Go Validate

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.

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	struct tags (validate:"required,email") driven by reflection
go-ozzo/ozzo-validation	rules composed in code, through a Rule interface over interface{}
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

// 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, 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 for that. This package deliberately stays small.

License

MIT. See LICENSE.md.