This site is updated infrequently. For up-to-date information, please visit the new OCaml website at ocaml.org.

[Caml-list] What's wrong with my parser ?
[ Home ] [ Index: by date | by threads ]
[ Search: ]

[ Message by date: previous | next ] [ Message in thread: previous | next ] [ Thread: previous | next ]
 Date: -- (:) From: John Prevost Subject: Re: [Caml-list] What's wrong with my parser ?
```>>>>> "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.

-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners

```