Go Validator

Go Validator

Generate validator files for GO modular architecture conventions.

When to Use

• Create domain validators (password, email, username, etc.)
• Input sanitization and business rule validation
• Any validation logic returning typed domain errors

Two-File Pattern

Every validator requires two files:

1. Port interface: internal/modules/<module>/ports/<validator_name>_validator.go 2. Validator implementation: internal/modules/<module>/validator/<validator_name>_validator.go

Port File Structure

The port file contains only the interface definition with its documentation comment.

Example structure:

package ports

// PasswordValidator validates password strength according to security policies.
type PasswordValidator interface {
	Validate(password string) error
}

Validator File Structure

The validator implementation file follows this order:

1. Package declaration and imports 2. Constants - validation rules, thresholds, limits 3. Struct definition - the validator implementation struct 4. Interface assertion - compile-time check with var _ ports.XxxValidator = (*XxxValidator)(nil) 5. Constructor - NewXxxValidator function 6. Methods - validation methods (e.g., Validate)

Example structure:

package validator

import (
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/errs"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/ports"
)

// 2. Constants
const (
	minLength = 8
	maxLength = 128
)

// 3. Struct definition
type PasswordValidator struct{}

// 4. Interface assertion
var _ ports.PasswordValidator = (*PasswordValidator)(nil)

// 5. Constructor
func NewPasswordValidator() *PasswordValidator {
	return &PasswordValidator{}
}

// 6. Methods
func (v *PasswordValidator) Validate(password string) error {
	// validation logic
	return nil
}

Port Interface Structure

Location: internal/modules/<module>/ports/<validator_name>_validator.go

package ports

// PasswordValidator validates password strength according to security policies.
type PasswordValidator interface {
	Validate(password string) error
}

Validator Variants

Stateless validator (no dependencies)

Most validators are stateless utilities with no external dependencies.

package validator

import (
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/errs"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/ports"
)

type EmailValidator struct{}

var _ ports.EmailValidator = (*EmailValidator)(nil)

func NewEmailValidator() *EmailValidator {
	return &EmailValidator{}
}

func (v *EmailValidator) Validate(email string) error {
	// Validation logic
	return nil
}

Stateful validator (with dependencies)

Use when validation requires external data or configuration.

package validator

import (
	"context"

	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/errs"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/ports"
)

type UsernameValidator struct {
	userRepo ports.UserRepository
	minLen   int
	maxLen   int
}

var _ ports.UsernameValidator = (*UsernameValidator)(nil)

func NewUsernameValidator(
	userRepo ports.UserRepository,
	minLen int,
	maxLen int,
) *UsernameValidator {
	return &UsernameValidator{
		userRepo: userRepo,
		minLen:   minLen,
		maxLen:   maxLen,
	}
}

func (v *UsernameValidator) Validate(ctx context.Context, username string) error {
	if len(username) < v.minLen {
		return errs.ErrUsernameTooShort
	}

	// Check uniqueness using repository
	exists, err := v.userRepo.ExistsByUsername(ctx, username)
	if err != nil {
		return err
	}
	if exists {
		return errs.ErrUsernameAlreadyExists
	}

	return nil
}

Multi-field validator

Use when validation involves multiple related fields.

Port interface:

package ports

// RegistrationValidator validates all fields for user registration.
type RegistrationValidator interface {
	ValidateEmail(email string) error
	ValidatePassword(password string) error
	ValidatePasswordMatch(password, confirmPassword string) error
}

Implementation:

package validator

import (
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/errs"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/ports"
)

type RegistrationValidator struct{}

var _ ports.RegistrationValidator = (*RegistrationValidator)(nil)

func NewRegistrationValidator() *RegistrationValidator {
	return &RegistrationValidator{}
}

func (v *RegistrationValidator) ValidateEmail(email string) error {
	// Email validation logic
	return nil
}

func (v *RegistrationValidator) ValidatePassword(password string) error {
	// Password validation logic
	return nil
}

func (v *RegistrationValidator) ValidatePasswordMatch(password, confirmPassword string) error {
	if password != confirmPassword {
		return errs.ErrPasswordMismatch
	}
	return nil
}

Validation Constants

Define validation rules as constants at the package level for clarity and maintainability.

const (
	minPasswordLength = 8
	maxPasswordLength = 128
	minUsernameLength = 3
	maxUsernameLength = 32
)

Error Handling

Validators MUST return typed domain errors from the module's errs package. When adding new custom errors, translations are mandatory in locale files.

