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A strange typing error with polymorphic variants
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Date: -- (:)
From: Vincent Aravantinos <vincent.aravantinos@g...>
Subject: Re: [Caml-list] A strange typing error with polymorphic variants
Oops, I pushed "send" button too early.

Le 27 oct. 09 à 19:38, Vincent Aravantinos a écrit :

> Le 27 oct. 09 à 19:24, Jake Donham a écrit :
>
>> On Tue, Oct 27, 2009 at 3:28 AM, Marc de Falco <marc@de-falco.fr>  
>> wrote:
>>> The following code :
>>> type 'a p = R of 'a t | E of float
>>>    and 'a t = { mutable p : 'a p; c : 'a }
>>> let f =
>>>    let x = sqrt(2.0) in
>>>    fun () -> { c = `A; p = E 0.0 }
>>>
>>> generates the error :
>>>  The type of this expression, unit -> _[> `A ] t,
>>>  contains type variables that cannot be generalized
>>>
>>> but if I change the x definition to "let x = 2.0 in" then it works.
>>
>> I think this is just the value restriction. The type of f is
>> generalized only if the right hand side is a value (rather than an
>> expression needing some computation); in your examples the one that
>> fails is not a value, the others are. It looks like there is a
>> relaxation to allow let bindings which are themselves values.
>
> With the -dlambda option, the "sqrt(2.0)" version gives:
>  (let
>    (f/92
>       (let (x/93 (caml_sqrt_float 2.0))
>         (function param/94 (makemutable 0 [1: 0.0] 65a))))
>
> whereas the "2.0" version gives:
>  (let (f/96 (let (x/97 2.0) (function param/98 (makemutable 0 [1:  
> 0.0] 65a))))
>
> i.e. this last version is inlined.

Do you think this can give a hint?

V.