Constraints for generics (generics details 3) (#818)

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>

docs/design/generics/goals.md

@@ -532,7 +532,8 @@ are complicated and
 
 ### Interfaces are nominal
 
-Interfaces can either be structural, as in Go, or nominal, as in Rust and Swift.
+Interfaces can either be [structural](terminology.md#structural-interfaces), as
+in Go, or [nominal](terminology.md#nominal-interfaces), as in Rust and Swift.
 Structural interfaces match any type that has the required methods, whereas
 nominal interfaces only match if there is an explicit declaration stating that
 the interface is implemented for that specific type. Carbon will support nominal

docs/design/generics/overview.md

@@ -27,12 +27,13 @@ pointers to other design documents that dive deeper into individual topics.
         -   [Generic type parameters](#generic-type-parameters)
     -   [Requiring or extending another interface](#requiring-or-extending-another-interface)
     -   [Combining interfaces](#combining-interfaces)
-        -   [Structural interfaces](#structural-interfaces)
+        -   [Named constraints](#named-constraints)
         -   [Type erasure](#type-erasure)
     -   [Adapting types](#adapting-types)
     -   [Interface input and output types](#interface-input-and-output-types)
         -   [Associated types](#associated-types)
         -   [Parameterized interfaces](#parameterized-interfaces)
+    -   [Constraints](#constraints)
 -   [Future work](#future-work)
 -   [References](#references)
 
@@ -84,9 +85,9 @@ Summary of how Carbon generics work:
 -   The `&` operation on type-of-types allows you conveniently combine
     interfaces. It gives you all the names that don't conflict.
 -   You may also declare a new type-of-type directly using
-    ["structural interfaces"](terminology.md#structural-interfaces). Structural
-    interfaces can express requirements that multiple interfaces be implemented,
-    and give you control over how name conflicts are handled.
+    ["named constraints"](terminology.md#named-constraints). Named constraints
+    can express requirements that multiple interfaces be implemented, and give
+    you control over how name conflicts are handled.
 -   Alternatively, you may resolve name conflicts by using a qualified syntax to
     directly call a function from a specific interface.
 
@@ -251,7 +252,7 @@ the constraint on the type is that it must implement the interface `Comparable`.
 A type-of-type also defines a set of names and a mapping to corresponding
 qualified names. You may combine interfaces into new type-of-types using
 [the `&` operator](#combining-interfaces) or
-[structural interfaces](#structural-interfaces).
+[named constraints](#named-constraints).
 
 ### Generic functions
 
@@ -406,16 +407,16 @@ fn BothDraws[T:! Renderable & EndOfGame](game_state: T*) {
 }
 ```
 
-#### Structural interfaces
+#### Named constraints
 
 You may also declare a new type-of-type directly using
-["structural interfaces"](terminology.md#structural-interfaces). Structural
-interfaces can express requirements that multiple interfaces be implemented, and
-give you control over how name conflicts are handled. Structural interfaces have
-other applications and capabilities not covered here.
+["named constraints"](terminology.md#named-constraints). Named constraints can
+express requirements that multiple interfaces be implemented, and give you
+control over how name conflicts are handled. Named constraints have other
+applications and capabilities not covered here.
 
 ```
-structural interface Combined {
+constraint Combined {
   impl as Renderable;
   impl as EndOfGame;
   alias Draw_Renderable = Renderable.Draw;
@@ -431,7 +432,7 @@ fn CallItAll[T:! Combined](game_state: T*, int winner) {
   }
   game_state->Draw_Renderable();
   // Can still use qualified syntax for names
-  // not defined in the structural interface
+  // not defined in the named constraint
   return game_state->(Renderable.Center)();
 }
 ```
@@ -561,9 +562,30 @@ fn FindInVector[T:! Type, U:! Equatable(T)](v: Vector(T), needle: U)
 fn CompileError[T:! Type, U:! Equatable(T)](x: U) -> T;
 ```
 
+### Constraints
+
+Type-of-types can be further constrained using a `where` clause:
+
+```
+fn FindFirstPrime[T:! Container where .Element == i32]
+    (c: T, i: i32) -> 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.
+  ...
+}
+```
+
+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.
+
 ## Future work
 
--   Other kinds of constraints will be finalized.
 -   Implementations can be parameterized to apply to multiple types. These
     implementations would be restricted to various conditions are true for the
     parameters. When there are two implementations that can apply, there is a
@@ -583,3 +605,4 @@ fn CompileError[T:! Type, U:! Equatable(T)](x: U) -> T;
 
 -   [#524: Generics overview](https://github.com/carbon-language/carbon-lang/pull/524)
 -   [#731: Generics details 2: adapters, associated types, parameterized interfaces](https://github.com/carbon-language/carbon-lang/pull/731)
+-   [#818: Constraints for generics (generics details 3)](https://github.com/carbon-language/carbon-lang/pull/818)

docs/design/generics/terminology.md

@@ -24,6 +24,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -   [Interface](#interface)
     -   [Structural interfaces](#structural-interfaces)
     -   [Nominal interfaces](#nominal-interfaces)
+    -   [Named constraints](#named-constraints)
 -   [Associated entity](#associated-entity)
 -   [Impls: Implementations of interfaces](#impls-implementations-of-interfaces)
 -   [Compatible types](#compatible-types)
@@ -298,6 +299,14 @@ We use the "structural" versus "nominal" terminology as a generalization of the
 same terms being used in a
 [subtyping context](https://en.wikipedia.org/wiki/Subtyping#Subtyping_schemes).
 
+### Named constraints
+
+Named constraints are "structural" in the sense that they match a type based on
+meeting some criteria rather than an explicit statement in the type's
+definition. The criteria for a named constraint, however, are less focused on
+the type's API and instead might include a set of nominal interfaces that the
+type must implement.
+
 ## Associated entity
 
 An _associated entity_ is a requirement in an interface that a type's
@@ -320,8 +329,10 @@ instead of associated entity.
 An _impl_ is an implementation of an interface for a specific type. It is the
 place where the function bodies are defined, values for associated types, etc.
 are given. Impls are needed for [nominal interfaces](#nominal-interfaces);
-[structural interfaces](#structural-interfaces) define conformance implicitly
-instead of by requiring an impl to be defined.
+[structural interfaces](#structural-interfaces) and
+[named constraints](#named-constraints) define conformance implicitly instead of
+by requiring an impl to be defined. In can still make sense to implement a named
+constraint as a way to implement all of the interfaces it requires.
 
 ## Compatible types
 

docs/design/lexical_conventions/words.md

@@ -43,6 +43,7 @@ The following words are interpreted as keywords:
 -   `break`
 -   `case`
 -   `class`
+-   `constraint`
 -   `continue`
 -   `default`
 -   `else`
@@ -55,11 +56,13 @@ The following words are interpreted as keywords:
 -   `impl`
 -   `import`
 -   `interface`
+-   `is`
 -   `let`
 -   `library`
 -   `match`
 -   `namespace`
 -   `not`
+-   `observe`
 -   `or`
 -   `override`
 -   `package`