Parameter Make_gen.T

type ('a, 'phantom) t
type 'a elt
val fold : ('a, 'phantom) t -> init:'acc -> f:('acc -> 'a elt -> 'acc) -> 'acc
val iter : [ `Define_using_fold | `Custom of ('a, 'phantom) t -> f:('a elt -> unit) -> unit ]

The iter argument to Container.Make specifies how to implement the container's iter function. `Define_using_fold means to define iter via:

iter t ~f = Container.iter ~fold t ~f

`Custom overrides the default implementation, presumably with something more efficient. Several other functions returned by Container.Make are defined in terms of iter, so passing in a more efficient iter will improve their efficiency as well.

val length : [ `Define_using_fold | `Custom of ('a, 'phantom) t -> int ]

The length argument to Container.Make specifies how to implement the container's length function. `Define_using_fold means to define length via:

length t ~f = Container.length ~fold t ~f

`Custom overrides the default implementation, presumably with something more efficient. Several other functions returned by Container.Make are defined in terms of length, so passing in a more efficient length will improve their efficiency as well.