There were some concerns about facet types leaking out of generic code in return types. Some initial fixes for this were done in [PR #900](https://github.com/carbon-language/carbon-lang/pull/900), but there remain concerns, for example when associated types are involved.
In particular, given an interface method with return type using an associated type, as in:
```
interface Deref {
let Result:! Type;
fn DoDeref[me: Self]() -> Result;
}
class IntHandle {
impl as Deref {
let Result:! Type = i32;
fn DoDeref[me: Self]() -> Result { ... }
}
}
```
Since `Result` has type `Type`, we had the problem that `IntHandle.DoDeref` would have to return `i32 as Type`, instead of the desired `i32`.
We also think we can simplify the model by eliminating the facet type concept and syntax.
This proposal removes facet types, introduces archetypes in their place, clarifies how associated types work outside of a generic function, and specifies how a generic `let` statement in a function body works.
Co-authored-by: Wolff Dobson <wolffg@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Add an appendix with the rationale for and alternatives to coherence, along with an entry in the terminology and updates to the coherence discussion in the goals.
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
This proposal describes `where` clauses that can add constraints on a type-of-type, for example define restrictions on its associated types. Example:
```
fn FindFirstPrime[T:! Container where .Element = i32]
(c: T) -> Optional(i32) {
// The elements of `c` have type `T.Element`, which is `i32`.
...
}
fn PrintContainer[T:! Container where .Element is Printable](c: T) {
// The type of the elements of `c` is not known, but we do know
// that type satisfies the `Printable` interface.
...
}
```
Some other constraints, such as `Sized` are defined as type-of-types directly, possibly parameterized.
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
This proposal defines the very basics of `class` types, primarily focused on:
- use cases including: data classes, encapsulated types, inheritance with and without `virtual`, interfaces as base classes, and mixins for code reuse;
- anonymous data types for called _structural data classes_ or _struct types_. Struct literals are used to initialize class values and ad-hoc parameter and return types with named components; and
- future work, including the provisional syntax already in use for features that have not been decided.
The intent is to both make some small incremental progress and get agreement on direction. As such it doesn't include things like nominal types, methods, access control, inheritance, etc.
It proposes this struct type and literal syntax:
```
var p: {.x: Int, .y: Int} = {.x = 0, .y = 1};
```
Note that it uses commas (`,`) between fields instead of semicolons (`;`), and no introducer for types or literal values.
Incorporates decisions from #665 , #653 , #651
Co-authored-by: Geoff Romer <gromer@google.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
To talk about generics as a programming language feature, you need a lot of
specialized terminology. We need to agree on the words we are using and their
meaning before we can meaningfully talk about the design of the feature itself.
There a number of problems a glossary solves:
Not everyone knows every term, so having a single place to look them up will
improve the ease of understanding, ease of contributing, and accessibility
of the project.
There may not be widespread agreement on the meaning of some terms. In
particular, individual programming languages tend to assign very specific
meanings to terms used within their ecosystem.
Some terms may be used in multiple ways, but we only use the term with one
specific meaning.
Some terms are our invention and we need to introduce them.
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
The "generics" feature of Carbon is a large design effort that needs to be broken up into manageable steps. The first thing we need is a high-level goals document. The goals here reflect the desirable properties that we have discovered as part of considering several alternative generics designs:
- Use cases:
- Generic programming
- Upgrade path from C++ abstract interfaces
- Dependency injection
- Generics instead of open overloading and ADL
- Performance
- Better compiler experience
- Encapsulation
- Predictability
- Dispatch control
- Upgrade path from templates
- Coherence
- No novel name lookup
- Learn from others
- Interfaces are nominal
- Interop and evolution
- Bridge for C++ customization points
Goals are summarized in [this presentation](https://docs.google.com/presentation/d/12yGyu5Pvdag7CJp-_yLVkmbhyvuRIilBTNlkO0LKaHo/edit?usp=sharing&resourcekey=0-JB9yrUO4-6J8-zzGhnIyNg).
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>
Co-authored-by: Matthew Riley <mdriley@gmail.com>
Co-authored-by: austern <austern@google.com>
Co-authored-by: Dmitri Gribenko <gribozavr@gmail.com>
Jon Meowandjosh11b
