is this a bug ?

is this a bug ?

[Christophe Raffalli]

hello,

--------------------------------
let f b l = match l with
   [] | [_] -> 1
| [a;_] | [_;a] when a = b-> 2
| _ -> 3

let _ =
   print_int (f 1 [1;2]);
   print_int (f 1 [2;1]);
   print_newline ()
--------------------------------

Do you think this code should have a useless pattern warning and print "23" instead of "22" ?

If someone tell me this is a bug then I fill a bug report ...


-- 
Christophe Raffalli
Université de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex

tél: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
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At least Enigmail/Mozilla, mutt or evolution
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Re: [Caml-list] is this a bug ?

[Remi Vanicat]

2006/5/9, Christophe Raffalli <christophe.raffalli@univ-savoie.fr>:
>
> hello,
>
> --------------------------------
> let f b l = match l with
>    [] | [_] -> 1
> | [a;_] | [_;a] when a = b-> 2
> | _ -> 3
>
> let _ =
>    print_int (f 1 [1;2]);
>    print_int (f 1 [2;1]);
>    print_newline ()
> --------------------------------
>
> Do you think this code should have a useless pattern warning ?

Yes. When you wrote a pattern as :  '| [a;_] | [_;a] when a = b' it
first match the pattern '| [a;_] | [_;a]' then try the condition. So
it will match [a;_] and never [_;a]. You have to wrote it as :
| [a;c] when a = b or c = b -> 2


> and print "23" instead of "22" ?

It is the same problem there. Only the first patttern to match is matched.

Re: [Caml-list] is this a bug ?

[Samuel Mimram]

Hi,

Christophe Raffalli wrote:
> --------------------------------
> let f b l = match l with
>   [] | [_] -> 1
> | [a;_] | [_;a] when a = b-> 2
> | _ -> 3
> 
> let _ =
>   print_int (f 1 [1;2]);
>   print_int (f 1 [2;1]);
>   print_newline ()
> --------------------------------
> 
> Do you think this code should have a useless pattern warning and print
> "23" instead of "22" ?
> 
> If someone tell me this is a bug then I fill a bug report ...

I guess that the priorities says that you should "read" the third line as

| [a;_] | ( [_;a] when a = b ) -> 2

This would justify the warning and explain the behaviour...

Cheers,

Samuel.

Re: [Caml-list] is this a bug ?

[Christophe Raffalli]

2006/5/9, Christophe Raffalli <christophe.raffalli@univ-savoie.fr>:
>
> hello,
>
> --------------------------------
> let f b l = match l with
>    [] | [_] -> 1
> | [a;_] | [_;a] when a = b-> 2
> | _ -> 3
>
> let _ =
>    print_int (f 1 [1;2]);
>    print_int (f 1 [2;1]);
>    print_newline ()
> --------------------------------
>
> Do you think this code should have a useless pattern warning ?

Yes. When you wrote a pattern as :  '| [a;_] | [_;a] when a = b' it
first match the pattern '| [a;_] | [_;a]' then try the condition. So
it will match [a;_] and never [_;a]. You have to wrote it as :
| [a;c] when a = b or c = b -> 2


> and print "23" instead of "22" ?

It is the same problem there. Only the first patttern to match is matched.

Re: [Caml-list] is this a bug ?

[Christophe Raffalli]

Samuel Mimram a écrit :
> Hi,
> 
> Christophe Raffalli wrote:
> 
>>--------------------------------
>>let f b l = match l with
>>  [] | [_] -> 1
>>| [a;_] | [_;a] when a = b-> 2
>>| _ -> 3
>>
>>let _ =
>>  print_int (f 1 [1;2]);
>>  print_int (f 1 [2;1]);
>>  print_newline ()
>>--------------------------------
>>
>>Do you think this code should have a useless pattern warning and print
>>"23" instead of "22" ?
>>
>>If someone tell me this is a bug then I fill a bug report ...
> 
> 
> I guess that the priorities says that you should "read" the third line as
> 
> | [a;_] | ( [_;a] when a = b ) -> 2
> 

but the you should be allowed to write

  | ([a;_] when a = b) | ( [_;a] when a = b ) -> 2

but this results in a syntax error ...

> This would justify the warning and explain the behaviour...
> 
> Cheers,
> 
> Samuel.

-- 
Christophe Raffalli
Université de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex

tél: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
---------------------------------------------
IMPORTANT: this mail is signed using PGP/MIME
At least Enigmail/Mozilla, mutt or evolution
can check this signature. The public key is
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Re: [Caml-list] is this a bug ?

[Yoann Padioleau]

Samuel Mimram <samuel.mimram@ens-lyon.org> writes:

> Hi,
>
> Christophe Raffalli wrote:
>> --------------------------------
>> let f b l = match l with
>>   [] | [_] -> 1
>> | [a;_] | [_;a] when a = b-> 2
>> | _ -> 3
>> 
>> let _ =
>>   print_int (f 1 [1;2]);
>>   print_int (f 1 [2;1]);
>>   print_newline ()
>> --------------------------------
>> 
>> Do you think this code should have a useless pattern warning and print
>> "23" instead of "22" ?
>> 
>> If someone tell me this is a bug then I fill a bug report ...
>
> I guess that the priorities says that you should "read" the third line as
>
> | [a;_] | ( [_;a] when a = b ) -> 2

I am not sure because because  | ([a;_] | [_;a]) when a = b    -> 2
issues the same warning.


I think | [a;_] | [_;a] when a = b-> 2    is  parsed  as 

| ( [a;_] | [_;a])  when a = b -> 2

but that the semantic is first to look at the pattern, 
and if one is found, then look at the condition, which is too late.

