#5609: Type checking arrow notation in the presence of deferred constraints
---------------------------------+------------------------------------------
Reporter: dreixel | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
In `TcArrows`, the following code
{{{
tc_cmd w_tv (cmd, i, b, tup_ty, s)
= do { tup_ty' <- zonkTcType tup_ty
; let (corner_ty, arg_tys) = unscramble tup_ty'
-- Check that it has the right shape:
-- ((w,s1) .. sn)
-- where the si do not mention w
; checkTc (corner_ty `eqType` mkTyVarTy w_tv &&
not (w_tv `elemVarSet` tyVarsOfTypes arg_tys))
(badFormFun i tup_ty')
; tcCmdTop (env { cmd_arr = b }) cmd arg_tys s }
}}}
is wrong. In particular, the check `elemVarSet w_tv (tyVarsOfTypes
arg_tys)` does not make sense in the presence of deferred constraints.
There is no testcase for this at the moment, but in the ghc-kinds branch
both T5283 and arrowform1 fail because of this. The temporary fix was:
{{{
; _bogus <- unifyType corner_ty (mkTyVarTy w_tv)
; checkTc (not (w_tv `elemVarSet` tyVarsOfTypes arg_tys))
(badFormFun i tup_ty')
}}}
This solves only T5283 and arrowform1; other problems might arise in the
future, so this should be fixed properly.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5609>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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