multirec-0.7.7: Generic programming for families of recursive datatypes

Copyright(c) 2009--2010 Universiteit Utrecht
LicenseBSD3
Maintainergenerics@haskell.org
Stabilityexperimental
Portabilitynon-portable
Safe HaskellSafe
LanguageHaskell2010

Generics.MultiRec.FoldAlgK

Contents

Description

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

Synopsis

The type family of convenient algebras.

type family Alg f r :: * Source

The type family we use to describe the convenient algebras.

Instances

type Alg U r = r Source

For a unit, no arguments are available.

type Alg (K a) r = a -> r Source

For a constant, we take the constant value to a result.

type Alg (I xi) r = r -> r Source

For an identity, we turn the recursive result into a final result. Note that the index can change.

type Alg (C c f) r = Alg f r Source

Constructors are ignored.

type Alg ((:>:) f xi) r = Alg f r Source

Tags are ignored.

type Alg ((:*:) (K a) g) r = a -> Alg g r Source

For a product where the left hand side is a constant, we take the value as an additional argument.

type Alg ((:*:) (I xi) g) r = r -> Alg g r Source

For a product where the left hand side is an identity, we take the recursive result as an additional argument.

type Alg ((:+:) f g) r = (Alg f r, Alg g r) Source

For a sum, the algebra is a pair of two algebras.

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

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

Instances

Fold U Source 
Fold (K a) Source 
Fold (I xi) Source 
Fold f => Fold (C c f) Source 
Fold f => Fold ((:>:) f xi) Source 
Fold g => Fold ((:*:) (K a) g) Source 
Fold g => Fold ((:*:) (I xi) g) Source 
(Fold f, Fold g) => Fold ((:+:) f g) Source 

Interface

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

Fold with convenient algebras.

Construction of algebras

(&) :: a -> b -> (a, b) infixr 5 Source

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