Equality check for types that respects type families? (#9582)

[Andreas Abel]

Thanks, Simon, for the quick answer (see ticket #9582). --Andreas

On 17.09.2014 23:58, Andreas Abel wrote:
> Looking at bug #9582
>
>    Associated Type Synonyms do not unfold in InstanceSigs
>    https://ghc.haskell.org/trac/ghc/ticket/9582
>
> I found that the given type in an instance signature is compared to the
> expected type using Type.eqType, which does not seem to know about type
> families.
>
> https://github.com/ghc/ghc/blob/master/compiler/typecheck/TcInstDcls.lhs
>
>      -- Check that any type signatures have exactly the right type
>      check_inst_sig hs_ty@(L loc _)
>         = setSrcSpan loc $
>           do { sig_ty <- tcHsSigType (FunSigCtxt sel_name) hs_ty
>              ; inst_sigs <- xoptM Opt_InstanceSigs
>              ; if inst_sigs then
>                  unless (sig_ty `eqType` local_meth_ty)
>                         (badInstSigErr sel_name local_meth_ty)
>              ...
>
> https://github.com/ghc/ghc/blob/master/compiler/types/Type.lhs
>
> eqType :: Type -> Type -> Bool
> -- ^ Type equality on source types. Does not look through @newtypes@ or
> -- 'PredType's, but it does look through type synonyms.
> -- Watch out for horrible hack: See Note [Comparison with OpenTypeKind]
>
> Question:  Is there a (monadic) function for checking type equality
> which knows about the current type family rules in scope and honors them?
> Maybe the better question is where to find it, since there should be
> such a beast (used in the type checker somewhere to compare expected
> with inferred types).
>
> Cheers,
> Andreas (new to the GHC source code)
>

-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www2.tcs.ifi.lmu.de/~abel/