options-gen

Code-generator that allows you to create a functional options like Dave Cheney's post.

Generate the options for your service/client/etc. All that you need is to define a struct with fields, that can be applied as Option then embed this struct into yours.

Installation

go install github.com/kazhuravlev/options-gen/cmd/options-gen@latest

Usage

package mypkg

import (
	"io"
	"log"
)

//go:generate options-gen -out-filename=options_generated.go -from-struct=Options
type Options struct {
	logger     log.Logger `option:"mandatory"`
	listenAddr string     `option:"mandatory" validate:"required,hostname_port"`
	closer     io.Closer  `validate:"required"`
}

go generate ./...

This will generate out-filename file with options constructor. Like this:

// options_generated.go
package mypkg

import (
	"log"
)

func NewOptions(
	// mandatory options. you cannot ignore or forget them because they are arguments.
	logger log.Logger,
	listenAddr string,
	// optional: you can leave them empty or not.
	other ...Option,
) {
	// ...
}

// Validate will check that all options are in desired state
func (o *Options) Validate() error {
	// ...
}

And you can use generated options as follows:

package mypkg

import "fmt"

type Component struct {
	opts Options // struct that you define as struct with options 
}

func New(opts Options) (*Component, error) { // constructor of your service/client/component
	if err := opts.Validate(); err != nil {    // always add only these lines for all your constructors
		return nil, fmt.Errorf("cannot validate options: %w", err)
	}

	return &Component{opts: opts}, nil // embed options into your component
}

And after that you can use new constructor in (for ex.) main.go:

package main

func main() {
	c, err := mypkg.New(mypkg.NewOptions( /* ... */))
	if err != nil {
		panic(err)
	}
}

Usage with Generics

package mypkg

//go:generate options-gen -from-struct=Options
type Options[T any] struct {
	addr string   `option:"mandatory" validate:"required,hostname_port"`
	ch   <-chan T `option:"mandatory"`
}

And just go generate ./....

Examples

See an examples to get real-world examples.

Configuration

To configure this tool you should know two things: how to work with cli tool and how to define options in your Options struct.

CLI tool

All the tool needs is the information about source and target files and packages. Tool can be invoked by options-gen (after Installation) and it will have the following arguments:

• filename - is a source filename that contains Options struct relative to the current dir. For example ./pkg/github-client/options.go. Default: $GOFILE (file where you placed //go:generate).
• pkg - name of output filename package. In most cases we can just use the same package as the filename file. For example githubclient. Default: $GOPACKAGE. Package name same as file where you placed //go:generate.
• from-struct - name of structure that contains our options. For example Options.
• out-filename - specifies an output filename. This filename will be rewritten with options-gen specific content. For example ./pkg/github-client/options_generated.go.

See an Examples.

Option tag

You can control two important things. The first is about the options constructor

• how options-gen will generate NewOptions constructor. The second is about how to validate data, that has been passed as value for this field.

Control the constructor

options-gen can generate a constructor that can receive all option fields as separate arguments. It will force the user to pass each (or someone) option field to the constructor. Like this:

// Mark Field1 as mandatory
type Options struct {
	field1 string `option:"mandatory"`
}

// options-gen will generate constructor like this
func NewOptions(field1 string, otherOptions ...option)...

But, if we do not want to force the user to pass each argument - we can remove the option:"mandatory" feature for this field and get something like this:

// Do not mark Field1 as mandatory
type Options struct {
	field1 string
}

// options-gen will generate constructor like this
func NewOptions(otherOptions ...option)...

So, this allows setting only those options fields that user is want to set.

Validate field data

After we define the fields, we want to restrict the values of these fields. To do that we can use a well-known library validator

Just read the docs for validator library and add tag to fields like this:

type Options struct {
	maxDbConn int `validate:"required,min=1,max=16"`
}

Default values

options-gen provide several ways to define defaults for options. You can choose which mechanism you need by providing a flag -defaults-from. By default, this flag is set to tag=default.

