multirec-0.4.1: Generic programming for families of recursive datatypes





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 :: *Source

The type family we use to describe the convenient algebras.

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

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 whereSource

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.


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


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) 


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

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.