Version 193 (modified by chak, 10 years ago) (diff)



Type Functions: Implementation Status

Debugging of type family patch:

  1. #1651; fixed the panic, remains the bogus missing instance error
  2. Check that the restrictions on equality constraints in instance and class contexts are enforced. We should have tests for that in the testsuite. Document the exact restrictions on the Haskell wiki tutorial page. [Tom is working at this.]
  3. boxySplitTyConApp and friends must be able to deal with orig_tys that have outermost type family applications; i.e., they need to try to normalise and possibly have to defer. They also need to defer on skolems. Consequently, they also need to return a coercion. This , in particular, affects the treatment of literal lists, parallel arrays, and tuples inTcExpr.tcExpr is fishy.
  4. Can't we now allow non-left-linear declarations; e.g., instance type F a a = ..?
  5. Fix export list problem (ie, export of data constructors introduced by orphan data instances):
    • Change HscTypes.IfaceExport to use Name instead of OccName.
    • Then, there is also no need for the grouping of the identifiers by module anymore (but sort it to avoid spurious iface changes dur to re-ordering when re-compiling).
    • We still need to have the name parent map, though.
    • See email for example.
  6. Allow data family GADT instances.
  7. Fix core-lint breakage in cholewo-eval.
  8. The tests tcfail068 and rw used to raise more type errors right away. Now, we see less recovery.
  9. To move GADT type checking from refinements to using equalities, proceed as follows (as suggested by SPJ):
    • In TcPat.tcConPat:
      • set eq_spec' = [], to get an empty refinement
      • add the equalities from eq_spec to theta' (to propagate them instead of the refinement)
    • Test whether this works (it basically disables the refinement mechanism without deleting it)
    • In TcUnify, make all occurs checks more elaborate. They should only defer if the checked variable occurs as part of an argument to a type family application; in other cases, still fail right away.
    • TcGadt.tcUnifyTys can now probably be replaced again by the non-side-effecting unifier that was in types/Unify.hs (recover from previous repo states).
  10. Emit a warning if there are two identical type instances.


  • Add some trac wiki documentation of how inference with type families works.

Parsing and Renaming

Todo (low-level): None.

Todo (high-level):

  1. 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 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.
  • Import/export lists: ATs can be listed as subnames of classes and the data constructors of instances of a data family are subnames of that family.
  • Parsing and renaming of equational constraints in contexts.

Type Checking

Todo (low-level):

  • Allow data family GADT instances.
  • Deriving Typeable for data families.
  • 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.)

Todo (high-level):

  1. Type checking of type families; routines in TcUnify that still need to be extended:
    • boxySplitTyConApp: The second argument (BoxyRhoType) can be a synonym family application. Then, we must produce a wanted coercion and return a HsWrapper value that applies that coercion.
    • boxySplitAppTy: Basically, the same deal as the previous.
  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, including the generation of representation tycons.
  • Wrapper generation and type checking of pattern matching for indexed data and newtypes.
  • Consistency checking for family instances.
  • Enforce syntactic constraints on type instances needed to ensure the termination of constraint entailment checking.


Todo (low-level): None.

Todo (high-level): None.


  • 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.
  • Equational constraints in contexts.

*OLD* Regression tests of type family patches

[This is outdated pre-merge info just kept around for a while.]

Current validate result:

Unexpected passes:

Unexpected failures:
  • Class1: VALID. (Only marked to fail in head to keep validate happy.)
  • GADT2: VALID. (Only marked to fail in head to keep validate happy.)
  • Relf2: VALID. Type family BUG.
  • Simple5a: VALID. Error message changed.
  • break001: INVALID. GHC panic instead of printing error message about ambiguous variable.
  • break006: INVALID. Seems to be the same problem as break001.
  • print019: INVALID. Seems to be the same problem as break001.
  • rw: VALID. Changed error message. Reports one error less. Other one is pruned, but still raised if the first one is removed.
  • tc210: INVALID (matching forall a.a -> Int against `Int -> Int fails).
  • tc211: INVALID (tests impredicative types).
  • tcfail046: VALID. Changed error message, BUT the new error message has one more type synonym unfolding, which should be avoided.
  • tcfail065: VALID. Cosmetic difference, as tidy names are assigned in different order.
  • tcfail068: VALID. Reports two errors less (probably due to different recovery points). Yes, I checked that its not that it admits incorrect programs, but just due to different order of checking/pruning of errors.
  • tcfail071: VALID. Now reports one instead of two errors as deferred unification is checked only after the contexts of mutually recursive groups have been unified. (The latter is what this test case is really about, and it still works fine for that.)
  • tcfail076: VALID. Same as tcfail065.
  • tcfail102: VALID.
  • tcfail103: VALID. Error message is actually better!
  • tcfail128: VALID. Same as tcfail046.
  • tcfail145: VALID. Error message got worse.
  • tcfail153: VALID. Error message is different, but equally correct and accurate. The type mismatch manifests itself at two different subexpressions. Due to a different traversal order, we now report the error at the other subexpression.
  • tcfail179: VALID. If anything, the error message improved.
  • while: VALID. Works if definition of succeed gets a type signature Monad m => a -> m a. The error seems to be due to the new GADT rules about annotations, but the error message is a bit strange; ie, need to be improved.

Regression tests that are not part of the fast run:

  • cholewo-eval: core-lint breakage