The iter
argument to Container.Make
says 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.