[Caml-list] Matching when

[Caml-list] Matching when

[Nicolas Cannasse]

Hi !

I was thinking that the following should be possible :

match e with
| Any
| Int x when x > 0 ->
        .... (* here we don't use x *)
| ....

But the typechecker reject it because of the unbound 'x' in the "Any"
matching.
I don't know what kind of difficulty this modification require, but it would
be nice if the compiler was able to resolve such cases.

:-)
Nicolas Cannasse

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Re: [Caml-list] Matching when

[Luc Maranget]

> 
> Hi !
> 
> I was thinking that the following should be possible :
> 
> match e with
> | Any
> | Int x when x > 0 ->
>         .... (* here we don't use x *)
> | ....
> 
> But the typechecker reject it because of the unbound 'x' in the "Any"
> matching.
> I don't know what kind of difficulty this modification require, but it would
> be nice if the compiler was able to resolve such cases.
> 
> :-)
> Nicolas Cannasse
> 

Hello,

This has already been discussed.
The modifications required are important and probably not worth the
trouble.

More precisely the guard ``when expression'' is not part of patterns but rather
part of clauses.

That is your example is to to be parsed as
 Any | Int x <-- pattern                \
 when x > 0   <-- guard                 | clause
 ->                                     |
  ...         <-- right hand side       /

What you are asking is ``pattern when expression'' to be a pattern.
Then your example would be parsed as

That is your example is to to be parsed as
 Any              <-- pattern   | 
|                               | Pattern (or pat)
 Int x when x > 0 <-- pattern   |
->
  ...             <-- right hand side


Modifications are really important because this would make patterns and
expression mutually recursive, which they are not at the moment.



--Luc
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Re: [Caml-list] Matching when

[Pierre Weis]

Hi !

> I was thinking that the following should be possible :
> 
> match e with
> | Any
> | Int x when x > 0 ->
>         .... (* here we don't use x *)
> | ....
> 
> But the typechecker reject it because of the unbound 'x' in the "Any"
> matching.
> I don't know what kind of difficulty this modification require, but it would
> be nice if the compiler was able to resolve such cases.

I would require to analyze the set of free variables of expression
parts of match clauses to restrict the checks on variables of patterns
to this set of variables. In this case, the set of (typed) variables of
all the patterns of an ``or'' pattern should have the same intersection
with the set of free variables of the corresponding expression of the
clause, and this would solve your problem in a simple and elegant way.

This is simple and does not involve any modification of the semantics
of the language or of the pattern matching algorithm.

Regards,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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Re: [Caml-list] Matching when

[John Max Skaller]

Nicolas Cannasse wrote:

>Hi !
>
>I was thinking that the following should be possible :
>
>match e with
>| Any
>| Int x when x > 0 ->
>        .... (* here we don't use x *)
>| ....
>
>But the typechecker reject it because of the unbound 'x' in the "Any"
>matching.
>
Hmm. to rewrite it so it works:

    | Int _ when (match e with Int x -> x > 0 | _ -> assert false) -> ...

:-)


-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850




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Re: [Caml-list] Matching when

[Remi VANICAT]

"Nicolas Cannasse" <warplayer@free.fr> writes:

> Hi !
>
> I was thinking that the following should be possible :
>
> match e with
> | Any
> | Int x when x > 0 ->
>         .... (* here we don't use x *)
> | ....

well, I won't say it's beautiful, but this work :

match (e, 10) with
  | (Any, t)
  | (Int t, _) when t > 0 -> 
               ....
  | ....

-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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Re: [Caml-list] Matching when

[John Max Skaller]

Luc Maranget wrote:

>>What you are asking is ``pattern when expression'' to be a pattern.
>>Then your example would be parsed as
>>
>>That is your example is to to be parsed as
>> Any              <-- pattern   | 
>>|                               | Pattern (or pat)
>> Int x when x > 0 <-- pattern   |
>>->
>>  ...             <-- right hand side
>>
>>
>>Modifications are really important because this would make patterns and
>>expression mutually recursive, which they are not at the moment.
>>
The follwing code works in Felix [it prints "YES"]

#include <std.flx>

match (2,1) with
| ( (?x when x>=1), (?y when y >=1)) when x >=2 => { print "YES"; endl; }
| _ => { print "NO"; endl; }
endmatch;

wherein 'when' expressions are part of patterns, and as you can see,
can be nested within a pattern -- making the grammar for patterns
and expressions mutually recursive.

