foggo

日本語版 README

foggo is generator of Functional Option Pattern And Applicable Functional Option Pattern from struct field in Golang code.

Installation

$ go install golang.org/x/tools/cmd/goimports@latest  # foggo use 'goimports' command
$ go install github.com/s14t284/foggo@latest

Usage

foggo provides fop and afop subcommand.

Usage:
  foggo (fop|afop) [flags]

Flags:
  -h, --help   help for fop

Global Flags:
  -p, --package string   Package name having target struct (default ".")
  -s, --struct string    Target struct name (required)

Generate with command line

1. prepare a struct type.

   // ./image/image.go
   package image
   
   type Image struct {
       Width  int
       Height int
       // don't want to create option, specify `foggo:"-"` as the structure tag 
       Src    string `foggo:"-"`
       Alt    string
   }

2. execute foggo fop command.

   # struct must be set struct type name 
   # package must be package path
   $ foggo fop --struct Image --package image

3. then foggo generates Functional Option Pattern code to ./image/image_gen.go.

   // Code generated by foggo; DO NOT EDIT.
   
   package image
   
   type ImageOption func(*Image)
   
   func NewImage(options ...ImageOption) *Image {
       s := &Image{}
   
       for _, option := range options {
           option(s)
       }
   
       return s
   }
   
   func WithWidth(Width int) ImageOption {
       return func(args *Image) {
           args.Width = Width
       }
   }
   
   func WithHeight(Height int) ImageOption {
       return func(args *Image) {
           args.Height = Height
       }
   }
   
   func WithAlt(Alt string) ImageOption {
       return func(args *Image) {
           args.Alt = Alt
       }
   }

4. write Golang code using functional option parameter

   package main
   
   import "github.com/user/project/image"
   
   func main() {
       image := NewImage(
       	WithWidth(1280),
       	WithHeight(720),
       	WithAlt("alt title"),
       )
       image.Src = "./image.png"
       ...
   }

Generate with go:generate

1. prepare a struct type with go:generate.

   // ./image/image.go
   package image
   
   //go:generate foggo fop --struct Image 
   type Image struct {
       Width  int
       Height int
       // don't want to create option, specify `foggo:"-"` as the structure tag 
       Src    string `foggo:"-"`
       Alt    string
   }

2. execute go generate ./... command.

   $ go generate ./...

3. the foggo generate Functional Option Pattern code to all files written go:generate.

Generate with afop command

afop is the method to generate Applicable Functional Option Pattern code.

1. prepare a struct type with go:generate. (use afop subcommand)

   // ./image/image.go
   package image
   
   //go:generate foggo afop --struct Image 
   type Image struct {
       Width  int
       Height int
       // don't want to create option, specify `foggo:"-"` as the structure tag 
       Src    string `foggo:"-"`
       Alt    string
   }

2. execute go generate ./... command.

   $ go generate ./...

3. the foggo generate Applicable Functional Option Pattern code to all files written go:generate.

   // Code generated by foggo; DO NOT EDIT.
   
   package image
   
   type ImageOption interface {
       apply(*Image)
   }
   
   type WidthOption struct {
       Width int
   }
   
   func (o WidthOption) apply(s *Image) {
       s.Width = o.Width
   } 
   
   type HeightOption struct {
       Height int
   }
   
   func (o HeightOption) apply(s *Image) {
       s.Height = o.Height
   }
   
   type AltOption struct {
       Alt string
   }
   
   func (o AltOption) apply(s *Image) {
       s.Alt = o.Alt
   }
   
   func NewImage(options ...ImageOption) *Image {
       s := &Image{}
   
       for _, option := range options {
           option.apply(s)
       }
   
       return s
   }

4. write Golang code using Applicable Functional Option Parameter

   package main
   
   import "github.com/user/project/image"
   
   func main() {
       image := NewImage(
       	WidthOption(1280),
       	HeightOption(720),
       	AltOption("alt title"),
       )
       image.Src = "./image.png"
       ...
   }

Functional Option Pattern ?

Functional Option Pattern(FOP) is one of the most common design patterns used in Golang code.

Golang cannot provide optional arguments such as keyword arguments (available in python, ruby, ...). FOP is the technique for achieving optional arguments.

For more information, please refer to the following articles.

• https://commandcenter.blogspot.jp/2014/01/self-referential-functions-and-design.html
• https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

Applicable Functional Option Pattern ?

Applicable Functional Option Pattern(AFOP) is testable FOP. FOP express options to function. For that reason, comparing to option function with same arguments fails (not testable).

AFOP express options to struct type and options have a parameter and apply method. Struct type is comparable in Golang, options followed AFOP are testable.

AFOP proposed by following articles.

• https://github.com/uber-go/guide/blob/master/style.md#functional-options
• https://ww24.jp/2019/07/go-option-pattern (in Japanese)

References

• https://github.com/moznion/gonstructor