[antlr-interest] Réf. : Re: Pass parameters to DFAs for semantic predicate (or AntLR 3.3 wish list? :o) )
loic.lefevre at bnpparibas.com
loic.lefevre at bnpparibas.com
Wed Dec 16 08:03:04 PST 2009
Yes SWIFT parsing by hand is very easy (at least I've almost finished it
in 30 minutes).
But here come my problems with ambiguities to treat.
For example, we can have a tag named 53B; its format is [/1!a][/34x]#[35x]
which means:
- optionally (absolutely 1 upper case letter preceded by a /)
- optionally (between 1 and 34 chars preceded by a /)
- then a carriage return (CrLf); this \r\n can be optional if both
previous fields are not present or if the following field does not exist
- optionally (between 1 and 35 chars)
Now with the input String:
/YOH
LCN484841
I need to detect:
- field 1 is not present
- field 2's value is YOH
- field 3's value is LCN484841
and of course, you may have very complex regular expression to handle
(this one is pretty simple)
Regards,
Loïc
Internet
jimi at temporal-wave.com
Envoyé par : antlr-interest-bounces at antlr.org
16/12/2009 16:53
Pour
antlr-interest at antlr.org
cc
Objet
Re: [antlr-interest] Pass parameters to DFAs for semantic predicate (or
AntLR 3.3 wish list? :o) )
Your predicate is based on a local variable so the generated methods for
the DFA do not see it. You will have to set the length in to a scope
variable and use that in your predicate:
data_x[ int length ]
returns[ String s ]
@init {
final StringBuilder sb = new StringBuilder();
}
@after {
s = sb.toString();
}
scope {
int sLen = 0;
}
{ sb.append($d.text); $data_x::sLen = sb.length(); …
However I am not sure if it is safe for you to just return from the rule
yourself. It might be though as you are not building trees etc.
I wonder if rather than a lexer/parser, you just need to use a filtering
lexer, or whether in fact this format is really lending itself to be
parsed by something like ANTLR. Perhaps you just need hand crafted code.
Are these SWIFT records or something similar with fixed length/length
encoded fields? Something like awk may be better for this.
Jim
From: antlr-interest-bounces at antlr.org
[mailto:antlr-interest-bounces at antlr.org] On Behalf Of
loic.lefevre at bnpparibas.com
Sent: Wednesday, December 16, 2009 7:16 AM
To: antlr-interest at antlr.org
Subject: [antlr-interest] Pass parameters to DFAs for semantic predicate
(or AntLR 3.3 wish list? :o) )
Hello again,
I continue to struggle with AntLR :o)
I think I've got a real problem now.
I have a grammar that is absolutely ambiguous that's why I absolutely need
backtracking :o)
So ambiguous that I also need variable length tokens.
For example, when I need to parse at most 16 chars (for a given data
type), I've got:
data_x[ int length ]
returns[ String s ]
@init {
final StringBuilder sb = new StringBuilder();
}
@after {
s = sb.toString();
}
: ( ( d=DIGIT { sb.append($d.text);if( sb.length() == length
) { return sb.toString(); }} |
l=LETTER { sb.append($l.text);if( sb.length() ==
length ) { return sb.toString(); }} |
cl=CAPITAL_LETTER { sb.append($cl.text);if(
sb.length() == length ) { return sb.toString(); }} |
SLASH { sb.append('/');if( sb.length() == length ) {
return sb.toString(); }} |
SPACE { sb.append(' ');if( sb.length() == length ) {
return sb.toString(); }} |
ANTI_SLASH { sb.append('\\');if( sb.length() ==
length ) { return sb.toString(); }} |
MINUS { sb.append('-');if( sb.length() == length ) {
return sb.toString(); }} |
COLON { sb.append(':');if( sb.length() == length ) {
return sb.toString(); }} |
LPAREN { sb.append('(');if( sb.length() == length )
{ return sb.toString(); }} |
RPAREN { sb.append(')');if( sb.length() == length )
{ return sb.toString(); }} |
DOT { sb.append('.');if( sb.length() == length ) {
return sb.toString(); }} |
COMMA { sb.append(',');if( sb.length() == length ) {
return sb.toString(); }} |
PLUS { sb.append('+');if( sb.length() == length ) {
return sb.toString(); }} |
QUOTE { sb.append('\'');if( sb.length() == length )
{ return sb.toString(); }} |
QUESTION_MARK { sb.append('?');if( sb.length() ==
length ) { return sb.toString(); }}
)
)+
;
I know this is awful but at least it works or I should precise, it worked.
