where
extended_defaulting = interactive || dopt Opt_ExtendedDefaultRules dflags
+ -- See also Trac #1974
ovl_strings = dopt Opt_OverloadedStrings dflags
unaries :: [(Inst, Class, TcTyVar)] -- (C tv) constraints
-> TcM () -- Just does unification, to fix the default types
disambigGroup default_tys dicts
- = try_default default_tys
+ = do { mb_chosen_ty <- try_default default_tys
+ ; case mb_chosen_ty of
+ Nothing -> return ()
+ Just chosen_ty -> do { unifyType chosen_ty (mkTyVarTy tyvar)
+ ; warnDefault dicts chosen_ty } }
where
(_,_,tyvar) = ASSERT(not (null dicts)) head dicts -- Should be non-empty
classes = [c | (_,c,_) <- dicts]
- try_default [] = return ()
+ try_default [] = return Nothing
try_default (default_ty : default_tys)
= tryTcLIE_ (try_default default_tys) $
do { tcSimplifyDefault [mkClassPred clas [default_ty] | clas <- classes]
-- For example, if Real a is reqd, but the only type in the
-- default list is Int.
- -- After this we can't fail
- ; warnDefault dicts default_ty
- ; unifyType default_ty (mkTyVarTy tyvar)
- ; return () -- TOMDO: do something with the coercion
+ ; return (Just default_ty) -- TOMDO: do something with the coercion
}