Lifting of the Hashable class to unary and binary type constructors.
These classes are needed to express the constraints on arguments of
types that are parameterized by type constructors. Fixed-point data
types and monad transformers are such types.
class (Eq1 t, forall a. Hashable a => Hashable (t a)) => Hashable1 tliftHashWithSalt :: (Int -> a -> Int) -> Int -> t a -> IntLift a hashing function through the type constructor.
class (Eq2 t, forall a. Hashable a => Hashable1 (t a)) => Hashable2 tliftHashWithSalt2 :: (Int -> a -> Int) -> (Int -> b -> Int) -> Int -> t a b -> IntLift a hashing function through the binary type constructor.
hashWithSalt1 :: (Hashable1 f, Hashable a) => Int -> f a -> IntLift the hashWithSalt function through the type constructor.
hashWithSalt1 = liftHashWithSalt hashWithSalthashWithSalt2 :: (Hashable2 f, Hashable a, Hashable b) => Int -> f a b -> IntLift the hashWithSalt function through the type constructor.
hashWithSalt2 = liftHashWithSalt2 hashWithSalt hashWithSaltdefaultLiftHashWithSalt :: (Hashable2 f, Hashable a) => (Int -> b -> Int) -> Int -> f a b -> IntLift the hashWithSalt function halfway through the type constructor.
This function makes a suitable default implementation of liftHashWithSalt,
given that the type constructor t in question can unify with f a.
This type classes provided in this module are used to express constraints
on type constructors in a Haskell98-compatible fashion. As an example, consider
the following two types (Note that these instances are not actually provided
because hashable does not have transformers or free as a dependency):
newtype WriterT w m a = WriterT { runWriterT :: m (a, w) }
data Free f a = Pure a | Free (f (Free f a))The Hashable1 instances for WriterT and Free could be written as:
instance (Hashable w, Hashable1 m) => Hashable1 (WriterT w m) where
liftHashWithSalt h s (WriterT m) =
liftHashWithSalt (liftHashWithSalt2 h hashWithSalt) s m
instance Hashable1 f => Hashable1 (Free f) where
liftHashWithSalt h = go where
go s x = case x of
Pure a -> h s a
Free p -> liftHashWithSalt go s pThe Hashable instances for these types can be trivially recovered with
hashWithSalt1:
instance (Hashable w, Hashable1 m, Hashable a) => Hashable (WriterT w m a) where
hashWithSalt = hashWithSalt1
instance (Hashable1 f, Hashable a) => Hashable (Free f a) where
hashWithSalt = hashWithSalt1Community contribution. The service has not verified this documentation. The generator reported 6 diagnostics.
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