# Associated constants ## Table of contents - [Overview](#overview) - [Declaration checking](#declaration-checking) - [Specifying rewrite constraints](#specifying-rewrite-constraints) - [Definition of associated constant values](#definition-of-associated-constant-values) - [Use of associated constants](#use-of-associated-constants) - [Simple member access](#simple-member-access) - [Compound member access](#compound-member-access) - [Forming the constant value](#forming-the-constant-value) ## Overview _Note:_ This document only describes non-function associated constants. An associated constant is declared within an interface scope with the syntax: ```carbon [MODIFIERS] let NAME:! TYPE [= INITIALIZER] ; ``` Associated constants introduce a slot in the witness table for an interface that contains a value of type `TYPE`. Associated constants are always generic entities, because they're always parameterized at least by the `Self` type of the interface, as well as any other enclosing generic parameters. Note that the interface itself is _not_ parameterized by its `Self`. Associated constant entities are held in the `associated_constants` value store as objects of type `AssociatedConstant`. Each declaration of an associated constant is modeled by an `AssociatedConstantDecl` instruction. Each such instruction is then wrapped in an `AssociatedEntity` instruction which represents the slot within an interface witness where the constant's value can be found. ## Declaration checking Because associated constants share the syntax of `let` declarations, a lot of the checking logic is also shared. This logic is in [handle_let_and_var.cpp](/toolchain/check/handle_let_and_var.cpp). Associated constant declaration handling proceeds as follows: 1. ```carbon let NAME:! TYPE [= INITIALIZER] ; ^ ``` `StartAssociatedConstant` is called at the start of an interface-scope `let` declaration. This: - Starts a generic declaration region. - Pushes an instruction block to hold instructions within the declaration of the constant. These form the body of the generic. 2. ```carbon let NAME:! TYPE [= INITIALIZER] ; ~~~~^~~~~~~ ``` Process the symbolic binding pattern. This is done in [handle_binding_pattern.cpp](/toolchain/check/handle_binding_pattern.cpp), which detects that we are at interface scope, and creates an `AssociatedConstantDecl` and corresponding `AssociatedConstant` entity. This binding is then produced as the instruction associated with the binding pattern. _Note:_ This is somewhat unusual: usually, a pattern instruction would be associated with a pattern parse node. 3. ```carbon let NAME:! TYPE ; ^ let NAME:! TYPE = INITIALIZER ; ^ ``` When we reach the end of the pattern in an interface-scope `let` binding, either because we reached the `=` or because we reached the `;` and there was no initializer, `EndAssociatedConstantDeclRegion` is called. This: - Ends the generic declaration region. - Builds an `AssociatedEntity` object, reserving a slot in the interface's witness table for the constant. - Adds the associated constant to name lookup. _Note:_ The pattern might not be valid for an associated constant. In this case, we won't have built an `AssociatedConstantDecl` in the previous step. When this happens, we instead just discard the generic declaration region and continue. The invalid pattern will be diagnosed later. 4. ```carbon let NAME:! TYPE = INITIALIZER ; ^ ``` If there is an initializer, we start the generic definition region. 5. ```carbon let NAME:! TYPE [= INITIALIZER] ; ^ ``` At the end of the declaration, `FinishAssociatedConstant` is called to finalize the declaration. This: - Diagnoses if the pattern handling didn't create an `AssociatedConstantDecl`. - Finishes handling the initializer, if it's present: - Converts the initializer to the type of the constant. - Ends the generic definition region. - Pops the inst block created by `StartAssociatedConstant` and attaches it to the `AssociatedConstantDecl`. - Adds the `AssociatedConstantDecl` to the enclosing inst block. ## Specifying rewrite constraints TODO: Fill this out. In particular, note that we do not convert the rewrite to the type of the associated constant as part of forming a `where` expression if the constant's type is symbolic, and instead defer that until the facet type is resolved. ## Definition of associated constant values Associated constant values are stored into witness tables as part of impl processing in [impl.cpp](/toolchain/check/impl.cpp). TODO: Fill this out once the new model is implemented. ## Use of associated constants The work to handle uses of associated constants starts in [member_access.cpp](/toolchain/check/member_access.cpp). When an `AssociatedEntity` is the member in a member access, impl lookup is performed to find the corresponding impl witness. The self type in impl lookup depends on how the member name was found. ### Simple member access In `LookupMemberNameInScope`, if lookup for `y` in `x.y` finds an associated constant from interface `I`, then a witness is determined as follows: - If the lookup scope is the type `T` of `x`, then: - If `T` is a non-type facet, the witness for that facet is used. TODO: That facet might not contain a witness for `I`. In that case we will need to perform impl lookup for `T as I` instead. - Otherwise, impl lookup for `T as I` is performed to find the witness. - If the lookup scope is `x` itself, then: - If `x` is a facet type or a namespace, impl lookup is not performed, and the result is simply `y`. This happens for cases such as `Interface.AssocConst`. - Otherwise, `x` must be a type other than a facet type, and impl lookup for `x as I` is performed to find the witness. ### Compound member access In `PerformCompoundMemberAccess` for `x.(y)`, if `y` is an associated constant then impl lookup is performed for `T as I`, where `T` is the type of `x` and `I` is the interface in which `y` is declared to find the witness containing the constant value. ### Forming the constant value Once the witness is determined, `AccessMemberOfImplWitness` is called to find the value of the associated constant in the witness. In the case where an impl lookup is needed, `PerformImplLookup` calls `AccessMemberOfImplWitness`, otherwise it's called directly. `AccessMemberOfImplWitness` uses `GetTypeForSpecificAssociatedEntity` to form the type of the constant. This substitutes both the generic arguments (if any) for the interface and the `Self` type into the type of the associated constant. Then, an `ImplWitnessAccess` instruction is created to extract the relevant slot from the witness. Constant evaluation of this instruction reads the associated constant from the witness table.