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Re: [Caml-list] Pattern matcher no more supposed to warn on non
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Date: -- (:)
From: Dan Grossman <danieljg@c...>
Subject: Re: [Caml-list] Pattern matcher no more supposed to warn on non

Luc Maranget wrote:
 
> Argl....
> 
> Dear Dan, which result would you consider to be correct ?
> (1111 ?)
> Please tell me...
> (Note that I see no consitency either in 3.02 or 3.00,)
> 
> Cheers, thanks,  and argl again !
> 
> --Luc

If this problem is worth fixing, I suggest that only 1111 or 4444 are
reasonable answers.  I tend to prefer 1111.  For fixing the
exhaustiveness-checker and pattern-compiler, it's probably easiest to do the
most expensive thing: the presence of a when clause basically invalidates
the failure of all previous cases -- it's like they weren't there.

Note that no change needs to be made except when the pattern's type includes
a mutable field.

Further note that threads are even harder to deal with -- they effectively
can put evil when clauses everywhere.  My familiarity with the ocaml
run-time is rusty, but perhaps a pattern-match (where the pattern's type
includes a mutable field) could be "rolled back" when the thread is
restarted.  Of course, you can't re-execute any when clauses! :-)

--Dan

> 
> It is worth a warning in the manual, at least...
> 
> >
> > ===============
> >
> > type y = {mutable c:bool}
> > type x = {a:y; b:y}
> >
> > let f1 (v:x) =
> >   match v with
> >     {a = _; b = _} when (v.a.c <- false; v.b.c <- false; false) -> 0
> >   | {a = {c=false}; b = {c=false}} -> 1
> >   | {a = {c=false}; b = {c=true}}  -> 2
> >   | {a = {c=true};  b = {c=false}} -> 3
> >   | {a = {c=true};  b = {c=true}}  -> 4
> >
> > let f2 (v:x) =
> >   match v with
> >     {a = {c=true}; b = _} when (v.a.c <- false; v.b.c <- false; false) -> 0
> >   | {a = {c=false}; b = {c=false}} -> 1
> >   | {a = {c=false}; b = {c=true}}  -> 2
> >   | {a = {c=true};  b = {c=false}} -> 3
> >   | {a = {c=true};  b = {c=true}}  -> 4
> >
> > let f3 (v:x) =
> >   match v with
> >     {a = _; b = {c=true}} when (v.a.c <- false; v.b.c <- false; false) -> 0
> >   | {a = {c=false}; b = {c=false}} -> 1
> >   | {a = {c=false}; b = {c=true}}  -> 2
> >   | {a = {c=true};  b = {c=false}} -> 3
> >   | {a = {c=true};  b = {c=true}}  -> 4
> >
> > let f4 (v:x) =
> >   match v with
> >     {a = {c=true}; b={c=true}} when (v.a.c <- false; v.b.c <- false; false)
> > -> 0
> >   | {a = {c=false}; b = {c=false}} -> 1
> >   | {a = {c=false}; b = {c=true}}  -> 2
> >   | {a = {c=true};  b = {c=false}} -> 3
> >   | {a = {c=true};  b = {c=true}}  -> 4
> >
> > let main () =
> >   let v1:x = {a = {c=true}; b = {c=true}} in
> >   let v2:x = {a = {c=true}; b = {c=true}} in
> >   let v3:x = {a = {c=true}; b = {c=true}} in
> >   let v4:x = {a = {c=true}; b = {c=true}} in
> >   print_int (f1 v1);
> >   print_int (f2 v2);
> >   print_int (f3 v3);
> >   print_int (f4 v4)
> >
> > let _ = main ()
> > -------------------
> > Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
> > To unsubscribe, mail caml-list-request@inria.fr  Archives: http://caml.inria.fr
> >

-- 
| Dan Grossman     www.cs.cornell.edu/home/danieljg H:607 256 0724 |
| 5157 Upson Hall  danieljg@cs.cornell.edu          O:607 255 9834 |
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