[Caml-list] strange behaviour with variants and "cannot be generalized"

[Caml-list] strange behaviour with variants and "cannot be generalized"

[Beck01, Wolfgang]

I have three files

vtop.ml --------------------------

type 'a t = {
    t_c : 'a;
}

v1.ml ----------------------------

open Vtop

type t = {
    v1_x : int array;
}

let init = `V1 { v1_x = [| 0 |] }

v2.ml ----------------------------

open Vtop

type t= {
    v2_x : int;
}

let init = `V2 { v2_x = 0 }
---------------------------------

compilation fails with "The type of this expression _[> `V1 of t ] Vtop.t,
contains type variables that cannot be generalized"

However, if I change the type of v1_x to a string or some large struct,
it compiles without warning and works. The error seems only to occur
with arrays. I am using version 3.06. 

Is there an explanation for this behaviour?

Wolfgang Beck

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Re: [Caml-list] strange behaviour with variants and "cannot be generalized"

[Jacques Garrigue]

From: "Beck01, Wolfgang" <BeckW@t-systems.com>

> v1.ml ----------------------------
> 
> open Vtop
> 
> type t = {
>     v1_x : int array;
> }
> 
> let init = `V1 { v1_x = [| 0 |] }
> 
> compilation fails with "The type of this expression _[> `V1 of t ] Vtop.t,
> contains type variables that cannot be generalized"
> 
> However, if I change the type of v1_x to a string or some large struct,
> it compiles without warning and works. The error seems only to occur
> with arrays. I am using version 3.06. 
> 
> Is there an explanation for this behaviour?

Sure: [| 0 |] creates a mutable data structure, and as such is
considered as a side-effecting expression. As a result the type of
init cannot be generalized.

OCaml 3.07 is more clever about that, and the above program would be
accepted with no problem.

Jacques Garrigue

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