multirec-0.2: Generic programming with systems of recursive datatypes Source code Contents Index

Generics.MultiRec.FoldAlg Portability non-portable Stability experimental Maintainer generics@haskell.org

Contents The type family of convenient algebras. The class to turn convenient algebras into standard algebras. Interface Construction of algebras

Description

A variant of fold that allows the specification of the algebra in a convenient way.

Synopsis

type family Alg f s r ix :: * type Algebra s r = forall ix. Ix s ix => s ix -> Alg (PF s) s r ix class Fold f where alg :: Ix s ix => Alg f s r ix -> f s r ix -> r ix fold_ :: forall s ix r. (Ix s ix, HFunctor (PF s), Fold (PF s)) => s ix -> Algebra s r -> ix -> r ix fold :: forall s ix r. (Ix s ix, HFunctor (PF s), Fold (PF s)) => Algebra s r -> ix -> r ix (&) :: a -> b -> (a, b)

The type family of convenient algebras.

type family Alg f s r ix :: * Source

The type family we use to describe the convenient algebras.

type Algebra s r = forall ix. Ix s ix => s ix -> Alg (PF s) s r ix Source

The algebras passed to the fold have to work for all index types in the system. The additional witness argument is required only to make GHC's typechecker happy.

The class to turn convenient algebras into standard algebras.

class Fold f where Source

The class fold explains how to convert a convenient algebra Alg back into a function from functor to result, as required by the standard fold function. Methods alg :: Ix s ix => Alg f s r ix -> f s r ix -> r ix Source Instances Fold U Fold (K a) Fold (I xi) Fold f => Fold (C c f) Fold f => Fold (f :>: xi) Fold g => Fold (K a :*: g) Fold g => Fold (I xi :*: g) (Fold f, Fold g) => Fold (f :+: g)

Interface

fold_ :: forall s ix r. (Ix s ix, HFunctor (PF s), Fold (PF s)) => s ix -> Algebra s r -> ix -> r ix Source

Variant of fold that takes an additional witness argument.

fold :: forall s ix r. (Ix s ix, HFunctor (PF s), Fold (PF s)) => Algebra s r -> ix -> r ix Source

Fold with convenient algebras.

Construction of algebras

(&) :: a -> b -> (a, b) Source

For constructing algebras that are made of nested pairs rather than n-ary tuples, it is helpful to use this pairing combinator.

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