go-codegen, a simple code generation system

go-codegen is a simple template-based code generation system for go. By annotating structs with special-format fields, go-codegen will generate code based upon templates provided alongside your package.

Example usage

go-codegen works by building a catalog of available code templates, and then any anonymous fields on a struct whose type match a template name will be invoked with a struct that provides information about the struct that the template is being invoked upon.

Take for example, the following go file:

package main

import "fmt"

//go:generate go-codegen

// cmd is a template. Any type can have a template bound to it by creating the
// appropriate file on disk.
//
// aside: Blank interfaces are good to use for targeting templates
// as they do not affect the compiled package.
type cmd interface {
	Execute() (interface{}, error)
	MustExecute() interface{}
}

type HelloCommand struct {
	// HelloCommand needs to have the `cmd` template invoked upon it.
	// By mixing in cmd, we tell go-codegen so.
	cmd
	Name string
}

func (cmd *HelloCommand) Execute() (interface{}, error) {
	return "Hello, " + cmd.Name, nil
}

type GoodbyeCommand struct {
	cmd
	Name string
}

func (cmd *GoodbyeCommand) Execute() (interface{}, error) {
	return "Goodbye, " + cmd.Name, nil
}

func main() {
	var c cmd
	c = &HelloCommand{Name: "You"}
	fmt.Println(c.MustExecute())
	c = &GoodbyeCommand{Name: "You"}
	fmt.Println(c.MustExecute())
}

Notice that HelloCommand doesn't have a MustExecute method. This code will be generated by go-codegen. Now we need to write the cmd template.

Create a new file named cmd.tmpl in the same package:

// MustExecute behaves like Execute, but panics if an error occurs.
func (cmd *{{ .Name }}) MustExecute() interface{} {
	result, err := cmd.Execute()

  if err != nil {
    panic(err)
  }

  return result
}

Notice the {{ .Name }} expression: It's just normal go template code.

Now, given both files, lets generate the code. Run go-codegen in the package directory, and you'll see a file called main_generated.go Whose content looks like:

package main

// MustExecute behaves like Execute, but panics if an error occurs.
func (cmd *HelloCommand) MustExecute() interface{} {
	result, err := cmd.Execute()

	if err != nil {
		panic(err)
	}

	return result
}

// MustExecute behaves like Execute, but panics if an error occurs.
func (cmd *GoodbyeCommand) MustExecute() interface{} {
	result, err := cmd.Execute()

	if err != nil {
		panic(err)
	}

	return result
}

Installation

Presently, we only support installing from source. You'll first need to installing gb, used for building this project:

go get -u github.com/constabulary/gb/...

Then:

git clone [email protected]:nullstyle/go-codegen.git
cd go-codegen
gb build
cp ./bin/go-codegen /usr/local/bin

Notes on template invocation

TODO

Template Data

The "data" available in a template invocation is represented with a TemplateContext struct. You can see it here.

Finding templates

At present, a template is searched for within the same directory as a struct invoking the template, using a name of the form TemplateName.tmpl. For example, invoking go-codegen in a directory that has a file named "FooBar.tmpl" will cause that template to be invoked for any struct that has an anonymous field with a type of FooBar. FooBar may be any type: interface, struct or otherwise.

Arguments

TODO

Adding Imports

A template may add additional imports into generated go giles by calling the AddImport method on the TemplateContext like so:

{{ $.AddImport "net/http" }}

TODO

• Add additional data about the invoking structs fields.
• Add more facilities to interpret struct tags: Extract different named tags, for example.
• Template search paths.
• Better examples

Example usages

Below is a list of some example usages of go-codegen. Open up a github issue if you would like to add your own.

• go-horizon uses this to remove some boilerprate from their HTTP Handler code
• Some examples from this project illustrate simple usage

Contributing

1. Fork it ( https://github.com/nullstyle/go-codegen/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request