'when' does not "distribute" over Or-patterns :( 



let f b l = match l with
   [] | [_] -> 1
| [a;_] when a = b-> 2
| [_;a] when a = b-> 2
| _ -> 3

works.

>
> This would justify the warning and explain the behaviour...
>
> Cheers,
>
> Samuel.
>
> _______________________________________________
> Caml-list mailing list. Subscription management:
> http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
> Archives: http://caml.inria.fr
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> Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>

-- 
pad

Re: [Caml-list] is this a bug ?

[Luc Maranget]

> 
> hello,
> 
> --------------------------------
> let f b l = match l with
>   [] | [_] -> 1
> | [a;_] | [_;a] when a = b-> 2
            ^^^^
> | _ -> 3
Warning U: this pattern is unused.

> 
> let _ =
>   print_int (f 1 [1;2]);
>   print_int (f 1 [2;1]);
>   print_newline ()
> --------------------------------
23


> 
> Do you think this code should have a useless pattern warning and print "23" 
> instead of "22" ?
> 
> If someone tell me this is a bug then I fill a bug report ...
> 


You may object the semantics, but it is consitent w.r.t. both
documentation (deterministic left-to-right matching of or-patterns,
semantics of when as match first, then test)
and supplied warning (which you omit).

So I am tempted to say not a bug.

However, there is indeed a difficulty, if we expand or-pat as:

let f b l = match l with
| [] -> 1
| [_] -> 1
| [a;_] when a = b -> 2
| [_;a] when a = b -> 2
| _ -> 3

Then, we have the other behavior (no warning, 22).

Oups, another oddity of 'when...'

--Luc

Re: [Caml-list] is this a bug ?

[Samuel Mimram]

Christophe Raffalli wrote:
> Samuel Mimram a écrit :
>> Christophe Raffalli wrote:
>>
>>> --------------------------------
>>> let f b l = match l with
>>>  [] | [_] -> 1
>>> | [a;_] | [_;a] when a = b-> 2
>>> | _ -> 3
>>>
>>> let _ =
>>>  print_int (f 1 [1;2]);
>>>  print_int (f 1 [2;1]);
>>>  print_newline ()
>>> --------------------------------
>>>
>>> Do you think this code should have a useless pattern warning and print
>>> "23" instead of "22" ?
>>>
>>> If someone tell me this is a bug then I fill a bug report ...
>>
>> I guess that the priorities says that you should "read" the third line as
>>
>> | [a;_] | ( [_;a] when a = b ) -> 2
> 
> but the you should be allowed to write
> 
>  | ([a;_] when a = b) | ( [_;a] when a = b ) -> 2
> 
> but this results in a syntax error ...

Yes, in fact I answered too quickly. The answers would have been 22 in
both cases. Moreover,

let f x = match x with
  | z | y when y = 0 -> 0
  | _ -> 1
;;

results in "Variable y must occur on both sides of this | pattern"...

Samuel.

Re: [Caml-list] is this a bug ?

[Arne Ehrlich]

Am Dienstag, 9. Mai 2006 15:09 schrieb Christophe Raffalli:
> hello,
>
> --------------------------------
> let f b l = match l with
>    [] | [_] -> 1
>
> | [a;_] | [_;a] when a = b-> 2
> | _ -> 3
>
> let _ =
>    print_int (f 1 [1;2]);
>    print_int (f 1 [2;1]);
>    print_newline ()
> --------------------------------
>
> Do you think this code should have a useless pattern warning and print "23"
> instead of "22" ?
You found an semantic problem in the gramar ^^
or-patterns do not play well with "when"

I think it's a bug in the semantics... 

Re: [Caml-list] is this a bug ?

[Alain Frisch]

Christophe Raffalli wrote:
> 
> hello,
> 
> --------------------------------
> let f b l = match l with
>   [] | [_] -> 1
> | [a;_] | [_;a] when a = b-> 2
> | _ -> 3
> 
> let _ =
>   print_int (f 1 [1;2]);
>   print_int (f 1 [2;1]);
>   print_newline ()
> --------------------------------
> 
> Do you think this code should have a useless pattern warning and print
> "23" instead of "22" ?

Well, both happens, right?  I don't see any problem.

The pattern [_;a] is useless because [a;_] matches the same values and
the first match (left-to-right) policy is specificed
(http://caml.inria.fr/pub/docs/manual-ocaml/manual014.html).

The guard is checked after the binding, and there is no backtracking.
This explains why the function returns 3 when b=1 l=[2;1].

-- Alain

Re: [Caml-list] is this a bug ?

[Jonathan Roewen]

> You found an semantic problem in the gramar ^^
> or-patterns do not play well with "when"
>
> I think it's a bug in the semantics...

I don't believe so. It depends on the type. In fact, it works very
well with sum types (I did a very small inference engine which used or
patterns with when to great effect).

The problem is the two match clauses are not different. They look
different for the fact that the variables are bound in a different
order -- which is why the warning occurs. For matching, variable
naming has no semantic impact, hence the confusion.

Jonathan

Is this a bug?

[Thomas Fischbacher]

If we look at the following piece of code:

===>
type dof_region_spec = (string * bool * (int option array)) array;;
type opt_field_restriction = dof_region_spec option;;

let make_field ?(name="Field") ?restriction ?constant_value mwe =
   let restriction:opt_field_restriction = restriction in
	failwith "FOO";;
<===

...then make_field types as:

===>
val make_field :
   ?name:string -> ?restriction:<hidden> -> ?constant_value:'a -> 'b -> 'c
<===

The <hidden> actually should not show up here. What is going on?

-- 
best regards,
Thomas Fischbacher
tf@functionality.de