• tag[=TagName]. This mechanism will try to find a tag TagName in field tags. By default TagName is equal to default
• var[=VariableName]. This mechanism will copy variable VariableName fields to your Options instance. By default VariableName is equal to default<StructName>. This variable should contain Options struct.
• func[=FunctionName]. The same as var, but for the function name. Function FunctionName will be called once per NewOptions constructor. This function should return an Options struct.
• none to disable defaults.

Using tag

For numbers, strings, and time.Duration you can set the default value:

// simple example
//go:generate options-gen -from-struct=Options
type Options struct {
	pingPeriod  time.Duration `default:"3s" validate:"min=100ms,max=30s"`
	name        string        `default:"unknown" validate:"required"`
	maxAttempts int           `default:"10" validate:"min=1,max=10"`
	eps         float32       `default:"0.0001" validate:"gt=0"`
}

// custom default tag
//go:generate options-gen -from-struct=Options --default-from=tag=mydefaulttag
type Options struct {
	pingPeriod  time.Duration `mydefaulttag:"3s" validate:"min=100ms,max=30s"`
	name        string        `mydefaulttag:"unknown" validate:"required"`
	maxAttempts int           `mydefaulttag:"10" validate:"min=1,max=10"`
	eps         float32       `mydefaulttag:"0.0001" validate:"gt=0"`
}

It would be relevant if the field were not filled either explicitly or through functional option.

The default value must be valid for the field type and must satisfy validation rules.

Using variable

Tags allow you to define defaults for simple types like string, number , time.Duration. When you want to define a variable with prefilled values - you can do this like that:

// simple example
//go:generate options-gen -from-struct=Options -defaults-from=var
type Options struct {
	httpClient *http.Client
}

var defaultOptions = Options{
	httpClient: &http.Client{},
}

// custom variable name
//go:generate options-gen -from-struct=Options -defaults-from=var=myDefaults
type Options struct {
	httpClient *http.Client
}

var myDefaults = Options{
	httpClient: &http.Client{},
}

Using function

The same as variable. See an examples:

// simple example
//go:generate options-gen -from-struct=Options -defaults-from=func
type Options struct {
	httpClient *http.Client
}

func getDefaultOptions() Options {
	return Options{
		httpClient: &http.Client{},
	}
}

// custom function name
//go:generate options-gen -from-struct=Options -defaults-from=func=myDefaults
type Options struct {
	httpClient *http.Client
}

func myDefaults() Options {
	return Options{
		httpClient: &http.Client{},
	}
}

Disable defaults

If you want to be sure that defaults will not be parsed - you can specify the none for -defaults-from flag.

// defaults will now be parsed at all
//go:generate options-gen -from-struct=Options -defaults-from=none
type Options struct {
	name string `default:"joe"`
}

Which fields are set?

options-gen can produce additional code that allows you to check which fields were set. To do this, simply add the -with-isset flag to options-gen.

For example, this code with the specified option...

package app

//go:generate options-gen -from-struct=Options -with-isset
type Options struct {
	name string
}

...will produce function func (o *Options) IsSet(field optField) bool{...}.

Custom validator

You can override options-gen validator for specific struct by implementing the Validator() method:

import "github.com/mycoolmodule/internal/validator"

// ...

func (Options) Validator() *validator.Validate {
return validator.Validator
}

Or you can override options-gen validator globally:

package validator

import (
	goplvalidator "github.com/go-playground/validator/v10"
	optsValidator "github.com/kazhuravlev/options-gen/pkg/validator"
)

var Validator = goplvalidator.New()

func init() {
	must(Validator.RegisterValidation( /* ... */))
	must(Validator.RegisterAlias( /* ... */))

	optsValidator.Set(Validator)
}

Contributing

The development process is pretty simple:

• Fork the repo on GitHub
• Clone your copy of the repo
• Create a new branch for your goals
• Install the Task. It's like Make, but simple
• Check that your working copy is ready to start development by running task check in repo workdir
• Reach your goals!
• Check that all is ok by task check
• Create a Pull Request