Generics as builtin typeclasses

I love the debate around the generics proposal for Go 2.

In the spirit of Go’s ‘good enough’ type system, here’s a thought experiment for ‘good enough’ generics. I think we can get 80% of the power for 20% of the complexity.

(‘Good enough’ is a compliment – I come from a C# background and find the simplicity of Go’s type system refreshing.)

I propose that the first version of Go’s generics consist of a finite-but-thorough set of builtin type classes, instead of user-defined contracts. Usage might look like:

func Sum(type T numeric)(x []T) T {
	var total T
	for _, v := range x {
		total += v
	return total


type Set(type T comparable) []T

…where numeric and comparable are typeclasses, defined below. In fact, the go/types package already defines many type classes, without using that terminology.

Instead of covering the full design space of what generics can do, we focus on an intuitive set of typeclasses that meets a broad set of needs.


numeric = integer | float | complex
  // supports +, -, *, / operators, enabling generic Sum, Average, etc

comparable = [requires logic]
   // supports == operator, enabling generic Set
   // Defined here:

orderable = integer | float | string
  // supports < and > operators, enabling generic Min, Max, Sort

integer = int | int8 | int16 | int32 | int64| uint | uint8 | uint16 | uint32 | uint64 | uintptr | byte | rune

float = float32 | float64

complex = complex64 | complex128


Typeclasses are not assignable types, they are (optional) constraints on generic type declarations.

// Wrong
type Foo struct {
	Age numeric

// Right
type Foo(type T numeric) struct {
	Age T

// Constructor example
func NewFoo(type T numeric)(age T) Foo(T) {
	foo := Foo(T) {
		Age : age,
	return foo

What this reminds me of

Typeclasses look a lot like union types. int8 gets unioned into integer which gets unioned into numeric, etc.

Another way to think of type classes is as interfaces for operators. Imagine that the + operator were an Add() method instead. The numeric type above is a lot like an Adder interface.

Published by Matt Sherman on September 4, 2018