Version 100 (modified by chak, 10 years ago) (diff)


Type Functions: Implementation Status

Back to TypeFunctions.


  • Where should the family instance consistency check go for GHCi? tcGetModuleExports? (Where is that function called?)
  • There really is no reason anymore to disallow family instances where all arguments are type variables. Such instances are rarely useful, but may be when creating stub files or so; so why disallow them. Check that they are working.

Parsing and Renaming

Todo (low-level):

  • Should family declarations be optional with ATs, too? (See comment at patch making kinds optional at toplevel declarations.)

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.)
  3. Import/export lists:
    • We need to be able to write something like GMapKey(GMap,empty).
    • Export and import of data constructors declarated in data instances. We should be able to use the same syntax for the entity specs as for closed data types, but the meaning is somewhat different.


  • Parsing and renaming of kind signatures (toplevel and in classes).
  • Parsing and renaming of indexed type declarations (toplevel and in classes).
  • Using new syntax with family and instance on top level.
  • Added -findexed-types switch.
  • Allowing type tag in export lists to list associated types in the sub-binder list of an import/export item for a class.

Type Checking

Todo (low-level):

  • RHS of a type instance must be a tau type.
  • Check that patterns of type indexes don't contain type functions.
  • Construct TyCon for type equation in tcIdxTyInstDecl1. This needs to be a synonym tycon, which still needs to be extended to include family information.
  • Implement the equation of conflict, which is local to FamInst.addLocalFamInst, for synonyms; ie, check that the left hand sides coincide under the substitution.
  • If an associated synonym has a default definition, use that in the instances. In contrast to methods, this cannot be overridden by a specialised definition. (Confluence requires that any specialised version is extensionally the same as the default.)
  • It should be ok to allow newtype instances for data families. (NB: the rhs of a newtype is guaranteed to be a lifted type.) Is this desirable?

Todo (high-level):

  1. Type checking of type functions (and hence, associated type synonyms); forget about iso for the moment.
  2. Type checking in the presence of associated synonym defaults. (Default AT synonyms are only allowed for ATs defined in the same class.)
  3. Type check functional dependencies as type functions.


  • Kind and type checking of kind signatures.
  • Kind and type checking of instance declarations of indexed types.
  • Wrapper generation and type checking of pattern matching for indexed data and newtypes.
  • Consistency checking for family instances.


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. Extend interface files to include equality axioms:
    • How do we exactly want to represent type equations in interface files?
      • SPJ pointed out that instances are maintained in InstEnv.InstEnv with different values for the home packages and others. Type instances may have to be maintained in a similar way, as they are also incrementally collected during compiling a program. (We probably include them in the same structure, as they will also be of type InstInfo.)
      • IfaceInst contains the instance declaration information for interfaces.
      Answer: We don't put anything extra into interface files. Instead, we derive the information corresponding toIfaceInst list in ModIface and the Instance list in ModDetails from the interface declarations and type environment, respectively. I.e., it is the type instances that carry the whole payload. Update: We may actually want to put a rough match signature in the iface seperate from the full instance declaration, so we can delay type checking the full instance declaration until we get a rough match. (This makes only sense for type instances, not for data instances, as the latter are loaded when their constructors are mentioned. Well actually, it does make sense for data instances as far as loading them for overlap checking is concerned.)
  1. Desugar type functions and equality constraints.


  • Representation of family kind signatures as TyCon.TyCons.
  • Extension of Class.Class by associated TyCons.
  • Extension of TyCon.TyCon with a reference to the parent TyCon for data instances.
  • Extension of DataCon.DataCon with instance types for constructors belonging to data instances.
  • Extension of TyCon.TyCon such that the parent of a data instance is paired with a coercion identifying family instance and representation type.
  • For indexed data types, the datacon wrapper uses data instance coercion and pattern matching casts the scrutinee via an ExprCoFn in a CoPat.
  • Import and exporting.
  • Generation and plumbing through of rough matches.



  • Compile libraries with CoreLint.
  • Convert TyFuns.hs and the various GMaps to tests in the testsuite. SPJ already created testsuite/tests/ghc-regress/indexed-types.