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Fo is an experimental language which adds functional programming features to Go. The name is short for "Functional Go".

Table of Contents

Current Status

Fo is no longer being actively worked on or maintained.

When this project was first created, Go did not support generics. However, as of Go version 1.18, that has changed. For more information about how to use generics in the latest version of Go, see this official tutorial.

Since Go supports generics, there is much less reason to continue working on Fo. It is sort of true that Fo supports some things that Go does not (like generic methods or omitting the type parameters on a receiver type when it is not used). But overall Fo is still fairly unstable and has the major limitation of being restricted to compiling a single file. This likely makes it unsuitable for any real world applications.

I have decided to leave this repository and the playground up for. It might be helpful for anyone who wants to hack on the Go compiler or work with ASTs.


If you want to give Fo a try without installing anything, you can visit The Fo Playground.


The Fo compiler is written in Go, so you can install it like any other Go program:

go get -u

Command Line Usage

For now, the CLI for Fo is extremely simple and only works on one file at a time. There is only one command, run, and it works like this:

fo run <filename>

<filename> should be a source file ending in .fo which contains a main function.


You can see some example programs showing off various features of the language in the examples directory of this repository.

Language Features

In terms of syntax and semantics, Fo is a superset of Go. That means that any valid Go program is also a valid Fo program (provided you change the file extension from ".go" to ".fo").

Generic Named Types


Fo extends the Go grammar for type declarations to allow the declaration of generic types. Generic types expect one or more "type parameters", which are placeholders for arbitrary types to be specified later.

The extended grammar looks like this (some definitions omitted/simplified):

TypeDecl   = "type" identifier [ TypeParams ] Type .
TypeParams = "[" identifier { "," identifier } "]" .

In other words, type parameters should follow the type name and are surrounded by square brackets. Multiple type parameters are separated by a comma (e.g., type A[T, U, V]).

Here's the syntax for a generic Box which can hold a value of any arbitrary type:

type Box[T] struct {
	v T

Type parameters are scoped to the type definition and can be used in place of any type. The following are all examples of valid type declarations:

type A[T] []T

type B[T, U] map[T]U

type C[T, U] func(T) U

type D[T] struct{
	a T
	b A[T]

In general, any named type can be made generic. The only exception is that Fo does not currently allow generic interface types.


When using a generic type, you must supply a number of "type arguments", which are specific types that will take the place of the type parameters in the type definition. The combination of a generic type name and its corresponding type arguments is called a "type argument expression". The grammar looks like this (some definitions omitted/simplified):

TypeArgExpr = Type TypeArgs .
TypeArgs    = "[" Type { "," Type } "]" .

Like type parameters, type arguments follow the type name and are surrounded by square brackets, and multiple type arguments are separated by a comma (e.g. A[string, int, bool]). In general, type argument expressions can be used anywhere you would normally use a type.

Here's how we would use the Box type we declared above to initialize a Box which holds a string value:

x := Box[string]{ v: "foo" }

Fo does not currently support inference of type arguments, so they must always be specified.

Generic Functions


The syntax for declaring a generic function is similar to named types. The grammar looks like this:

FunctionDecl = "func" FunctionName [ TypeParams ] Signature [ FunctionBody ] .
TypeParams   = "[" identifier { "," identifier } "]"

As you might expect, type parameters follow the function name. Both the function signature and body can make use of the given type parameters, and the type parameters will be replaced with type arguments when the function is used.

Here's how you would declare a MapSlice function which applies a given function f to each element of list and returns the results.

func MapSlice[T](f func(T) T, list []T) []T {
	result := make([]T, len(list))
	for i, val := range list {
		result[i] = f(val)
	return result


Just like generic named types, to use a generic function, you must supply the type arguments. If you actually want to call the function, just add the function arguments after the type arguments. The grammar looks like this (some definitions omitted/simplified):

CallExpr = FunctionName ( TypeArgs ) Arguments .
TypeArgs = "[" Type { "," Type } "]" .

Here's how you would call the MapSlice function we defined above:

func incr(n int) int {
	return n+1

// ...

MapSlice[int](incr, []int{1, 2, 3})

Generic Methods


Fo supports a special syntax for methods with a generic receiver type. You can optionally include the type parameters of the receiver type and those type parameters can be used in the function signature and body. Whenever a generic method of this form is called, the type arguments of the receiver type are passed through to the method definition.

The grammar for methods with generic receiver types looks like this (some definitions omitted/simplified):

MethodDecl = "func" Receiver MethodName [ TypeParams ] Signature [ FunctionBody ] .
Receiver   = "(" [ ReceiverName ] Type [ TypeParams ] ")" .
TypeParams = "[" identifier { "," identifier } "]" .

Here's how we would define a method on the Box type defined above which makes use of receiver type parameters:

type Box[T] struct {
  v T

func (b Box[T]) Val() T {
  return b.v

You can also omit the type parameters of the receiver type if they are not needed. For example, here's how we would define a String method which does not depend on the type parameters of the Box:

func (b Box) String() string {
  return fmt.Sprint(b.v)

A method with a generic receiver can define additional type parameters, just like a function. Here's an example of a method on Box which requires additional type parameters.

func (b Box[T]) Map[U] (f func(T) U) Box[U] {
  return Box[U]{
    v: f(b.v),


When calling methods with a generic receiver type, you do not need to specify the type arguments of the receiver. Here's an example of calling the Val method we defined above:

x := Box[string]{ v: "foo" }

However, if the method declaration includes additional type parameters, you still need to specify them. Here's how we would call the Map function defined above to convert a Box[int] to a Box[string].

y := Box[int] { v: 42 }
z := y.Map[string](strconv.Itoa)


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