DISCLAIMER: NEVER USE THIS EXCEPT FOR FUN.
Require constraints Optionally. See this blog
post for implementation details and limitations.
NOTE Works only with optimisations enabled so don't forget to pass -fobject-code -O if you
want to test it in ghci.
Example:
class Foo
instance Foo
class Bar
class Slow a where
slow :: a -> String
class Fast a where
fast :: a -> String
instance Slow () where
slow _ = "slow @()"
instance Slow Bool where
slow _ = "slow @Bool"
instance Fast Bool where
fast _ = "fast @Bool"
class Baz (a :: k)
instance Baz 1
instance Baz 'True
tryShow :: forall a . Show? a => a -> Maybe String
tryShow a = tryC @(Show a) $ show a
fastOrSlow :: forall a . (Slow a, Fast? a) => a -> String
fastOrSlow = orC @(Fast a) fast slow
nubSmart :: forall a . (Eq a, Ord? a) => [a] -> [a]
nubSmart a = orC @(Ord a) (trace "nubOrd" $ nubOrd a) (trace "nubEq" $ nub a)
nubForceEq :: forall a . Eq a => [a] -> [a]
nubForceEq = discard @(Ord a) nubSmart
nubForceOrd :: forall a . Ord a => [a] -> [a]
nubForceOrd = give @(Ord a) nubSmart
newtype NoOrd a = NoOrd a
deriving stock (Show, Eq)isSatisfied @Foo: True
isSatisfied @Bar: False
isSatisfied @(Baz 1): True
isSatisfied @(Baz 2): False
isSatisfied @(Baz 'True): True
isSatisfied @(Baz 'False): False
isSatisfied @(Show Int): True
isSatisfied @(Show [Int]): True
isSatisfied @(Show [Int -> Int]): False
isSatisfied @(Monad IO): True
isSatisfied @(MonadReader Int IO): False
isSatisfied @(MonadReader Int (ReaderT Int IO)): True
tryShow True: Just "True"
tryShow $ id @Bool: Nothing
tryShow (True, ()): Just "(True,())"
tryShow (True, id @Bool): Nothing
fastOrSlow (): "slow @()"
fastOrSlow True: "fast @Bool"
-- traces are printed too early but you got the idea
nubOrd
nubSmart [1, 2 :: Int]: [1,2]
nubEq
nubSmart [NoOrd 1, NoOrd 2 :: NoOrd Int]: [NoOrd 1,NoOrd 2]
nubOrd
nubForceOrd [1, 2 :: Int]: [1,2]
nubEq
nubForceEq [NoOrd 1, NoOrd 2 :: NoOrd Int]: [NoOrd 1,NoOrd 2]