I'm curious as to what problems you perceive with
the mutual recursion.

-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850




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Re: [Caml-list] Matching when

[John Max Skaller]

Remi VANICAT wrote:

>"Nicolas Cannasse" <warplayer@free.fr> writes:
>
>>Hi !
>>
>>I was thinking that the following should be possible :
>>
>>match e with
>>| Any
>>| Int x when x > 0 ->
>>        .... (* here we don't use x *)
>>| ....
>>
>
>well, I won't say it's beautiful, but this work :
>
>match (e, 10) with
>  | (Any, t)
>  | (Int t, _) when t > 0 -> 
>               ....
>  | ....
>
Heh! This wins first prize for ugliest ocaml hack I have ever seen :-))

-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850




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Re: [Caml-list] Matching when

[Oleg]

On Monday 05 August 2002 05:26 am, Nicolas Cannasse wrote:
> match e with
>
> | Any
> | Int x when x > 0 ->
>
>         .... (* here we don't use x *)
>
> | ....

What is "Any" ?  I looked in the keywords, values and types indexes of the 
O'Caml manual, and I did not find the answer.

Thanks
Oleg
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Re: [Caml-list] Matching when

[Luc Maranget]

> 
> #include <std.flx>
> 
> match (2,1) with
> | ( (?x when x>=1), (?y when y >=1)) when x >=2 => { print "YES"; endl; }
> | _ => { print "NO"; endl; }
> endmatch;
> 
> wherein 'when' expressions are part of patterns, and as you can see,
> can be nested within a pattern -- making the grammar for patterns
> and expressions mutually recursive.
> 
> I'm curious as to what problems you perceive with
> the mutual recursion.
> 
> -- 
> John Max Skaller, mailto:skaller@ozemail.com.au
> snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
> voice:61-2-9660-0850
> 
> 
> 
> 

Nothing special, it is just an important change for a questionable
benefit....

Changing that would change many things in the front end, in
particular, the type-checker.

May be easy, may not be...



--Luc
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[Caml-list] Sharing Files between OCaml and C

[Michael Tucker]

Hi,

  I would like to share access to a file between a C function and an OCaml
function in an application put together with CamlIDL. If I have a C
function with prototype:

int manip_file(FILE* f, int x);

  Is there any support in CamlIDL for passing a file that was opened in
OCaml. I checked the archives, and found a message that described pulling
out the integer file descriptor, and passing that to fdopen on the C side,
but that was it -- the same message asked about better support from
CamlIDL (this was winter 2000), but I couldn't find a response. Has this
changed over the past year and a half, or is that the best method?

Thanks,
Mike


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Re: [Caml-list] Matching when

[Florian Hars]

Oleg wrote:
> What is "Any" ?  I looked in the keywords, values and types indexes of the 
> O'Caml manual, and I did not find the answer.

It is a cconstr-name:

   cconstr-name ::=  capitalized-ident
                  |  false
                  |  true
                  |  [ ]
                  |  ( )


It's probably part of some

   type t = Any | Int of int | Float of float | ...

Yours, Florian.
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Re: [Caml-list] Sharing Files between OCaml and C

[Olivier Andrieu]

 Michael Tucker [Monday 5 August 2002] :
 >
 > Hi,
 > 
 >   I would like to share access to a file between a C function and an
 > OCaml function in an application put together with CamlIDL. If I
 > have a C function with prototype:
 > 
 > int manip_file(FILE* f, int x);
 > 
 >   Is there any support in CamlIDL for passing a file that was opened in
 > OCaml. I checked the archives, and found a message that described
 > pulling out the integer file descriptor, and passing that to fdopen
 > on the C side, but that was it -- the same message asked about
 > better support from CamlIDL (this was winter 2000), but I couldn't
 > find a response. Has this changed over the past year and a half, or
 > is that the best method?

