Reusing source code with Go templates

In one of my previous blog posts, we discovered go generate command line tool. Lets take the next step and evaluate its advanced benefits to generate a source code by using our own templates. We will explore gotemplate command line tool.

Overview

This command line tool manages package based Go templates using go generate. By default it provides a set of templates that can be used out of the box:

  • set - a template that generates a set struct for a type
  • list - a template that generates a list struct for a type
  • sort - a template that provides a sort primitivies for a type
  • heap - a template that provides heap operations for a type

Installation

It is simple to install by using go get command. Note that the command installs the predefined templates as well.

$ go get github.com/ncw/gotemplate/...

Usage

To instaciate a particular template, you must use it using a special comment in your code:

//go:generate gotemplate "github.com/ncw/gotemplate/list" StudentList(Student)
type Student struct {
	FirstName string
	LastName  string
	BirthDate time.Time
}

Afte executing go generate command, a file gotemplate_StudentList.go is generated. It contains the StudentList type that defines a list struct that works with Student type. It has the following methods and functions:

+StudentList : struct
  [methods]
  +Back() : *StudentListElement
  +Front(): *StudentListElement
  +Init() : *StudentList
  +InsertAfter(v Student, mark *StudentListElement) : *StudentListElement
  +InsertBefore(v Student, mark *StudentListElement) : *StudentListElement
  +Len() : int
  +MoveToBack(e *StudentListElement)
  +MoveToFront(e *StudentListElement)
  +PushBack(v Student) : *StudentListElement
  +PushBackList(other *StudentList)
  +PushFront(v Student) : *StudentListElement
  +PushFrontList(other *StudentList)
  +Remove(e *StudentListElement) : Student
  [functions]
  +NewStudentList() : *StudentList

You can use it in the following manner:

package main

import (
	"fmt"
	"spike/education"
)

func main() {
	student := education.Student{
		FirstName: "John",
		LastName:  "Smith",
	}

	list := education.NewStudentList()
	list.PushFront(student)

	fmt.Println(list.Front().Value.FirstName)
}

Using an initial capital when you name your template instantiation will make any external functions and types public. If you want to generate them as private you must use lower case like:

//go:generate gotemplate "github.com/ncw/gotemplate/set" stringSet(string)
//go:generate gotemplate "github.com/ncw/gotemplate/set" floatSet(float64)

Then code generation produces gotemplate_stringSet.go and gotemplate_floateSet.go files.

$ go generate
substituting "github.com/ncw/gotemplate/set" with stringSet(string) into package main
Written 'gotemplate_stringSet.go'
substituting "github.com/ncw/gotemplate/set" with floatSet(float64) into package main
Written 'gotemplate_floatSet.go'

Creating a custom templates

Templates must be valid go packages. They should compile and have tests and be usable as-is.

To make the package a valid template it should have one or more declarations and a special comment that declares the its template name and parameters.

The line below indicates that the base name for the template is TemplateType and it has one type parameter TParameter. Supported parameterized declarations are type, const, var and func.

// template type TemplateType(TParameter)
type TParameter int

Lets implement a template for Stack data structure.

package stack

import "errors"

// template type Stack(TValue)
type TValue *int

type Stack struct {
	data []TValue
}

func (s *Stack) Push(value TValue) {
	s.data = append(s.data, value)
}

func (s *Stack) Pop() (TValue, error) {
	length := len(s.data)
	if length == 0 {
		return nil, errors.New("Stack is empty")
	}

	value := s.data[length-1]
	s.data = s.data[:length-1]
	return value, nil
}

Lets declare it for Student struct:

//go:generate gotemplate "github.com/svett/gotemplate/stack" StudentStack(*Student)
type Student struct {
	FirstName string
	LastName  string
	BirthDate time.Time
}

Then you can instatiate:

$ go get github.com/svett/gotemplate/stack
$ go generate

When the template is instantiated, a new file gotemplate_StudentStack.go is create. It is a result of substition of actual template with StudentStack(*Student) declaration. All TValue occurances are replaced with Student. The Stack struct is changed to StudentStack.

The template can be downloaded from here.

Conclusion

Gotemplate is great tool for automatic a common development tasks. Because of its extensibility, we can focus on what should be generated instead of how to generate it.

Published by in programming languages and tutorial and tagged #code generation and #go using 613 words.