Data.Generics.Genifunctors

Generate (derive) generalized fmap, foldMap and traverse for Bifunctors, Trifunctors, or a functor with any arity

Example:

data U a b c d
    = L [U a b c d]               -- polymorphic recursion
    | M (V (a,b) (Either c d))    -- mutually recursive
    | a :+: Int                   -- infix syntax, record syntax, type synonyms
    | R { c :: c, d :: String }   -- and primitive data types supported

data V u v = X (U v v u u) | Z u

fmapU :: (a -> a') -> (b -> b') -> (c -> c') -> (d -> d') -> U a b c d -> U a' b' c' d'
fmapU = $(genFmap ''U)

foldU :: Monoid m => (a -> m) -> (b -> m) -> (c -> m) -> (d -> m) -> U a b c d -> m
foldU = $(genFoldMap ''U)

travU :: Applicative f => (a -> f a') -> (b -> f b') -> (c -> f c') -> (d -> f d') -> U a b c d -> f (U a' b' c' d')
travU = $(genTraverse ''U)
 

genFoldMapT and genTraverseT allow for specifying custom functions to handle subparts of a specific type. The compiler will throw an error if any of the types is actually a type synonym.

Documentation

genFmap :: Name -> Q Exp

Generate generalized fmap for a type

bimapTuple :: (a -> a') -> (b -> b') -> (a,b) -> (a',b')
bimapTuple = $(genFmap ''(,))
 

genFoldMap :: Name -> Q Exp

Generate generalized foldMap for a type

foldMapEither :: Monoid m => (a -> m) -> (b -> m) -> Either a b -> m
foldMapEither = $(genFoldMap ''Either)
 

genFoldMapT :: [(Name, Name)] -> Name -> Q Exp

Generate generalized foldMap for a type, optionally traversing subparts of it with custom implementations.

foldTupleRev :: Monoid m => (a -> m) -> (b -> m) -> (a,b) -> m
foldTupleRev f g (a,b) = g b <> f a

foldUCustom :: Monoid m => (a -> m) -> (b -> m) -> (c -> m) -> (d -> m) -> U a b c d -> m
foldUCustom = $(genFoldMapT [(''(,), 'foldTupleRev)] ''U)
 

genTraverse :: Name -> Q Exp

Generate generalized traversable for a type

travTriple :: Applicative f => (a -> f a') -> (b -> f b') -> (c -> f c') -> (a,b,c) -> f (a',b',c')
travTriple = $(genTraverse ''(,,))
 

genTraverseT :: [(Name, Name)] -> Name -> Q Exp

Generate generalized traversable for a type, optionally traversing subparts of it with custom implementations.

travTupleRev :: Applicative f => (a -> f a') -> (b -> f b') -> (a,b) -> f (a',b')
travTupleRev f g (a,b) = (\b a -> (a,b)) <$> g b <*> f a

travUCustom :: Applicative f => (a -> f a') -> (b -> f b') -> (c -> f c') -> (d -> f d') -> U a b c d -> f (U a' b' c' d')
travUCustom = $(genTraverseT [(''(,), 'travTupleRev), (''V, 'travVCustom)] ''U)

travVCustom :: Applicative f => (a -> f a') -> (b -> f b') -> V a b -> f (V a' b')
travVCustom = $(genTraverseT [(''U, 'travUCustom)] ''V)