* Re: [Caml-list] What's wrong with my parser ?
@ 2002-07-13 19:44 Ryan Tarpine
0 siblings, 0 replies; 6+ messages in thread
From: Ryan Tarpine @ 2002-07-13 19:44 UTC (permalink / raw)
To: nicolas.francois; +Cc: caml-list
>From: Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr>
>To: Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr>
>CC: caml-list@inria.fr
>Subject: Re: [Caml-list] What's wrong with my parser ?
>Date: Sat, 13 Jul 2002 13:27:34 +0200
>
>OK, this is my new polynom parser, it seems to work OK, thanks to your
>explanations.
>
> let rec parse s =
><snip>
> let parse_coeff = parser
> | [< n = R.parse >] -> n
> | [< >] -> R.one
> in
> let parse_monome = parser
> | [< n = parse_coeff; e = parse_x >] ->
> monome n e
> | [< e = parse_x >] ->
> monome R.one e
><snip>
> in
> match s with parser
> | [< m = parse_monome >] ->
> match parse_suite s with
> | p -> m ++ p
>
><snip>
># print (P.parse (Stream.of_string ""));;
>[1: 1]- : unit = ()
>
Parsing the empty string calls parse_monome, which calls parse_coeff, which
returns R.one. My take at this is that you need 2 versions of parse_x: one
that requires an X to be there, and one that can parse an X but doesn't need
it.
If there is no leading coefficient (the second option to parse_monome), the
X *must* be there. If the coeffecient is there (the first option), then the
X doesn't have to be. The empty string, which has no coefficient, should
fail parse_monome because it doesn't have an X either. If passed the string
"X" or "2", though, parse_monome should succeed.
Ryan Tarpine, rtarpine@hotmail.com
"To err is human, to compute divine. Trust your computer but not its
programmer."
- Morris Kingston
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^ permalink raw reply [flat|nested] 6+ messages in thread
* [Caml-list] What's wrong with my parser ?
@ 2002-07-12 2:41 Nicolas FRANCOIS
2002-07-12 2:48 ` John Prevost
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Nicolas FRANCOIS @ 2002-07-12 2:41 UTC (permalink / raw)
To: Caml List
I define a polynom parser like this :
let rec parse =
let parse_puissance = parser
| [< ''^'; e = parse_int >] -> e
| [< >] -> 1
in
let parse_x = parser
| [< ''X'; e = parse_puissance >] -> e
| [< >] -> 0
in
let parse_coeff = parser
| [< n = R.parse >] -> n
| [< >] -> R.one
in
let parse_monome = parser
| [< n = R.parse; e = parse_x >] ->
monome n e
| [< e = parse_x >] ->
monome R.one e
in
parser
| [< ''+'; m = parse_monome; p = parse >] ->
m ++ p
| [< ''-'; m = parse_monome; p = parse >] ->
(opp m) ++ p
| [< m = parse_monome; p = parse >] ->
m ++ p
| [< >] -> zero
parse_int and the R.parse functions work OK, and I'll add all the
"_=parse_space" after. But :
# let p1 = P.parse (Stream.of_string "1+X");;
Stack overflow during evaluation (looping recursion?).
What's the problem ?
\bye
--
Nicolas FRANCOIS
http://nicolas.francois.free.fr
A TRUE Klingon programmer does NOT comment his code
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [Caml-list] What's wrong with my parser ?
2002-07-12 2:41 Nicolas FRANCOIS
@ 2002-07-12 2:48 ` John Prevost
2002-07-12 2:59 ` John Prevost
2002-07-13 11:27 ` Nicolas FRANCOIS
2 siblings, 0 replies; 6+ messages in thread
From: John Prevost @ 2002-07-12 2:48 UTC (permalink / raw)
To: Nicolas FRANCOIS; +Cc: Caml List
>>>>> "nf" == Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr> writes:
nf> I define a polynom parser like this : let rec parse = let
{...}
nf> What's the problem ?
What's the definition of R.parse? It isn't in the code you excerpted.
John.
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [Caml-list] What's wrong with my parser ?
