[Caml-list] Strange type inference error

[Caml-list] Strange type inference error

[Maxence Guesdon]

Hello,

I encountered a strange typing problem and reduced it to the following
code example:

====[t.ml]====
let mk_env () = None
let list_of_string (s: string) = [s]

let apply : 'a -> 'a option -> string list -> 'a * string list =
  fun acc env l -> (acc, ["hello"])

let f =
   let tmpl = ["coucou"] in
    fun ?(env=mk_env()) ->
      fun ~x ->
        fun () -> apply () env tmpl

let g =
   let tmpl = list_of_string "coucou" in
    fun ?(env=mk_env()) ->
      fun ~x ->
        fun () -> apply () env tmpl
====
Note that f and g only differ on the way tmpl is defined.

When I compile this file with ocaml 4.02.1, I get the following error:
File "t.ml", line 14, characters 3-119:
Error: The type of this expression,
       ?env:unit option -> x:'_a -> unit -> unit * string list,
       contains type variables that cannot be generalized

Am I missing something or should I file a bug report ?

Regards,

Maxence

Re: [Caml-list] Strange type inference error

[Arnaud Spiwack]

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The error message is correct in that `g` is not a value (and so the value
restriction bites you). At the end of a module, no non-generalisable
variables must be left, so Ocaml complains (you will notice, indeed, that
in a toplevel, ocaml will give you the non-generalisable type without
complaining).

I'm guessing ocaml has a relaxed view of values where `let x = <value> in
<value>` is considered a value, which would be why `f` properly generalises.

On 23 January 2015 at 15:43, Maxence Guesdon <Maxence.Guesdon@inria.fr>
wrote:

> Hello,
>
> I encountered a strange typing problem and reduced it to the following
> code example:
>
> ====[t.ml]====
> let mk_env () = None
> let list_of_string (s: string) = [s]
>
> let apply : 'a -> 'a option -> string list -> 'a * string list =
>   fun acc env l -> (acc, ["hello"])
>
> let f =
>    let tmpl = ["coucou"] in
>     fun ?(env=mk_env()) ->
>       fun ~x ->
>         fun () -> apply () env tmpl
>
> let g =
>    let tmpl = list_of_string "coucou" in
>     fun ?(env=mk_env()) ->
>       fun ~x ->
>         fun () -> apply () env tmpl
> ====
> Note that f and g only differ on the way tmpl is defined.
>
> When I compile this file with ocaml 4.02.1, I get the following error:
> File "t.ml", line 14, characters 3-119:
> Error: The type of this expression,
>        ?env:unit option -> x:'_a -> unit -> unit * string list,
>        contains type variables that cannot be generalized
>
> Am I missing something or should I file a bug report ?
>
> Regards,
>
> Maxence
>
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>
>

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