Modulehashable-1.4.7.0Haskell2010

Data.Hashable.Lifted

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.

2 classes
3 values

Packagehashable-1.4.7.0
Exports5
LanguageHaskell2010
LicenceBSD-3-Clause
SourceLifted.hs

Type Classes

2 declarations

classclass Eq1 t => Hashable1 (t :: Type -> Type) where

Methods

liftHashWithSalt :: (Int -> a -> Int) -> Int -> t a -> Int
Lift a hashing function through the type constructor.

Instances21Hashable1, …

Hashable1 ComplexDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 IntMapDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 SeqDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 SetDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 TreeDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 NonEmptyDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 IdentityDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 MaybeDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 SoloDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 HashedDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 []Defined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable a => Hashable1 (Either a)Defined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable a1 => Hashable1 (Tuple2 a1)Defined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable k => Hashable1 (Map k)Defined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable1 ProxyDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable a => Hashable1 (Const a)Defined in hashable-1.4.7.0 · Data.Hashable.Class
(Hashable a1, Hashable a2) => Hashable1 (Tuple3 a1 a2)Defined in hashable-1.4.7.0 · Data.Hashable.Class
(Hashable a1, Hashable a2, Hashable a3) => Hashable1 (Tuple4 a1 a2 a3)Defined in hashable-1.4.7.0 · Data.Hashable.Class
(Hashable1 f, Hashable1 g) => Hashable1 (Product f g)Defined in hashable-1.4.7.0 · Data.Hashable.Class
(Hashable1 f, Hashable1 g) => Hashable1 (Sum f g)Defined in hashable-1.4.7.0 · Data.Hashable.Class
(Hashable1 f, Hashable1 g) => Hashable1 (Compose f g)Defined in hashable-1.4.7.0 · Data.Hashable.Class

classclass Eq2 t => Hashable2 (t :: Type -> Type -> Type) where

Methods

liftHashWithSalt2 :: (Int -> a -> Int) -> (Int -> b -> Int) -> Int -> t a b -> Int
Lift a hashing function through the binary type constructor.

Instances6Hashable2

Hashable2 MapDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable2 EitherDefined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable2 Tuple2Defined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable a1 => Hashable2 (Tuple3 a1)Defined in hashable-1.4.7.0 · Data.Hashable.Class
Hashable2 ConstDefined in hashable-1.4.7.0 · Data.Hashable.Class
(Hashable a1, Hashable a2) => Hashable2 (Tuple4 a1 a2)Defined in hashable-1.4.7.0 · Data.Hashable.Class

Auxiliary Functions

3 declarations

valuehashWithSalt1 :: (Hashable1 f, Hashable a) => Int -> f a -> Int

Lift the hashWithSalt function through the type constructor.

hashWithSalt1 = liftHashWithSalt hashWithSalt

valuehashWithSalt2

:: (Hashable2 f, Hashable a, Hashable b)
=> Int
-> f a b
-> Int

Lift the hashWithSalt function through the type constructor.

hashWithSalt2 = liftHashWithSalt2 hashWithSalt hashWithSalt

valuedefaultLiftHashWithSalt

:: (Hashable2 f, Hashable a)
=> Int -> b -> Int
-> Int
-> f a b
-> Int

Lift 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.

Motivation

0 declarations

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