Some playing around with go validation ideas.

foob.go

package main

import (
	"errors"
	"fmt"
	)

type BaseField interface {
	NameOf() string
	IsOptional() bool
}

type Foo struct {
	Name string
	Optional bool
}

type Bar struct {
	Name string
	Optional bool
	Maximum int
}

func BaseValid(f BaseField, inp string) (out string, err error) {
	if f.IsOptional() && inp == "" {
		return "", nil
	} else if inp == "" {
		return "", errors.New(fmt.Sprintf("%v is not optional", f.NameOf()))
	} 
	return inp, nil
}

func (f *Bar) Valid(inp string) (out string, err error) {
	out, err = BaseValid(f, inp)
	return
}

func (f *Bar) NameOf() string {
	return f.Name
}

func (f *Bar) IsOptional() bool {
	return f.Optional
}

func main() {
	f := Bar { Name: "Foob", Optional: false }
	o, e := f.Valid("red")
	fmt.Printf("out: %v, err: %v\n", o, e)

	o, e = f.Valid("")
	fmt.Printf("out: %v, err: %v\n", o, e)

	f.Optional = true

	o, e = f.Valid("")
	fmt.Printf("out: %v, err: %v\n", o, e)
}