Commit 1d051fbc authored by's avatar
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FIX #1688: Givens in checkLoop are not that rigid after all

- This patch re-instates the policy of 6.6.1 to zonk the given constraints
  in the simplifier loop.

parent d831632b
......@@ -1085,10 +1085,11 @@ checkLoop :: RedEnv
checkLoop env wanteds
= go env wanteds []
where go env wanteds needed_givens
= do { -- Givens are skolems, so no need to zonk them
wanteds' <- zonkInsts wanteds
= do { -- We do need to zonk the givens; cf Note [Zonking RedEnv]
; env' <- zonkRedEnv env
; wanteds' <- zonkInsts wanteds
; (improved, binds, irreds, more_needed_givens) <- reduceContext env wanteds'
; (improved, binds, irreds, more_needed_givens) <- reduceContext env' wanteds'
; let all_needed_givens = needed_givens ++ more_needed_givens
......@@ -1101,10 +1102,44 @@ checkLoop env wanteds
-- Using an instance decl might have introduced a fresh type variable
-- which might have been unified, so we'd get an infinite loop
-- if we started again with wanteds! See Note [LOOP]
{ (irreds1, binds1, all_needed_givens1) <- go env irreds all_needed_givens
{ (irreds1, binds1, all_needed_givens1) <- go env' irreds all_needed_givens
; return (irreds1, binds `unionBags` binds1, all_needed_givens1) } }
Note [Zonking RedEnv]
It might appear as if the givens in RedEnv are always rigid, but that is not
necessarily the case for programs involving higher-rank types that have class
contexts constraining the higher-rank variables. An example from tc237 in the
testsuite is
class Modular s a | s -> a
wim :: forall a w. Integral a
=> a -> (forall s. Modular s a => M s w) -> w
wim i k = error "urk"
test5 :: (Modular s a, Integral a) => M s a
test5 = error "urk"
test4 = wim 4 test4'
Notice how the variable 'a' of (Modular s a) in the rank-2 type of wim is
quantified further outside. When type checking test4, we have to check
whether the signature of test5 is an instance of
(forall s. Modular s a => M s w)
Consequently, we will get (Modular s t_a), where t_a is a TauTv into the
Given the FD of Modular in this example, class improvement will instantiate
t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
the givens, we will get into a loop as improveOne uses the unification engine
TcGadt.tcUnifyTys, which doesn't know about mutable type variables.
Note [LOOP]
class If b t e r | b t e -> r
......@@ -1655,6 +1690,12 @@ data WantSCs = NoSCs | AddSCs -- Tells whether we should add the superclasses
-- of a predicate when adding it to the avails
-- The reason for this flag is entirely the super-class loop problem
zonkRedEnv :: RedEnv -> TcM RedEnv
zonkRedEnv env
= do { givens' <- mappM zonkInst (red_givens env)
; return $ env {red_givens = givens'}
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