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.a...@gu.se
http://www2.tcs.ifi.lmu.de/~abel/
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