Validation helpers and i18n middleware for the GoFlash framework

Lightweight validation helpers powered by go-playground/validator and a tiny i18n middleware for the GoFlash framework. It gives you a ready-to-use validator instance, helper mappers to field->message maps, and a middleware to plug app-level translations without pulling locale packages into the framework.

Features

• Uses github.com/go-playground/validator/v10 under the hood
• Global validate.Validator with JSON tag name support out of the box
• Helpers to map validator.ValidationErrors into map[field]message
• Pluggable message function: global or per-request via context
• i18n middleware that wires your translators in one place

Installation

go get github.com/goflash/validator/v2

Go version: requires Go 1.23+. The module sets go 1.23 and can be used with newer Go versions. If you use GOTOOLCHAIN=auto, the toolchain directive will ensure a compatible toolchain is used.

Quick start

import (
    "github.com/goflash/flash/v2"
    "github.com/goflash/validator/v2/validate"
)

type User struct {
    Name string `json:"name" validate:"required,min=2"`
    Age  int    `json:"age"  validate:"gte=0,lte=130"`
}

func main() {
    a := flash.New()
    a.POST("/users", func(c flash.Ctx) error {
        var u User
        if err := c.BindJSON(&u); err != nil {
            return c.JSON(map[string]any{"message": "invalid payload", "error": err.Error()})
        }
        if err := validate.Struct(u); err != nil {
            return c.JSON(map[string]any{"message": "validation failed", "fields": validate.ToFieldErrors(err)})
        }
        return c.JSON(u)
    })
}

Configuration

Use ValidatorI18n to attach localized messages per request:

import (
    "github.com/goflash/flash/v2"
    v10 "github.com/go-playground/validator/v10"
    "github.com/goflash/validator/v2/validate"
    mw "github.com/goflash/validator/v2"
)

app := flash.New()
app.Use(mw.ValidatorI18n(mw.ValidatorI18nConfig{
    DefaultLocale: "en",
    MessageFuncFor: func(locale string) func(v10.FieldError) string {
        // Look up your translator for the locale and return fe.Translate(trans)
        return func(fe v10.FieldError) string { return fe.Error() }
    },
    SetGlobal: true, // optionally set global fallback to DefaultLocale
}))

Messages and mapping

• Register custom tags and tag-name functions directly on validate.Validator.
• Map errors with validate.ToFieldErrors(err) or validate.ToFieldErrorsWithContext(ctx, err).
• Provide request-scoped message function via middleware or validate.WithMessageFunc(ctx, fn).

Default messages

Built-in minimal fallback messages cover common tags like required, min/max/len, email, oneof, gte/lte, url, uuid, alpha/alphanum/numeric, contains/excludes, startswith/endswith, base64, json, ip/cidr, ascii/printascii/multibyte, isbn/isbn10/isbn13.

Context

The middleware stores a request-scoped message function on the request context. Use validate.MessageFuncFromContext(c.Context()) to retrieve it if needed.

Errors

When mapping errors, non-validation errors are returned under the _error key. You can also pass your own validate.FieldErrors map.

Examples

Three runnable examples are included:

• examples/binding_json: simple JSON bind and respond
• examples/validation: struct validation and error mapping
• examples/validation_with_i18n: localized messages using the middleware

Try them locally (they use this module):

cd ../validation && go run .
cd ../validation_with_i18n && go run .

Versioning and compatibility

• Module path: github.com/goflash/validator/v2
• Requires Go 1.23+
• Versioning starts at v2.0.0

Contributing

Issues and PRs are welcome. Please run tests before submitting:

go test ./...

License

MIT