A strange typing error with polymorphic variants

[Marc de Falco-2]

Hi, I've come across a very strange error, and I'm not sure if it is
a bug or a feature.

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.

Another solution is to add a dummy parameter "let f ?(dummy=())" this works too.
When I say that it works, I mean that the resulting type of f is
 val f : unit -> [> `A ] t
I've seen that in the dev version of ocaml this error has been removed but
the type of f is still "val f : unit -> _[> `A] t" which is not the thing that I want.

Is that an expected behavior ?

Marc

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[Jake Donham-4]

On Tue, Oct 27, 2009 at 3:28 AM, Marc de Falco <marc@...> wrote:

> Hi, I've come across a very strange error, and I'm not sure if it is
> a bug or a feature.
>
> 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.
>
> Another solution is to add a dummy parameter "let f ?(dummy=())" this works
> too.

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.

Jake

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[Vincent Aravantinos-2]

Le 27 oct. 09 à 19:24, Jake Donham a écrit :

> On Tue, Oct 27, 2009 at 3:28 AM, Marc de Falco <marc@...>  
> 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.

I thought the yping was done before (??)

V.






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[Vincent Aravantinos-2]

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@...>  
>> 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.
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