diff options
Diffstat (limited to 'testsuite/tests/polykinds/T9569.hs')
| -rw-r--r-- | testsuite/tests/polykinds/T9569.hs | 30 |
1 files changed, 27 insertions, 3 deletions
diff --git a/testsuite/tests/polykinds/T9569.hs b/testsuite/tests/polykinds/T9569.hs index 634d742803..112037461a 100644 --- a/testsuite/tests/polykinds/T9569.hs +++ b/testsuite/tests/polykinds/T9569.hs @@ -9,18 +9,42 @@ data Proxy (c :: Constraint) class Deferrable (c :: Constraint) where defer :: Proxy c -> (c => a) -> a -deferPair :: (Deferrable c1, Deferrable c2) => - Proxy (c1,c2) -> ((c1,c2) => a) -> a +deferPair :: (Deferrable c1, Deferrable c2) + => Proxy (c1,c2) -> (((c1,c2) :: Constraint) => a) -> a + -- NB: ((c1,c2) :: Constraint) => blah + -- is different form + -- (c1,c2) => blah + -- The former has dict, the latter has two deferPair _ _ = undefined instance (Deferrable c1, Deferrable c2) => Deferrable (c1,c2) where -- defer p f = deferPair p f -- Succeeds defer = deferPair -- Fails +{- Notes Apr 2020. +~~~~~~~~~~~~~~~~~ +Note the careful type for deferPair! You can also say + +deferPair :: (Deferrable c1, Deferrable c2, d ~ (c1,c2)) + => Proxy (c1,c2) -> (d => a) -> a + +but NOT + +deferPair :: (Deferrable c1, Deferrable c2) + => Proxy (c1,c2) -> ((c1,c2) => a) -> a + +The point is that + (c1,c2) => a +is short for + c1 => c2 => a +-} + {- [G] Deferrable c1, Deferrable c2 - [W] Proxy (c1,c2) -> ((c1,c2) => a) -> a ~ Proxy (c1x,c2x) -> ((c1x,c2x) => ax) -> ax + [W] Proxy (c1,c2) -> ((c1,c2) => a) -> a + ~ + Proxy (c1x,c2x) -> ((c1x,c2x) => ax) -> ax [w] Deferrable c1x [w] Deferrable c2x -} |