Yes, I think that's still the best method :
- get the descriptor with Unix.descr_of_in_channel or
  Unix.descr_of_out_channel 
- fdopen it on the C side
You might also want to call dup() on the fd so that your FILE* stream
is still working if the original caml channel is closed.

But maybe CamlIDL has improved, I don't know.

-- 
   Olivier
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Re: [Caml-list] Sharing Files between OCaml and C

[Bruno.Verlyck]

   From: Olivier Andrieu <andrieu@ijm.jussieu.fr>
   Date: Tue, 6 Aug 2002 15:04:11 +0200

   Michael Tucker [Monday 5 August 2002] :
   > I would like to share access to a file between a C function and an OCaml
   > function in an application put together with CamlIDL.  If I have a C
   > function with prototype:
   >  int manip_file(FILE* f, int x);
   > Is there any support in CamlIDL for passing a file that was opened in
   > OCaml.  I checked the archives, and found a message that described pulling
   > out the integer file descriptor, and passing that to fdopen on the C side,
   > but that was it -- the same message asked about better support from CamlIDL
   > (this was winter 2000), but I couldn't find a response.  Has this changed
   > over the past year and a half, or is that the best method?
   Yes, I think that's still the best method:
   - get the descriptor with Unix.descr_of_in_channel or
     Unix.descr_of_out_channel
   - fdopen it on the C side
   You might also want to call dup() on the fd so that your FILE* stream is still
   working if the original caml channel is closed.
But of course you now have two concurrent buffering mechanisms, so flushing when
switching the writing side (C vs Caml) for out_channels is mandatory, and
seeking cautiously when switching the reading side (for in_channels) too.

Bruno.
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Re: [Caml-list] Sharing Files between OCaml and C

[Lex Stein]

On POSIX compliant systems, Dup() does not, in fact, create a new vnode
for a file. It merely creates a new file descriptor table entry (in the
process's in-kernel descriptor table) that points to exactly the same
vnode. The two descriptors share the same buffered blocks in the global
buffer cache.

So if the C and Caml sides are sharing the same vnode, through dup'ed
descriptors, it should be sufficient to flush once on either the Caml
or C side.

If however, the two references to the file are independently created
using open, then the two descriptor table entries will point to
*distinct* vnodes with independent buffering and duplicate flushing
will be necessary (although one would have to be very careful not
to overwrite changes made to the same block by the other side-- a
motivator for using dup rather than open).

Lex

On Tue, 6 Aug 2002 Bruno.Verlyck@inria.fr wrote:

>    Yes, I think that's still the best method:
>    - get the descriptor with Unix.descr_of_in_channel or
>      Unix.descr_of_out_channel
>    - fdopen it on the C side
>    You might also want to call dup() on the fd so that your FILE* stream is still
>    working if the original caml channel is closed.
> But of course you now have two concurrent buffering mechanisms, so flushing when
> switching the writing side (C vs Caml) for out_channels is mandatory, and
> seeking cautiously when switching the reading side (for in_channels) too.
>
> Bruno.
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Re: [Caml-list] Sharing Files between OCaml and C

[Bruno.Verlyck]

   Date: Tue, 6 Aug 2002 14:50:44 -0400 (EDT)
   From: Lex Stein <stein@eecs.harvard.edu>

   On POSIX compliant systems, dup() does not, in fact, create a new
   vnode for a file.  It merely creates a new file descriptor table
   entry (in the process's in-kernel descriptor table) that points to
   exactly the same vnode.  The two descriptors share the same
   buffered blocks in the global buffer cache.
The problem I was alluding to is a local (inside the Caml process)
buffering one: the Caml runtime doesn't use C stdio, and has its own
buffering scheme.  As the original poster told us about FILE * and
channels, I assumed he would mix I/O on channels and FILE * pointing
to the same file.  Then he has to be cautious.

Bruno.
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