--
-- 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
; ex_tvs' <- tcInstSkolTyVars skol_info ex_tvs -- Get location from monad,
-- not from ex_tvs
; let tenv = zipTopTvSubst (univ_tvs ++ ex_tvs)
- (ctxt_res_tys ++ mkTyVarTys ex_tvs')
+ (ctxt_res_tys ++ mkTyVarTys ex_tvs')
eq_spec' = substEqSpec tenv eq_spec
theta' = substTheta tenv theta
arg_tys' = substTys tenv arg_tys
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,
- ppr pats
+ ptext SLIT("The body has type:") <+> ppr tidy_body_ty
]) }
where
bound_ids = collectPatsBinders pats
show_ids = filter is_interesting bound_ids
- is_interesting id = any (`elemVarSet` idFreeTyVars id) bound_tvs
+ is_interesting id = any (`elemVarSet` varTypeTyVars id) bound_tvs
ppr_id id ty = ppr id <+> dcolon <+> ppr ty
-- Don't zonk the types so we get the separate, un-unified versions