Generics.MultiRec.FoldAlgK

A variant of fold that allows the specification of the algebra in a convenient way, and that fixes the result type to a constant.

The type family of convenient algebras.

type family Alg f r :: *

The type family we use to describe the convenient algebras.

type Algebra phi r = forall ix. phi ix -> Alg (PF phi) r

The algebras passed to the fold have to work for all index types in the family. 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

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 :: Alg f r -> f (K0 r) ix -> r

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 phi ix r. (Fam phi, HFunctor phi (PF phi), Fold (PF phi)) => Algebra phi r -> phi ix -> ix -> r

Fold with convenient algebras.

Construction of algebras

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

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