0 {
 		return fmt.Sprintf(e.Message, e.Data...)
@@ -34,6 +34,10 @@ func (e Error) Is(target error) bool {
 	return false
 }
 
+// With returns a copy of the Error with value appended to Data, so it
+// lands in the message when Message contains a formatting verb:
+//
+//	ErrMustBeLonger.With(4) // "must contain at least 4 characters"
 func (e Error) With(value any) Error {
 	if e.Data == nil {
 		e.Data = []any{}

example_test.go

@@ -0,0 +1,66 @@
+package validate_test
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"code.aneur.in/go/validate"
+)
+
+// A validator is any func(T) error. Compose the built-ins with your own
+// closures using All (stop at the first failure) or Collect (report all).
+func Example() {
+	username := validate.All(
+		validate.MinLength(3),
+		validate.MaxLength(16),
+		validate.Chars("abcdefghijklmnopqrstuvwxyz0123456789_"),
+		func(s string) error {
+			if strings.HasPrefix(s, "_") {
+				return errors.New("must not start with an underscore")
+			}
+			return nil
+		},
+	)
+
+	fmt.Println(username("ok_name"))
+	fmt.Println(username("_nope"))
+	fmt.Println(username("Nope"))
+	// Output:
+	// <nil>
+	// must not start with an underscore
+	// contains disallowed characters
+}
+
+// Collect runs every validator and joins the failures; errors.Is still
+// matches each one.
+func Example_collectAll() {
+	password := validate.Collect(
+		validate.MinLength(8),
+		validate.Chars("abcdefghijklmnopqrstuvwxyz"),
+	)
+
+	err := password("Ab1")
+	fmt.Println(err)
+	fmt.Println(errors.Is(err, validate.ErrMustBeLonger))
+	fmt.Println(errors.Is(err, validate.ErrDisallowedChars))
+	// Output:
+	// must contain at least 8 characters
+	// contains disallowed characters
+	// true
+	// true
+}
+
+// Errors are plain sentinels, so errors.Is works with no library-specific
+// machinery. Err matches any error from this package.
+func Example_errorsIs() {
+	err := validate.Email("not-an-email")
+
+	fmt.Println(err)
+	fmt.Println(errors.Is(err, validate.ErrInvalidEmail))
+	fmt.Println(errors.Is(err, validate.Err))
+	// Output:
+	// invalid email address
+	// true
+	// true
+}

in.go

@@ -4,7 +4,7 @@ var (
 	ErrValueNotAllowed = NewError("not allowed")
 )
 
-// In validates whether a value is found in a slice of allowed values.
+// In validates that a value equals one of the allowed values.
 func In[T comparable](allow ...T) func(T) error {
 	return func(value T) error {
 		for _, cmp := range allow {
@@ -16,10 +16,10 @@ func In[T comparable](allow ...T) func(T) error {
 	}
 }
 
-// NotIn validates whether a value is not found in a slice of disallowed values.
-func NotIn[T comparable](allow ...T) func(T) error {
+// NotIn validates that a value equals none of the disallowed values.
+func NotIn[T comparable](disallow ...T) func(T) error {
 	return func(value T) error {
-		for _, cmp := range allow {
+		for _, cmp := range disallow {
 			if cmp == value {
 				return ErrValueNotAllowed
 			}

length.go

@@ -1,26 +1,57 @@
 package validate
 
+import "unicode/utf8"
+
 var (
-	ErrMustBeLonger  = NewError("must contain at least %d characters")
-	ErrMustBeShorter = NewError("must contain no more than %d characters")
+	ErrMustBeLonger       = NewError("must contain at least %d characters")
+	ErrMustBeShorter      = NewError("must contain no more than %d characters")
+	ErrMustHaveMoreBytes  = NewError("must have at least %d bytes")
+	ErrMustHaveFewerBytes = NewError("must have no more than %d bytes")
 )
 
-// MaxLength validates the length of a string as being less than or equal to a given maximum.
+// MaxLength validates that a string is no longer than a given maximum.
+// Length is counted in runes, so multi-byte characters count as one.
 func MaxLength(l int) func(string) error {
 	return func(value string) error {
-		if len(value) > l {
+		if utf8.RuneCountInString(value) > l {
 			return ErrMustBeShorter.With(l)
 		}
 		return nil
 	}
 }
 
