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Go generics

5 Oct 2021 Summary: Go generics

Michal Vyskocil BE Developer, Platform Team michal.vyskocil@90poe.io

Go road to generics

  • Go was released without generics in 2009
  • Since then it was the most requested feature
  • First attempt in 2010
  • Official proposal submitted 12th January 2021: golang/go#43651
  • Accepted in 10th February 2021

More info at https://go.dev/blog/generics-proposal

Can I try generics?

Can I really try generics?

Why it took so long?

  • Retrofitting of generics into existing language is not easy
  • Go team has a special relationship with a C++
  • Wanted to avoid C++ templates gotchas
  • And there are many opinions in the community
  • Paper https://arxiv.org/abs/2005.11710 described Go generics in a formal way
  • And there are many more opinions in the community

Why? It's complicated

  1. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#type-parameters
  2. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#constraints
  3. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#defining-constraints
  4. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#generic-types
  5. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#methods-may-not-take-additional-type-arguments
  6. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#type-sets-of-constraints
  7. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#comparable-types-in-constraints
  8. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#mutually-referencing-type-parameters
  9. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#type-inference
  10. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#type-unification
  11. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#function-argument-type-inference
  12. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#constraint-type-inference
  13. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#using-types-that-refer-to-themselves-in-constraints
  14. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#more-on-type-sets
  15. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#pointer-method-example
  16. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#composite-types-in-constraints
  17. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#type-parameters-in-type-sets
  18. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#type-conversions
  19. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#untyped-constants
  20. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#interface-types-in-union-elements
  21. https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#empty-type-sets
  22. type assert + reflect package

Does Go needs generics?

Should Go 2.0 support generics?

https://dave.cheney.net/2017/07/22/should-go-2-0-support-generics

So my first answer is: Go should have some form of generics because it is a mainstream, imperative, block scoped language and it is expected these days.

Will Go change a lot?

Why Go Getting Generics Will Not Change Idiomatic Go

http://www.jerf.org/iri/post/2955

Generics in Go will give us data structures, tools to manipulate channels, maps, and slices in various generic ways beyond what we have now, possibly type-safe iterators, and a handful of other useful things. I suspect there are going to be some changes to the web frameworks, some moves on the numeric computation front, and some other similar things. I've got a few little places where I can avoid a bit of casting. But overall, what is "idiomatic" isn't going to change much.

Go already has generic functions

  • append
  • copy
  • delete
  • make
  • new

Yet the language itself can't express them, those are runtime hacks!

Syntax

[Name constraint]

func new2[T any]() *T {
   return new(T)
}
type klass[T any] struct { value T }
func (k *klass[T]) foo() { }

constraint is mandatory or compiler will complain syntax error: missing type constraint

And that's

all folks'

Lets try some real code

  • 01/ no generics
  • 02/ generic new2
  • 03/ type sets in constraints
  • 04/ interface constraints
  • 05/ generic types
  • 06/ befare of int
  • 07/ comparable types
  • 08/ more on type sets

Lets try some real code

  • 09/ composition types
  • 10/ mutually referencing type parameters
  • 11/ type conversions and untyped constants
  • 12/ interface types in union
  • 13/ empty type sets
  • 14/ (function argument) type inference
  • 15/ constraint type inference
  • 16/ pointer method example
  • 17/ type asserts and a reflec package

more examples

func [E any]magic(a T) []T {

return []T{a}

}