generics

Generics for Go

Generics for Go generates strongly typed Go code by expanding generic templates from a library.

A template is a piece of generic code with type place-holders. Template expansion then replaces type place-holders with concrete types. This creates a strongly typed version of a generic algorithm.

Because generics are not supported by Go, a tool to generate code is needed. Install it as follows:

go get github.com/reactivego/generics/cmd/jig

Example

Let's implement a generic stack.

We start with two templates that use foo as a type place-holder:

package stack

type foo int

//jig:template <Foo>Stack

type FooStack []foo
var zeroFoo foo

//jig:template <Foo>Stack Push

func (s *FooStack) Push(v foo) {
  *s = append(*s, v)
}

Note that this library contains valid Go code that compiles normally.

This code is online and can be imported as follows:

import _ "github.com/reactivego/generics/example/stack/generic"

Given some code that uses this Library in a file main.go.

package main

import _ "github.com/reactivego/generics/example/stack/generic"

func main() {
  var stack StringStack
  stack.Push("Hello, World!")
}

Running jig will expand the two templates and write them to the file stack.go.

// Code generated by jig; DO NOT EDIT.

//go:generate jig

package main

//jig:name StringStack

type StringStack []string
var zeroString string

//jig:name StringStackPush

func (s *StringStack) Push(v string) {
  *s = append(*s, v)
}

The generated code is a type-safe stack generated by expanding templates for the type string

Quick Start

In this Quick Start you will learn to write a minimal program that uses generics. The generator tool jig will generate code from the generics library rx (Reactive eXtensions for Go). The purpose of this is to showcase how to use a generics library in Go.

Dependencies

You must to have Go installed.

Installation

To install jig and rx, open a terminal and from the command-line and run:

$ go get github.com/reactivego/generics/cmd/jig
$ go get github.com/reactivego/rx

Note the first line will install the jig command in your GOPATH bin directory and the second one will download rx.

To test whether jig was installed go to the command-line and run jig -h:

$ jig -h
Usage of jig [flags] [<dir>]:
  -c, --clean     Remove files generated by jig
  -r, --regen     Force regeneration of all code by jig (default)
  -m, --missing   Only generate code that is missing
  -v, --verbose   Print details of what jig is doing

How it works

• You write code that references templates from the rx library.
• You run the jig command in the directory where your code is located.
• Now jig analyzes your code and determines what additional code is needed to make it build.
• Jig takes templates from the rx library and specializes them on specific types.
• Specializations are generated into the file rx.go alongside your own code.
• If all went well, your code will now build.

Prepare Folder

Let's create a new folder for our simple program and start editing the file main.go.

$ cd $(go env GOPATH)
$ mkdir -p ./src/helloworld
$ cd ./src/helloworld
$ subl main.go

NOTE we use subl to open Sublime Text, but any text editor will do.

Write Code

Now that you have your main.go file open in your editor of choice, type the following code:

If you would now go to the command-line and run jig it would do the following:

1. Import github.com/reactivego/rx/generic to access the generics in that library.
2. Generate FromString by specializing the template From<Foo> from the library on type string. Generate dependencies of FromString like the type ObservableString that is returned by FromString.
3. Generate the MapString method of ObservableString by specializing the template Observable<Foo> Map<Bar> for Foo as type string and Bar (the type to map to) also as type string.
4. Map function from string to string just concatenates two strings and returns the result.
5. Print every string returned by the Map function.
6. The output you can expect when you run the program.

Now actually go to the command-line and run jig -v. Use the verbose flag -v because otherwise jig will be silent.

$ jig -v
found 126 templates in package "rx" (github.com/reactivego/rx/generic)
found 16 templates in package "multicast" (github.com/reactivego/multicast/generic)
generating "FromString"
  Scheduler
  Subscriber
  StringObserveFunc
  zeroString
  ObservableString
  FromSliceString
  FromString
generating "ObservableString MapString"
  ObservableString MapString
generating "ObservableString Println"
  Schedulers
  ObservableString Println
writing file "rx.go"

Now we can try to run the code and see what it does.

$ go run *.go
Hello, You!
Hello, Gophers!
Hello, World!

Success! jig generated the code into the file rx.go and we were able to run the program. Turns out the generated file rx.go contains less than 250 lines of code.

If you add additional code to the program that uses different generics of the rx library, then you should run jig again to generate specializations of those generics.