packagen

packagen : Generate Go code from packages or files

Overview

Before Go adopts generics in one form or another (if it ever does!), duplicating code for different types is the current way of dealing with it. The goal of this package is to be able to generate that code from a well written and tested package, and incidentally from any go file.

Install

go get github.com/pierrec/packagen/cmd/packagen

Usage

The main idea is to define a pivot type, eventually with its own set of methods, that works for the implemented algorithm. That type is then replaced when generating the code with a custom one that works in the same way, implementing the same methods if necessary.

The generated code contains the same types, functions etc than the source, but prefixed so that they do not collide with the code in the same package.

The available command line options are:

• single
  • -const list of integer constants to be updated (constname=integer[, ...])
  • -mvtype list of named types to be renamed (old=new[, ...])
  • -newpkg new package name (default=current working dir package)
  • -nogen - do not add the generate directive
  • -o file - write output to file (default=standard output)
  • -prefix prefix used to rename declarations (default=packageName_)
  • -rmtype list of named types to be removed (typename[, ...])

Example

Given a sorting algorithm implemented in the package domain/user/sort, generate the code for another integer type with the following command:

packagen single -o int32s_gen.go -prefix Int32 -mvtype Numbers=Int32s -rmtype Numbers domain/user/sort

Source package:

package sort

type Numbers []int

func (s Numbers) Len() int { return len(s) }
func (s Numbers) Less(i, j int) bool { return s[i] < s[j] }
func (s Numbers) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

func Sort(s Numbers) {
    // ... sorting algorithm
}

In the new package domain/user/myapp, there is an existing int32s.go file:

package myapp

type Int32s []int32

// Int32s implements the methods required by the sort algorithm.
func (s Int32s) Len() int { return len(s) }
func (s Int32s) Less(i, j int) bool { return s[i] < s[j] }
func (s Int32s) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

The generated file int32s_gen.go contains:

//go:generate go run github/pierrec/packagen/cmd/packagen single -o int32s_gen.go -prefix Int32 -mvtype Numbers=Int32s -rmtype Numbers domain/user/sort

package myapp

// Note that the package name has been set automatically.
// Also, the go:generate directive is added by default, but this can be disabled.

// All the code from the source is preserved and top level declarations are prefixed, except for the pivot type and its 
// methods which have been removed.
func Int32Sort(s Int32s) {
    // ... sorting algorithm
}

Contributing

Contributions welcome via pull requests. Please provide tests.