The problem here is that I can't use a disambiguating semantic predicate
such as:
data_x[ int length ]
returns[ String s ]
@init {
final StringBuilder sb = new StringBuilder();
}
@after {
s = sb.toString();
}
: (
{sb.length() < length}?
( d=DIGIT { sb.append($d.text);if( sb.length() ==
length ) { return sb.toString(); }} |
l=LETTER { sb.append($l.text);if( sb.length() ==
length ) { return sb.toString(); }} |
...
since the sb and length variables are not pushed inside the DFA :o(
It could be interesting to have at least the length parameter "pushed"
into the dfa via a generated setter for example:
class DFA149 extends DFA {
private int length;
public DFA149(BaseRecognizer recognizer) {
...
}
public void setLength( int length ) {
this.length = length;
}
public String getDescription() {
return "()+ loopback of 1163:3: ({...}? (d= DIGIT | l= LETTER
| cl= CAPITAL_LETTER | SLASH | SPACE | ANTI_SLASH | MINUS | COLON | LPAREN
| RPAREN | DOT | COMMA | PLUS | QUOTE | QUESTION_MARK ) )+";
}
public int specialStateTransition(int s, IntStream _input) throws
NoViableAltException {
TokenStream input = (TokenStream)_input;
int _s = s;
switch ( s ) {
case 0 :
int LA149_14 = input.LA(1);
int index149_14 = input.index();
input.rewind();
s = -1;
if ( ((synpred230_SWIFTMT()&&(sb.length() < length
))) ) {s = 17;}
else if ( ((sb.length() < length)) ) {s = 1;}
...
Then the length parameter could be used inside the specialStateTransition
method and we could imagine such a principle used for the
synpred230_SWIFTMT() methods also.
One point I don't understand is why my predicate is not pushed before the
generated syntactic predicate like:
if ( (((sb.length() < length
)&&synpred230_SWIFTMT())) ) {s = 17;}
instead of
if ( ((synpred230_SWIFTMT()&&(sb.length() < length
))) ) {s = 17;}
Since my comparison is faster :o) Maybe there are some reasons for that,
could someone explain me?
Finally, I've got of course another problem with the kind of action I set:
if( sb.length() == length ) { return sb.toString(); }
I just return from the rule if I reached the maximum length. This work
well since there are the blocks catch and finally to handle properly what
needs to be done (backtracking / error handling).
However when backtracking, the action is not run, see generated code:
case 1 :
//
C:\\GRP_Head\\GRP_Dev\\Development\\frameworks\\Foxhound\\target\\generated\\com\\bnpparibas\\acetp\\foxhound\\spec2009\\parser\\SWIFTMT.g:1108:6:
cl= CAPITAL_LETTER
{
cl=(Token)match(input,CAPITAL_LETTER,FOLLOW_CAPITAL_LETTER_in_data_a8285);
if (state.failed) return s;
if ( state.backtracking==0 ) {
sb.append((cl!=null?cl.getText():null)); if(
sb.length() == length ) { return sb.toString(); }
}
}
break;
So this "trick" does not work anymore (it used to work however).
With a grammar managing 2 message types (see previous posts) no problem.
With a third one, I get the following error message:
line 2:5 no viable alternative at input 'C'
I begin to doubt that antlr v3 will be able to parse SWIFT MT messages :o(
Regards,
Loïc
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