[antlr-interest] Pass parameters to DFAs for semantic predicate (or AntLR 3.3 wish list? :o) )
Jim Idle
jimi at temporal-wave.com
Wed Dec 16 07:53:12 PST 2009
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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