Version 24 (modified by chak, 11 years ago) (diff)


Type Functions: Implementation Status

Back to TypeFunctions.


  • Testing, before having a go at type checking.

Parsing and Renaming

Todo (high-level):

  1. Parse and rename equality constraints in signatures.
  2. Defaults for associated type synonyms. (Having both a kind signature and vanilla synonym is problematic as in RnNames.getLocalDeclBinders its hard to see that not both of them are defining declarations, which leads to a multiple declarations error. Defaults are quite different from vanilla synonyms anyway, as they usually have tyvars on their rhs that do not occur on the lhs.)


  • Parsing and renaming of kind signatures (toplevel and in classes).
  • Parsing and renaming of indexed types declarations (toplevel and in classes).

Type Checking

Todo (low-level):

  • Applications of indexed types need to be applied to all type indexes.
  • Match definitions of indexed types against their signatures.
  • Default AT synonyms are only allowed for ATs defined in the same class.
  • We need to require -fglasgow-exts for ATs. (The type checker seems to be the place to check whether the flag was supplied.) To check this for classes, we need to have Class.Class suitably extended; the infrastructure for the check is already builtin in (see binding no_ats in checkValidClass). We already do this to some extend.
  • For each case scrutinising an associated data type, check that all constructors have been defined in a single instance. (Maybe we can just extend the existing check that ensures that case expressions don't mix constructors of different data types.)
  • Check that each AT definition mirrors the class arguments of the instance in its type indexes. This might be a bit more tricky if we want to allow that they can vary syntactically before expansion of type synonyms. (Do this in the type checker unless we find it is very hard to do there; then, revert to trying it during renaming.)
  • Check that each instance has a definition for every AT and also that all defined associated types are, in fact, declared by the class. (Do this in the type checker - GHC does the corresponding checks for methods in the type checker, too.) Also check that kind signatures that correspond to type variables in the AT declaration or class declaration match the kinds inferred for the AT declaration. (This certainly needs to be done in the type checker.)
  • We need to somehow change the signatures of data constructors of ATs around so that they mention the Name of the type constructor declared in the class (and not the one of the instance the data constructor was declared in). Might be able to do that via a GADT style signature. Rethink this under the new scheme.
  • Search for !!!TODO (eg, TcTyClsDecls.lhs)
  • Extend TcTyClDecls.tcTyAndClassDecls to properly do kind checking for kind signatures and type equations.

Todo (high-level):

  1. Type checking of associated data types.
  2. Type checking of type functions (and hence, associated type synonyms).
  3. Type check functional dependencies as type functions.


  • Kind checking for associated data types.


Todo (low-level):

  • Derivings on an associated data type declaration need to be inherited by all definitions of that data type in instances.

Todo (high-level):

  1. Desugar associated data types.
  2. Desugar type functions and equality constraints.
  3. Extend interface files.
    • How do we exactly want to represent ATs in interface files?
      • SPJ pointed out that instances are maintained in InstEnv.InstEnv with different values for the home packages and others. The definitions of ATs may have to be maintained in a similar way, as they are also incrementally collected during compiling a program.
      • IfaceInst contains the instance declaration information for interfaces.
    • Export and import lists: The name lists that may appear at class imports and exports can now also contain type names, which is tricky as data type names can carry a list of data constructors.
    • At the moment, we add as the parent name of the data constructors of associated data types defined in instances, the new name for the data type constructor, which is different from that of the data type constructor in the class (also their source representation is the same). We may need to fix that during renaming. (We can't easily fix it in getLocalDeclBinders, where the names of the data constructors are made, as we don't have the means to get at the right class at that point.)

Done: Nothing.



  • Convert AT.hs to tests in the testsuite.