Why are there no generics in Go?

Does anybody know why there is no real support for generics/templates/whatsInAName in Go? So there is a generic map, but that's supplied by the compiler, while a Go programmer can't write her own implementation. With all the talk about making Go as orthogonal as possible, why can I USE a generic type but not CREATE a new one?

Especially when it comes to functional programming, there are lambdas, even closures, but with a static type system lacking generics, how do I write, well, generic higher order functions like filter(predicate, list)? OK, Linked lists and the like can be done with interface{} sacrificing type safety.

It looks like generics will only be added to Go as an afterthought, if at all,. I do trust Ken Thompson to do way better than the Java implementers, but why keep generics out? Or are they planned and just not implemented yet?

^{Note: Generics were added to Go in version 1.18.}

You will find the answer here: http://golang.org/doc/faq#generics

> Why does Go not have generic types?
>
> Generics may well be added at some point. We don't feel an urgency for them, although we understand some programmers do.
>
> Generics are convenient but they come at a cost in complexity in the type system and run-time. We haven't yet found a design that gives value proportionate to the complexity, although we continue to think about it. Meanwhile, Go's built-in maps and slices, plus the ability to use the empty interface to construct containers (with explicit unboxing) mean in many cases it is possible to write code that does what generics would enable, if less smoothly.
>
> This remains an open issue.

A proposal for generics was implemented for the Go 1.18 release, which was released on 15 March 2022

"Go 2"

The generics design started under the umbrella of Go2, first at https://blog.golang.org/go2draft and there were several more draft specs over the next 3 years.

Go 1

Russ Cox, one of the Go veterans wrote a blog post entitled The Generic Dilemma, in which he asks

> …do you want slow programmers, slow compilers and bloated binaries, or slow execution times?

Slow programmers being the result of no generics, slow compilers are caused by C++ like generics and slow execution times stem from the boxing-unboxing approach that Java uses.

The fourth possibility not mentioned in the blog is going the C# route. Generating the specialized code like in C++, but at runtime when it is needed. I really like it, but Go is very unlike C# so this is probably not applicable at all…

I should mention that using the popular Java 1.4 like technique of generic programming in go that casts to interface{} suffers from exactly the same problems as boxing-unboxing (because that's what we are doing), besides the loss of compile time type safety. For small types (like ints) Go optimizes the interface{} type so that a list of ints that were cast to interface{} occupies a contiguous area of memory and takes only twice as much space as normal ints. There is still the overhead of runtime checks while casting from interface{}, though. Reference.

All projects that add generic support to go (there is several of them and all are interesting) uniformly go the C++ route of compile time code generation.

To add to and update the excellent answers by @Vinzenz and @user7610.

> Although it's far from certain, after over a decade of work it looks
> like a design for parametric polymorphism, what is colloquially but
> misleadingly called generics, is coming in the next year or two. It
> was a very hard problem to find a design that works within the
> existing language and feels as if it belongs, but Ian Taylor invested
> a phenomenal amount of energy into the problem and it looks like the
> answer is now in reach. See https://evrone.com/rob-pike-interview.

"Type Parameters - Draft Design" supports the use of type parameters where you can read functions that handle incoming parameters without depending on the type specified in the function declaration. See https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md.

For example, the PrintSlice function receives a slice of integers or strings and prints it. See https://www.jetbrains.com/help/go/how-to-use-type-parameters-for-generic-programming.html.

package main

import "fmt"

func PrintSlice(type T)(s []T) {
	for _, v := range s {

		fmt.Print(v)
	}
}

func main() {
	PrintSlice([]int{1, 2, 3, 4, 5, 6, 7, 8, 9})
	PrintSlice([]string{"a", "b", "c", "d"})
}

You can test this example here https://go2goplay.golang.org/p/I09qwKNjxoq. This playground works just like the usual Go playground, but it supports generic code. See https://blog.golang.org/generics-next-step.

Parametric polymorphism basically means 'this function or data structure works identically with any type". That's what we also call generics. Eg the length of an array doesn't depend on what's in the array. See https://news.ycombinator.com/item?id=23560798.

The earliest that generics could be added to Go would be the Go 1.17 release, scheduled for August 2021. See https://blog.golang.org/generics-next-step.

Actually, according to this post:

> Many people have concluded (incorrectly) that the Go team’s position is “Go will never have generics.” On the contrary, we understand the potential generics have, both to make Go far more flexible and powerful and to make Go far more complicated. If we are to add generics, we want to do it in a way that gets as much flexibility and power with as little added complexity as possible.

Parametric polymorphism (generics) is under consideration for Go 2.

This approach would introduce the concept of a contract, that can be used to express constraints on type parameters:

contract Addable(a T) {
  a + a // Could be += also
}

Such a contract could then be used thusly:

func Sum(type T Addable)(l []T) (total T) {
  for _, e := range l {
    total += e
  }
  return total
}

This is a proposal at this stage.

Your filter(predicate, list) function could be implemented with a type parameter like this:

func Filter(type T)(f func(T) bool, l []T) (result []T) {
  for _, e := range l {
    if f(e) {
      result = append(result, e)
    }
  }
  return result
}

In this case, there is no need to constrain T.