ApNormalize.DList

Type of applicative difference lists.

An applicative transformer which accumulates f-actions in a left-nested composition using (<*>).

ApDList represents a sequence of f-actions u1 :: f x1, ... un :: f xn as "term with a hole" (_ <*> u1 <*> ... <*> un) :: f r.

That hole must have type _ :: f (x1 -> ... -> un -> r); the variable number of arrows is hidden by existential quantification and continuation passing.

To help ensure that syntactic invariant, the Functor and Applicative instances for ApDList have no constraints. liftApDList is the only function whose signature requires an Applicative f constraint, wrapping each action u inside one (<*>).

A difference list with one element u, denoted _ <*> u.

Complete a difference list, filling the hole with the first argument.

A delayed application of fmap which can be fused with an inner fmap or liftA2.

This is the same definition as in the kan-extensions library, but we redefine it to not pay for all the dependencies.