ocaml - Why is this weakly polymorphic -

ocaml - Why is this weakly polymorphic -

i starting larn ocaml , trying practices:

# allow left x y = x;; val left : 'a -> 'b -> 'a = <fun> # allow first = list.fold_right left;; val first : '_a list -> '_a -> '_a = <fun>

why first weakly polymorphic instead of polymorphic?

this value restriction. first not value, it's function application.

to polymorphic version, utilize eta expansion:

# allow left x y = x;; val left : 'a -> 'b -> 'a = <fun> # allow first b = list.fold_right left b;; val first : 'a list -> 'a -> 'a = <fun>

as @ivg points out, commonly asked ocaml question.

update

here's function application that's unsafe generalize:

# allow f x = ref x;; val f : 'a -> 'a ref = <fun> # f [];; - : '_a list ref = {contents = []}

if pretend result has type 'a list ref can create code go wrong (i tried it).

here's partial application that's unsafe generalize:

# allow g x = allow z = ref x in fun () -> z;; val g : 'a -> unit -> 'a ref = <fun> # g [];; - : unit -> '_a list ref = <fun>

if pretend result has type unit -> 'a list ref can create code go wrong (i tried it).

ocaml