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.

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 p

The 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 = hashWithSalt1