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Generics: Set associated constants using where constraints (#1013)

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Change the syntax for setting the associated constants and types in an interface implementation for a type from using `let` declarations as in:
```
class Vector(T:! Type) {
  impl as Iterable {
    let ElementType:! Type = T;
    ...
  }
}
```
to using `where` clauses as in:
```
class Vector(T:! Type) {
  impl as Iterable where .ElementType = T {
    ...
  }
}
```

This is an attempt to simplify by removing redundancy, improve consistency by removing a use of `let` that was different than other examples, and better support forward declaration that a type implements an interface while retaining the information needed for type checking.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
This commit is contained in:
josh11b
2022-01-27 11:18:20 -08:00
committed by GitHub
co-authored by Richard Smith
parent 94265befee
commit 2d567f5824
4 changed files with 250 additions and 62 deletions
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docs/design/generics/overview.md
+12 -2
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@@ -197,8 +197,8 @@ class Song {
// ...
// Implementing `Printable` for `Song` inside the definition of `Song`
// means all names of `Printable`, such as `F`, are included as a part
// of the `Song` API.
// without the keyword `external` means all names of `Printable`, such
// as `F`, are included as a part of the `Song` API.
impl as Printable {
// Could use `Self` in place of `Song` here.
fn Print[me: Song]() { ... }
@@ -594,6 +594,15 @@ Constraints limit the types that the generic function can operate on, but
increase the knowledge that may be used in the body of the function to operate
on values of those types.
Constraints are also used when implementing an interface to specify the values
of associated types (and other associated constants).
```
class Vector(T:! Movable) {
impl as Stack where .ElementType = T { ... }
}
```
### Parameterized impls
Implementations can be parameterized to apply to multiple types. Those
@@ -634,3 +643,4 @@ priority order in a prioritization block.
- [#818: Constraints for generics (generics details 3)](https://github.com/carbon-language/carbon-lang/pull/818)
- [#920: Generic parameterized impls (details 5)](https://github.com/carbon-language/carbon-lang/pull/920)
- [#950: Generic details 6: remove facets](https://github.com/carbon-language/carbon-lang/pull/950)
- [#1013: Generics: Set associated constants using `where` constraints](https://github.com/carbon-language/carbon-lang/pull/1013)