# \[Solved\] Js\_array map typing issue - cannot map to a new type

**URL:** <https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392>\
**Category:** Learning\
**Tags:** js\_of\_ocaml, solved, ocsigen\
**Created:** [March 28, 2020, 10:12am UTC](https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392 "2020-03-28T10:12:46Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![orbitz](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/orbitz/32/110_2.png) [@orbitz](https://discuss.ocaml.org/u/orbitz)\
**Post date:** [March 28, 2020, 10:12am UTC](https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392/1 "2020-03-28T10:12:46Z")

</div>

The following has been tried on js\_of\_ocaml 3.5.1 and 3.5.2.

I’m trying to transform a js\_array from an `'a` to a `'b` however I’m getting errors which suggest that map can only transform an `'a` to an `'a`. The interface states the type is:

`method map : ('a -> int -> 'a js_array t -> 'b) callback -> 'b js_array t meth`

The important part being:

`('a -> int -> 'a js_array t -> 'b) callback`

But the top level gives me:

```auto
#show Js_of_ocaml.Js.js_array;;
    method map :
      ('a -> int -> 'a Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> 'a)
      Js_of_ocaml.Js.callback ->
      'a Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t Js_of_ocaml.Js.meth

```

The important part being:

`('a -> int -> 'a Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> 'a) Js_of_ocaml.Js.callback`

And just to try it:

```auto
open Js_of_ocaml;;
let arr = Js.array [| 1; 2; 3 |];;
val arr : int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t = <abstr>
arr##map (Js.wrap_callback (fun v _ _ -> float v));;
Line 1, characters 0-8:
Error: This expression has type
         (int -> int -> int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> int)
         Js_of_ocaml.Js.callback ->
         int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t Js_of_ocaml.Js.meth
       but an expression was expected of type
         (unit,
          int -> int -> int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> float)
         Js_of_ocaml.Js.meth_callback -> 'res Js_of_ocaml.Js.meth
       Type
         (int -> int -> int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> int)
         Js_of_ocaml.Js.callback =
           (unit,
            int -> int -> int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> int)
           Js_of_ocaml.Js.meth_callback
       is not compatible with type
         (unit,
          int -> int -> int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t -> float)
         Js_of_ocaml.Js.meth_callback 
       Type int is not compatible with type float 

```

What’s going on here? Is there a way to use `map`?

---

<div class="post-metadata">

**Author:** ![xvw](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/xvw/32/2041_2.png) [@xvw](https://discuss.ocaml.org/u/xvw)\
**Post date:** [March 28, 2020, 3:48pm UTC](https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392/2 "2020-03-28T15:48:30Z")

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Hi, I don’t think that the problem is about your usage of `map` but about your usage of `float`.  
`val float : float -> float` is a deprecated function and it should take a float and return a float. So I suggest `arr##map (Js.wrap_callback (fun v _ _ -> float_of_int v))`.

edit : There’s also `Js.array_map` where the callback is already wrapped : `Js.array_map float_of_int arr`

---

<div class="post-metadata">

**Author:** ![orbitz](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/orbitz/32/110_2.png) [@orbitz](https://discuss.ocaml.org/u/orbitz)\
**Post date:** [March 28, 2020, 5:23pm UTC](https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392/3 "2020-03-28T17:23:44Z")

</div>

`float_of_int` doesn’t work either and the type of float is `int -> float` in this toplevel, so I do not believe that is the issue.

But `Js.array_map` does work, thank you:

```auto
Js.array_map float arr;;
- : float #Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t = <abstr>

```

Looking at the code for `Js.array_map`, looks like it has to do some magic to get around some type restrictions:

```auto
let array_map_poly :
    'a #js_array t -> ('a -> int -> 'a #js_array t -> 'b) callback -> 'b #js_array t =
 fun a cb -> (Unsafe.coerce a)##map cb

let array_map f a = array_map_poly a (wrap_callback (fun x _idx _ -> f x))

let array_mapi f a = array_map_poly a (wrap_callback (fun x idx _ -> f idx x))

```

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<div class="post-metadata">

**Author:** ![xvw](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/xvw/32/2041_2.png) [@xvw](https://discuss.ocaml.org/u/xvw)\
**Post date:** [March 29, 2020, 1:15pm UTC](https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392/4 "2020-03-29T13:15:19Z")

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I maybe miss something but in your example :

```ocaml
let arr = Js.array [| 1; 2; 3 |];;
(* val arr : int Js_of_ocaml.Js.js_array Js_of_ocaml.Js.t = <abstr> *)

```

So your array is `int js_array Js.t`, and the type of `float` is, according the documentation `val float : float -> float`. So I suppose (but i’m maybe wrong) that you want having a `float js_array Js.t` from an `int Js.js_array Js.t` and `float_of_int` goes from `int` to `float`.

---

<div class="post-metadata">

**Author:** ![orbitz](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/orbitz/32/110_2.png) [@orbitz](https://discuss.ocaml.org/u/orbitz)\
**Post date:** [March 29, 2020, 2:55pm UTC](https://discuss.ocaml.org/t/solved-js-array-map-typing-issue-cannot-map-to-a-new-type/5392/5 "2020-03-29T14:55:27Z")

</div>

I’m not sure I follow your response. I was trying to build the function to pass to the `map` method. This operates on each individual element of the `Js.js_array`