gogen

go get github.com/rastech/gogen

gogen is a templating generics preprocessor for Go. It allows you to write pseudo-generic code like this:

package list

import "github.com/rastech/gogen/generics”

type T generic.Generic

type List struct {
    data T
    next *List
}

And, as it turns out, that's valid Go! generic.Generic resolves to interface{}, which means that this snippet could be used as-is with type assertions. However, if we run gogen:

gogen -o intlist github.com/path/to/list T=int

A new package intlist is created for us in the current directory with list.go as the only file:

package intlist

import "github.com/rastech/gogen/generic"

type _ generic.Generic

type List struct {
	data int
	next *List
}

intlist can now be imported and used normally.

Usage

Rename generic.Generic to create generic types. This declaration provides the generic types T and U:

type T generic.Generic
type U generic.Generic

You can define as many types as you want in this manner, as long as their names don't collide with any other identifiers in the package. To generate a concrete implementation of the package, run gogen:

gogen -o [outpkg] [inpkg] K=V K=V ... 

Where outpkg is the path of the package to be created, inpkg is the location of the generic package in your GOPATH (usually of the form github.com/username/packages...), and the K=V pairs map from generic type names to concrete types.

Note that because Go has no support for type parameters (e.g. func[U] fn() U, as you might be used to seeing in Java, Scala, C#, or other similar languages), all generic types are declared at the package scope; i.e. globally. So for any given func() T in your package, T represents the same type. Of course, T could be an interface type, or even interface{}, if you desire, but using interface{} somewhat defeats the point of using generics in the first place.

Go generate

As you're writing and maintaining your generic package, it may become tedious to write out your entire gogen command each time you make a change. It's suggested that you use go generate to handle this process for you. Simply add

//go:generate gogen -o [outpkg] github.com/path/to/generic/pkg Key=Val Key=Val ... 

to any source file in your package, and whenever you run go generate in your package directory (or go generate ./... in a parent), your package will be (re)generated. It may make sense to put this in a generate.go file or similar with // +build ignore.

You may also want to add gogen to your build process (go get github.com/rastech/gogen && go generate ./...) rather than committing the generated files to source control.

By default, gogen creates a .gitignore in the generated package set to ignore everything. To disable this behavior, pass the --no-gi flag to gogen.

Inner workings

Feel free to look at main.go for the bulk of the implementation. At the basic level, gogen:

• loads and type-checks the generic package G
• loads and type-checks github.com/rastech/gogen/generic
• gets the type T of generic.Generic from github.com/rastech/gogen/generic
• searches the file ASTs in G for type specs of type T, eliding those that match the commandline KV pairs with _
• searches the file ASTs in G for identifiers matching the KV pairs and replaces them with concrete types
• rewrites the package names for all file ASTs in G
• writes out the new package, then reloads and type-checks it

Caveats

Imports

gogen only supports Go builtin types at the moment; i.e. this is not possible (but it will be soon):

gogen -o myimpl github.com/path/to/pkg T=github.com/path/to/my/type:Type

In fact, due to an unresolved issue with the type checker, only scalar types are supported; i.e. []int, map[int]string, etc. are unfortunately not available. This is a top priority feature for the near future.

Zeroes and Equality

Since the underlying type of generic.Generic is interface{} and gogen does not make any assumptions when it makes its first type-check, the following may appear to be safe because nil is a valid value for interface{}:

func fn(in T) {
	if in == nil {
		// ...
	}
}

However, it will fail to type-check for T=int or any other value type. To handle this, something like the following would work:

func fn(in T) {
	var zero T
	if in == zero {
		// ...
	}
}

But this fails for T=[]int,T=map[int]string, etc. The more these comparisons can be factored out into non-generic code, the better, but reflect.DeepEqual is also available if needed. Automatic replacement of these formations may also be available in the future.

NB: In his gengen overview, joeshaw offers a nice solution using Equaler interfaces, but gogen does not support this pattern, because generic.Generic (by definition) does not implement any interfaces, and gogen will therefore fail on its first typecheck (before substitution). Support for something of this nature may be added in the future.

Credits

This tool started out as a fork of joeshaw's gengen with a few tweaks and fixes and ended up as a complete rewrite. Rather than manual AST loading and walking, gogen uses the loader package and ast.Walk to substitute and type-check references to generic types.

The idea for the

	type T generic.Generic

pattern came from this issue on gengen.