Data.Functor.Foldable.TH

Build base functor with a sensible default configuration.

e.g.

data Expr a
    = Lit a
    | Add (Expr a) (Expr a)
    | Expr a :* [Expr a]
  deriving (Show)

makeBaseFunctor ''Expr

will create

data ExprF a x
    = LitF a
    | AddF x x
    | x :*$ [x]
  deriving (Functor, Foldable, Traversable)

type instance Base (Expr a) = ExprF a

instance Recursive (Expr a) where
    project (Lit x)   = LitF x
    project (Add x y) = AddF x y
    project (x :* y)  = x :*$ y

instance Corecursive (Expr a) where
    embed (LitF x)   = Lit x
    embed (AddF x y) = Add x y
    embed (x :*$ y)  = x :* y

Notes:

makeBaseFunctor works properly only with ADTs. Existentials and GADTs aren't supported, as we don't try to do better than GHC's DeriveFunctor.

Allowing makeBaseFunctor to take both Names and Decs as an argument is why it exists as a method in a type class. For trickier data-types, like rose-tree (see also Cofree):

data Rose f a = Rose a (f (Rose f a))

we can invoke makeBaseFunctor with an instance declaration to provide needed context for instances. (c.f. StandaloneDeriving)

makeBaseFunctor [d| instance Functor f => Recursive (Rose f a) |]

will create

data RoseF f a r = RoseF a (f fr)
  deriving (Functor, Foldable, Traversable)

type instance Base (Rose f a) = RoseF f a

instance Functor f => Recursive (Rose f a) where
  project (Rose x xs) = RoseF x xs

instance Functor f => Corecursive (Rose f a) where
  embed (RoseF x xs) = Rose x xs

Some doctests:

>>> data Expr a = Lit a | Add (Expr a) (Expr a) | Expr a :* [Expr a]; makeBaseFunctor ''Expr

>>> :t AddFAddF :: r -> r -> ExprF a r

>>> data Rose f a = Rose a (f (Rose f a)); makeBaseFunctor $ asQ [d| instance Functor f => Recursive (Rose f a) |]

>>> :t RoseFRoseF :: a -> f r -> RoseF f a r

>>> let rose = Rose 1 (Just (Rose 2 (Just (Rose 3 Nothing))))>>> cata (\(RoseF x f) -> x + maybe 0 id f) rose6

Rules of renaming data names

Default BaseRules: append F or $ to data type, constructors and field names.

How to name the base functor type.

Default is to append F or $.

How to rename the base functor type constructors.

Default is to append F or $.

How to rename the base functor type field names (in records).

Default is to append F or $.