Documentation

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 Name s and Dec s 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 AddF
AddF :: 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 RoseF
RoseF :: 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) rose
6

makeBaseFunctor = makeBaseFunctorWith baseRules

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

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 $ .