= return () -- Short cut for case when there are no existentials
tcCheckExistentialPat pats ex_tvs pat_tys body_ty
- = addErrCtxtM (sigPatCtxt (collectPatsBinders pats) ex_tvs pat_tys body_ty) $
+ = addErrCtxtM (sigPatCtxt pats ex_tvs pat_tys body_ty) $
checkSigTyVarsWrt (tcTyVarsOfTypes (body_ty:pat_tys)) ex_tvs
data PatState = PS {
--
-- Nor should a lazy pattern bind any existential type variables
-- because they won't be in scope when we do the desugaring
+--
+-- Note [Hopping the LIE in lazy patterns]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- In a lazy pattern, we must *not* discharge constraints from the RHS
+-- from dictionaries bound in the pattern. E.g.
+-- f ~(C x) = 3
+-- We can't discharge the Num constraint from dictionaries bound by
+-- the pattern C!
+--
+-- So we have to make the constraints from thing_inside "hop around"
+-- the pattern. Hence the getLLE and extendLIEs later.
+
tc_pat pstate lpat@(LazyPat pat) pat_ty thing_inside
- = do { (pat', pat_tvs, res) <- tc_lpat pat pat_ty pstate $ \ _ ->
- thing_inside pstate
- -- Ignore refined pstate',
- -- revert to pstate
+ = do { (pat', pat_tvs, (res,lie))
+ <- tc_lpat pat pat_ty pstate $ \ _ ->
+ getLIE (thing_inside pstate)
+ -- Ignore refined pstate', revert to pstate
+ ; extendLIEs lie
+ -- getLIE/extendLIEs: see Note [Hopping the LIE in lazy patterns]
+
-- Check no existentials
; if (null pat_tvs) then return ()
else lazyPatErr lpat pat_tvs
text "In the binding group for"])
4 (ppr pat)
-sigPatCtxt bound_ids bound_tvs pat_tys body_ty tidy_env
+sigPatCtxt pats bound_tvs pat_tys body_ty tidy_env
= do { pat_tys' <- mapM zonkTcType pat_tys
; body_ty' <- zonkTcType body_ty
; let (env1, tidy_tys) = tidyOpenTypes tidy_env (map idType show_ids)
sep [ptext SLIT("When checking an existential match that binds"),
nest 4 (vcat (zipWith ppr_id show_ids tidy_tys)),
ptext SLIT("The pattern(s) have type(s):") <+> vcat (map ppr tidy_pat_tys),
- ptext SLIT("The body has type:") <+> ppr tidy_body_ty
+ ptext SLIT("The body has type:") <+> ppr tidy_body_ty,
+ ppr pats
]) }
where
+ bound_ids = collectPatsBinders pats
show_ids = filter is_interesting bound_ids
is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs