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proposal: spec: allow additional constraints on methods of generic types #64846

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leaxoy opened this issue Dec 22, 2023 · 6 comments
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generics Issue is related to generics LanguageChange Suggested changes to the Go language LanguageChangeReview Discussed by language change review committee Proposal
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@leaxoy
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leaxoy commented Dec 22, 2023

Proposal Details

Proposal

Generic structures can have type parameters and constraints, and generic structures can also have methods. but there is no way to add more constraints on the methods of generic structures.

Let's look at a classic example, sorting slices:

type Slice[E cmp.Ordered] struct {}

func (s *Slice[E]) Sort() {}

// or 

type Slice[E any] struct {}
func Sort[E cmp.Ordered](s *Slice[E]) {}

// but now, we can't write such code
type Slice[E any] struct {}

func (s *Slice[E cmp.Ordered]) Sort() {}

Some currently feasible methods have some inconveniences

Adding narrower constraints on types also narrows the usefulness of the structure.

Top-level functions will directly affect the user experience, and some chain operations will have to be split into multiple lines.

Therefore I propose that the language should provide the ability to add type constraints on methods of generic structures.


Some possible syntax:

Option 1

using where clause at the end of method signature, but before {

func (s *Slice[E]) Sort() where E: cmp.Ordered {}

This method can only be accessed when E satisfies the constraint cmp.Ordered, otherwise it cannot be accessed.

Other languages, such as c#/rust/swift, provide similar capabilities.

This approach is closer to common practice in other languages, but requires the introduction of a keyword.

Option 2

add constraints near E

func (s *Slice[E cmp.Ordered]) Sort() {}

As far as I know, this approach has not been used in other languages.

This method does not require the introduction of additional keywords, but it looks a bit strange.


This is the complete proposal and some attempts at syntax.

@gopherbot gopherbot added this to the Proposal milestone Dec 22, 2023
@seankhliao
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Duplicate of #49085

@seankhliao seankhliao marked this as a duplicate of #49085 Dec 22, 2023
@seankhliao seankhliao closed this as not planned Won't fix, can't repro, duplicate, stale Dec 22, 2023
@leaxoy
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leaxoy commented Dec 22, 2023

These are completely different things. Are you sure you have carefully read the community's proposals? @seankhliao

@leaxoy
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leaxoy commented Dec 22, 2023

That issue propose add new type parameters, but this is not add new type parameters. It't just narrow the existing type parameter in specific scenarios.

@seankhliao
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if you consider the fact that the receiver is just another argument to the function, then it's the same: you're introducing new constraints in a method

@ianlancetaylor ianlancetaylor added LanguageChange Suggested changes to the Go language v2 An incompatible library change generics Issue is related to generics labels Feb 7, 2024
@ianlancetaylor
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This is not a dup. Reopening.

@ianlancetaylor ianlancetaylor reopened this Feb 7, 2024
@ianlancetaylor
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Although this is (now) a dup of #65394, which has more comments.

@ianlancetaylor ianlancetaylor changed the title proposal: generics: allow additional constraints on method. proposal: spec: allow additional constraints on methods of generic types Aug 6, 2024
@ianlancetaylor ianlancetaylor added LanguageChangeReview Discussed by language change review committee and removed v2 An incompatible library change labels Aug 6, 2024
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