Here is a slightly more usable version, using helper functions to
avoid writing intermediate closures by hand.
type 'a base = {x : 'a; fn : 'a -> unit}
(* 4 next lines are boilerplate, could be auto-generated *)
type 'b base_op = {bop: 'a. 'a base -> 'b}
type base_wrapper = {base: 'b. 'b base_op -> 'b}
let wrap a = {base = fun x -> x.bop a}
let apply op w = w.base op
let l =
[wrap {x = 1; fn = print_int}; wrap {x = 1.2; fn = print_float}];;
List.iter (apply {bop = fun r -> r.fn r.x}) l
The only thing you cannot abbreviate is the {bop = ...} part, as this
is where universality is checked, to ensure that no cross application
can be done.
Jacques Garrigue
> Then what you are asking for is existential types. There is no syntax
> for them in ocaml, but they can be encoded through universal types
> (interestingly, the dual is not true).
>
> type 'a base = {x : 'a; fn : 'a -> unit}
> type 'b base_op = {bop: 'a. 'a base -> 'b}
> type base_wrapper = {base: 'b. 'b base_op -> 'b}
>
> let l =
> let a = {x = 1; fn = print_int}
> and b = {x = 1.2; fn = print_float} in
> [{base = fun x -> x.bop a}; {base = fun x -> x.bop b}]
>
> List.iter (fun w -> w.base {bop = fun r -> r.fn r.x}) l
>
> As you can see, the result is rather verbose, but this works.
> Fortunately, closure and objects are usually enough...
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