// In internal/modules/<module>/errs/errs.go
var (
	ErrPasswordTooShort         = errors.New("password must be at least 8 characters")
	ErrPasswordMissingUppercase = errors.New("password must contain at least one uppercase letter")
	ErrPasswordMissingLowercase = errors.New("password must contain at least one lowercase letter")
	ErrPasswordMissingDigit     = errors.New("password must contain at least one digit")
	ErrPasswordMissingSpecial   = errors.New("password must contain at least one special character")
)

For every new custom error added to internal/modules/<module>/errs/errs.go:

• Add the translation key to locales/en.json
• Add the same translation key to every other existing locale file (e.g., locales/pt_BR.json)

Context Usage

Validators that perform I/O operations (database lookups, API calls) MUST accept context.Context as the first parameter.

// Stateless validator - no context needed
func (v *PasswordValidator) Validate(password string) error

// Stateful validator with I/O - context required
func (v *UsernameValidator) Validate(ctx context.Context, username string) error

Naming

• Port interface: XxxValidator (in ports package)
• Implementation struct: XxxValidator (in validator package, same name — disambiguated by package)
• Constructor: NewXxxValidator, returns a pointer of the struct implementation
• Validation method: Validate for single-purpose validators, or descriptive names for multi-purpose validators

Fx Wiring

Add to internal/modules/<module>/fx.go:

Stateless validator:

fx.Provide(
	fx.Annotate(
		validator.NewPasswordValidator,
		fx.As(new(ports.PasswordValidator)),
	),
),

Stateful validator with dependencies:

fx.Provide(
	fx.Annotate(
		validator.NewUsernameValidator,
		fx.As(new(ports.UsernameValidator)),
	),
),

The stateful validator's dependencies (e.g., ports.UserRepository) are automatically injected by Fx. Constructor parameters that are primitive types (e.g., minLen, maxLen) should be provided via configuration or fx.Supply.

Dependencies

Validators depend on interfaces only. Common dependencies:

• ports.XxxRepository — for uniqueness checks or data lookups
• ports.XxxService — for external validation services
• Configuration values — passed as constructor parameters

Testing

Validators MUST have comprehensive unit tests covering:

1. Valid input passes validation 2. Each invalid condition returns the correct error 3. Edge cases (empty strings, boundary values, special characters)

Test file location: internal/modules/<module>/validator/<validator_name>_validator_test.go

package validator_test

import (
	"testing"

	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/errs"
	"github.com/cristiano-pacheco/pingo/internal/modules/<module>/validator"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

func TestPasswordValidator_ValidPassword_Passes(t *testing.T) {
	// Arrange
	v := validator.NewPasswordValidator()

	// Act
	err := v.Validate("SecureP@ssw0rd")

	// Assert
	require.NoError(t, err)
}

func TestPasswordValidator_TooShort_ReturnsError(t *testing.T) {
	// Arrange
	v := validator.NewPasswordValidator()

	// Act
	err := v.Validate("Ab1!")

	// Assert
	require.Error(t, err)
	assert.ErrorIs(t, err, errs.ErrPasswordTooShort)
}

Critical Rules

1. No standalone functions: When a file contains a struct with methods, do not add standalone functions. Use private methods on the struct instead. 2. Two files: Port interface in ports/, implementation in validator/ 2. Interface in ports: Interface lives in ports/<name>_validator.go 3. Interface assertion: Add var _ ports.XxxValidator = (*XxxValidator)(nil) below the struct 4. Constructor: MUST return pointer *XxxValidator 5. Stateless by default: Only add dependencies when validation requires external data 6. Context when needed: Accept context.Context only for validators performing I/O 7. Typed errors: Return domain errors from module's errs package 8. Error translations: Every new custom error must have entries in locales/en.json and all other existing locale files 9. Constants: Define validation rules as package-level constants 10. No comments on implementations: Do not add redundant comments above methods in the implementations 11. Add detailed comment on interfaces: Provide comprehensive comments on the port interfaces to describe their purpose and validation rules 12. Comprehensive tests: Test valid cases and all invalid conditions

Workflow

1. Create port interface in ports/<name>_validator.go 2. Create validator implementation in validator/<name>_validator.go 3. Define validation constants 4. Add typed errors to module's errs/errs.go if needed 5. Add translations for each new custom error in locales/en.json and all other existing locale files 6. Create comprehensive unit tests in validator/<name>_validator_test.go 7. Add Fx wiring to module's fx.go 8. Run make test to verify tests pass 9. Run make lint to verify code quality 10. Run make nilaway for static analysis