Module type Applicative.Basic

type 'a t
val return : 'a -> 'a t
val apply : ('a -> 'b) t -> 'a t -> 'b t
val map : [ `Define_using_apply | `Custom of 'a t -> f:('a -> 'b) -> 'b t ]

The map argument to Applicative.Make says how to implement the applicative's map function. `Define_using_apply means to define map t ~f = return f <*> t. `Custom overrides the default implementation, presumably with something more efficient.

Some other functions returned by Applicative.Make are defined in terms of map, so passing in a more efficient map will improve their efficiency as well.