-// MinLength validates the length of a string as being greater than or equal to a given minimum.
+// MinLength validates that a string is at least a given minimum length.
+// Length is counted in runes, so multi-byte characters count as one.
 func MinLength(l int) func(string) error {
 	return func(value string) error {
-		if len(value) < l {
+		if utf8.RuneCountInString(value) < l {
 			return ErrMustBeLonger.With(l)
 		}
 		return nil
 	}
 }
+
+// MaxLengthBytes validates that a string is no longer than a given maximum
+// number of bytes (len). Prefer [MaxLength] for a limit on visible
+// characters; use this when the budget is genuinely a byte count, such as
+// a fixed-width column or a wire-format field.
+func MaxLengthBytes(l int) func(string) error {
+	return func(value string) error {
+		if len(value) > l {
+			return ErrMustHaveFewerBytes.With(l)
+		}
+		return nil
+	}
+}
+
+// MinLengthBytes validates that a string is at least a given minimum
+// number of bytes (len). See [MaxLengthBytes] on when to prefer this over
+// [MinLength].
+func MinLengthBytes(l int) func(string) error {
+	return func(value string) error {
+		if len(value) < l {
+			return ErrMustHaveMoreBytes.With(l)
+		}
+		return nil
+	}
+}

length_test.go

@@ -20,7 +20,14 @@ func ExampleMinLength() {
 
 func TestMaxLength(t *testing.T) {
 	testCases := map[int]map[string]error{
-		8: {"abcd": nil, "abcdefgh": nil, "abcd efg": nil, "abcdefghi": ErrMustBeShorter.With(8)},
+		8: {
+			"abcd":      nil,
+			"abcdefgh":  nil,
+			"abcd efg":  nil,
+			"abcdéfgh":  nil, // 8 runes, 9 bytes
+			"abcdefghi": ErrMustBeShorter.With(8),
+			"abcdéfghi": ErrMustBeShorter.With(8), // 9 runes, 10 bytes
+		},
 	}
 
 	for setup, values := range testCases {
@@ -41,7 +48,14 @@ func TestMaxLength(t *testing.T) {
 
 func TestMinLength(t *testing.T) {
 	testCases := map[int]map[string]error{
-		8: {"abcd": ErrMustBeLonger.With(8), "abcdefgh": nil, "abcd efg": nil, "abcdefghi": nil},
+		8: {
+			"abcd":      ErrMustBeLonger.With(8),
+			"abcdéfg":   ErrMustBeLonger.With(8), // 7 runes, 8 bytes
+			"abcdefgh":  nil,
+			"abcdéfgh":  nil, // 8 runes, 9 bytes
+			"abcd efg":  nil,
+			"abcdefghi": nil,
+		},
 	}
 
 	for setup, values := range testCases {
@@ -59,3 +73,69 @@ func TestMinLength(t *testing.T) {
 		}
 	}
 }
+
+func ExampleMaxLengthBytes() {
+	testMaxLengthBytes := MaxLengthBytes(8)
+	fmt.Println(testMaxLengthBytes("cafés round the world"))
+	// Output: must have no more than 8 bytes
+}
+
+func ExampleMinLengthBytes() {
+	testMinLengthBytes := MinLengthBytes(8)
+	fmt.Println(testMinLengthBytes("2short"))
+	// Output: must have at least 8 bytes
+}
+
+func TestMaxLengthBytes(t *testing.T) {
+	testCases := map[int]map[string]error{
+		8: {
+			"abcd":      nil,
+			"abcdefgh":  nil,
+			"abcdéfg":   nil,                           // 7 runes, 8 bytes
+			"abcdéfgh":  ErrMustHaveFewerBytes.With(8), // 8 runes, 9 bytes
+			"abcdefghi": ErrMustHaveFewerBytes.With(8),
+		},
+	}
+
+	for setup, values := range testCases {
+		testMaxLengthBytes := MaxLengthBytes(setup)
+
+		for input, want := range values {
+			t.Run(fmt.Sprintf("%d/%s", setup, input), func(t *testing.T) {
+				got := testMaxLengthBytes(input)
+
+				if !errors.Is(got, want) {
+					t.Error("got", got)
+					t.Error("want", want)
+				}
+			})
+		}
+	}
+}
+
+func TestMinLengthBytes(t *testing.T) {
+	testCases := map[int]map[string]error{
+		8: {
+			"abcd":     ErrMustHaveMoreBytes.With(8),
+			"abcdefg":  ErrMustHaveMoreBytes.With(8),
+			"abcdéf":   ErrMustHaveMoreBytes.With(8), // 6 runes, 7 bytes
+			"abcdéfg":  nil,                          // 7 runes, 8 bytes
+			"abcdefgh": nil,
+		},
+	}
+
+	for setup, values := range testCases {
+		testMinLengthBytes := MinLengthBytes(setup)
+
+		for input, want := range values {
+			t.Run(fmt.Sprintf("%d/%s", setup, input), func(t *testing.T) {
+				got := testMinLengthBytes(input)
+
+				if !errors.Is(got, want) {
+					t.Error("got", got)
+					t.Error("want", want)
+				}
+			})
+		}
+	}
+}