2002-07-12 2:41 Nicolas FRANCOIS
2002-07-12 2:48 ` John Prevost
@ 2002-07-12 2:59 ` John Prevost
2002-07-12 7:14 ` Daniel de Rauglaudre
2002-07-13 11:27 ` Nicolas FRANCOIS
2 siblings, 1 reply; 6+ messages in thread
From: John Prevost @ 2002-07-12 2:59 UTC (permalink / raw)
To: Nicolas FRANCOIS; +Cc: Caml List
>>>>> "nf" == Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr> writes:
nf> I define a polynom parser like this : let rec parse = let
let rec parse =
let parse_puissance = parser
| [< ''^'; e = parse_int >] -> e
| [< >] -> 1
in
let parse_x = parser
| [< ''X'; e = parse_puissance >] -> e
| [< >] -> 0
in
let parse_coeff = parser
| [< n = R.parse >] -> n
| [< >] -> R.one
in
let parse_monome = parser
| [< n = R.parse; e = parse_x >] ->
monome n e
| [< e = parse_x >] ->
monome R.one e
in
parser
| [< ''+'; m = parse_monome; p = parse >] ->
m ++ p
| [< ''-'; m = parse_monome; p = parse >] ->
(opp m) ++ p
| [< m = parse_monome; p = parse >] ->
m ++ p
| [< >] -> zero
nf> parse_int and the R.parse functions work OK, and I'll add all
nf> the "_=parse_space" after. But :
# let p1 = P.parse (Stream.of_string "1+X");;
Stack overflow during evaluation (looping recursion?).
nf> What's the problem ?
Ahh. I didn't look closely enough. The problem can be seen by
looking at what happens when you try to parse the empty stream:
in parse:
[< >] -> cases 1 and 2 fail, tries [< m = parse_monome; p = parse >]
in m = parse_monome:
[< >] -> I assume R.parse fails, so it tries [< e = parse_x >]
in parse_x
[< >] -> case 1 fails, case 2 [< >] succeeds
parse_x returns 0
parse_monome returns monome R.one 0
[< m = parse_monome; ... >] succeeded, move to [< ...; p = parse >]
And at this point, parse is called on [< >] again and it loops.
Unfortunately, I can't remember how to explicitly check for the end of
the stream--and it's been removed from the O'Caml manual. That's what
you have to do to avoid this. A quick and dirty hack is to have a
special symbol (";", for instance) at the end of every phrase.
John.
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [Caml-list] What's wrong with my parser ?
2002-07-12 2:59 ` John Prevost
@ 2002-07-12 7:14 ` Daniel de Rauglaudre
0 siblings, 0 replies; 6+ messages in thread
From: Daniel de Rauglaudre @ 2002-07-12 7:14 UTC (permalink / raw)
To: Caml List
Hi,
On Thu, Jul 11, 2002 at 10:59:20PM -0400, John Prevost wrote:
> And at this point, parse is called on [< >] again and it loops.
> Unfortunately, I can't remember how to explicitly check for the end of
> the stream
It is not the point: even testing the end of the stream would not
help, since the example did not reach the end of the stream, it
took no token at all (no character at all) from the stream.
> --and it's been removed from the O'Caml manual.
It has been copied is in the Camlp4 manual, but perhaps not yet in
the 3.04 version.
http://caml.inria.fr/camlp4/manual/manual003.html
However the module "Stream" is still part of the OCaml manual. To test
the empty stream, call the parser Stream.empty:
[< _ = Stream.empty >] ->
--
Daniel de RAUGLAUDRE
daniel.de_rauglaudre@inria.fr
http://cristal.inria.fr/~ddr/
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Caml-list] What's wrong with my parser ?
2002-07-12 2:41 Nicolas FRANCOIS
2002-07-12 2:48 ` John Prevost
2002-07-12 2:59 ` John Prevost
@ 2002-07-13 11:27 ` Nicolas FRANCOIS
2 siblings, 0 replies; 6+ messages in thread
From: Nicolas FRANCOIS @ 2002-07-13 11:27 UTC (permalink / raw)
To: Nicolas FRANCOIS; +Cc: caml-list
OK, this is my new polynom parser, it seems to work OK, thanks to your
explanations.
let rec parse s =
let parse_puissance = parser
| [< ''^'; e = parse_int >] -> e
| [< >] -> 1
in
let parse_x = parser
| [< ''X'; e = parse_puissance >] -> e
| [< >] -> 0
in
let parse_coeff = parser
| [< n = R.parse >] -> n
| [< >] -> R.one
in
let parse_monome = parser
| [< n = parse_coeff; e = parse_x >] ->
monome n e
| [< e = parse_x >] ->
monome R.one e
in
let rec parse_suite = parser
| [< ''+'; m = parse_monome; p = parse_suite >] ->
m ++ p
| [< ''-'; m = parse_monome; p = parse_suite >] ->
(opp m) ++ p
| [< >] -> zero
in
match s with parser
| [< m = parse_monome >] ->
match parse_suite s with
| p -> m ++ p
and the results :
# print (P.parse (Stream.of_string "-1+3X+4X^2"));;
[3: -1, 3, 4]- : unit = ()
# print (P.parse (Stream.of_string ""));;
[1: 1]- : unit = ()
Oups !!
What do you think about this ?
\bye
--
Nicolas FRANCOIS
http://nicolas.francois.free.fr
A TRUE Klingon programmer does NOT comment his code
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