vgen

vgen

A type safe validation library using code generation

Motivation

When decoding json data it is impossible to differentiate between zero and nil values. One solution is to use pointers which become nil in the case of abscent fields. But using pointers can be tedious and creating new types leads to a lot of boilerplate.

vgen uses code generation to generate this boilerplate for you. Code generation also allows for compile time checking of validation rules

Installation

Go

Run the following command to download the binary using go install

go install github.com/stofffe/vgen/cmd/vgen@latest

Usage

Annotate a struct with the tag vgen(include)

// vgen(include)
type Person struct {
    name string
    age int
}

Run the vgen generate <filename> to generate the following types

type PersonVgen struct { ... }
type PersonRules struct { ... }

func (v PersonVgen) Validate(rules PersonRules) ErrorMap { ... }
func (v PersonVgen) Convert() Person { ... }
func (v PersonVgen) ValidatedConvert() (Person, ErrorMap) { ... }

...

Use type safe validation

func main() {
    // Create person manually or decode from json
    person := PersonVgen { ... }

	rules := PersonRules{
		name: vgen.RulesRequired(
			vgen.Eq("bob"),
		),
		age: vgen.RulesOptional(
			vgen.Gte(18),
		),
	}

	result, err := person.ValidatedConvert(rules)
	if err != nil {
		log.Fatal(err.Debug())
	}
	fmt.Println(result)
}

Output

{
    name: [
        "required",
    ],
    age: [
        "must be greater than or equal to 18",
    ]
}

More examples can be found in examples

Tags

Types and fields can be annotated with tags, the tags inform the vgen compiler what code to generate

Example

// vgen(i)
type Person struct {
    NAME string // vgen(alias=name)
}

Type Tags

Field Tags

Rules

Rule

vgen validation is based of the Rule interface

type Rule[T any] interface {
	Validate(key string, input T) ErrorMap
}

RuleFunc

Custom rules can be created by implementing this Rule, or by creating a RuleFunc which automatically implements the interface. See custom example for more information

type RuleFunc[T any] func(key string, input T) ErrorMap

Implemented rules

vgen includes a lot of default rules, here is the currently implemented rules