number.go

@@ -8,7 +8,7 @@ var (
 )
 
 // Max validates whether an integer is less than or equal to a given maximum.
-// If exclusive is true, an equal value will also produce an error.
+// If exclusive is true, an equal value also produces an error.
 func Max(n int, exclusive bool) func(int) error {
 	return func(value int) error {
 		if exclusive {
@@ -24,7 +24,7 @@ func Max(n int, exclusive bool) func(int) error {
 }
 
 // MaxFloat32 validates whether a float32 is less than or equal to a given maximum.
-// If exclusive is true, an equal value will also produce an error.
+// If exclusive is true, an equal value also produces an error.
 func MaxFloat32(n float32, exclusive bool) func(float32) error {
 	return func(value float32) error {
 		if exclusive {
@@ -40,7 +40,7 @@ func MaxFloat32(n float32, exclusive bool) func(float32) error {
 }
 
 // MaxFloat64 validates whether a float64 is less than or equal to a given maximum.
-// If exclusive is true, an equal value will also produce an error.
+// If exclusive is true, an equal value also produces an error.
 func MaxFloat64(n float64, exclusive bool) func(float64) error {
 	return func(value float64) error {
 		if exclusive {
@@ -55,8 +55,8 @@ func MaxFloat64(n float64, exclusive bool) func(float64) error {
 	}
 }
 
-// Min validates whether an integer is less than or equal to a given maximum.
-// If exclusive is true, an equal value will also produce an error.
+// Min validates whether an integer is greater than or equal to a given minimum.
+// If exclusive is true, an equal value also produces an error.
 func Min(n int, exclusive bool) func(int) error {
 	return func(value int) error {
 		if exclusive {
@@ -71,8 +71,8 @@ func Min(n int, exclusive bool) func(int) error {
 	}
 }
 
-// MinFloat32 validates whether a float32 is less than or equal to a given maximum.
-// If exclusive is true, an equal value will also produce an error.
+// MinFloat32 validates whether a float32 is greater than or equal to a given minimum.
+// If exclusive is true, an equal value also produces an error.
 func MinFloat32(n float32, exclusive bool) func(float32) error {
 	return func(value float32) error {
 		if exclusive {
@@ -87,8 +87,8 @@ func MinFloat32(n float32, exclusive bool) func(float32) error {
 	}
 }
 
-// MinFloat64 validates whether a float64 is less than or equal to a given maximum.
-// If exclusive is true, an equal value will also produce an error.
+// MinFloat64 validates whether a float64 is greater than or equal to a given minimum.
+// If exclusive is true, an equal value also produces an error.
 func MinFloat64(n float64, exclusive bool) func(float64) error {
 	return func(value float64) error {
 		if exclusive {

url.go

@@ -6,7 +6,9 @@ var (
 	ErrInvalidURL Error = NewError("invalid URL")
 )
 
-// URL validates a URL.
+// URL validates that a string is an absolute URL (with a scheme) or an
+// absolute path, per net/url.ParseRequestURI. "example.com" with no
+// scheme is rejected.
 func URL(value string) error {
 	if _, err := url.ParseRequestURI(value); err != nil {
 		return ErrInvalidURL

uuid.go

@@ -10,8 +10,9 @@ var (
 
 var uuidRegexp = regexp.MustCompile("^[a-f0-9]{8}(-[a-f0-9]{4}){3}-[a-f0-9]{12}$")
 
-// UUID validates a UUID string.
-// The UUID must be formatted with separators.
+// UUID validates a UUID string in the canonical 8-4-4-4-12 hyphenated
+// form. Only lowercase hexadecimal is accepted; the version and variant
+// bits are not checked.
 func UUID(value string) error {
 	if !uuidRegexp.MatchString(value) {
 		return ErrInvalidUUID