I am quite new to ANTLR, so this is likely a simple question.
I have defined a simple grammar which is supposed to include arithmetic expressions with numbers and identifiers (strings that start with a letter and continue with one or more letters or numbers.)
The grammar looks as follows:
grammar while;
#lexer::header {
package ConFreeG;
}
#header {
package ConFreeG;
import ConFreeG.IR.*;
}
#parser::members {
}
arith:
term
| '(' arith ( '-' | '+' | '*' ) arith ')'
;
term returns [AExpr a]:
NUM
{
int n = Integer.parseInt($NUM.text);
a = new Num(n);
}
| IDENT
{
a = new Var($IDENT.text);
}
;
fragment LOWER : ('a'..'z');
fragment UPPER : ('A'..'Z');
fragment NONNULL : ('1'..'9');
fragment NUMBER : ('0' | NONNULL);
IDENT : ( LOWER | UPPER ) ( LOWER | UPPER | NUMBER )*;
NUM : '0' | NONNULL NUMBER*;
fragment NEWLINE:'\r'? '\n';
WHITESPACE : ( ' ' | '\t' | NEWLINE )+ { $channel=HIDDEN; };
I am using ANTLR v3 with the ANTLR IDE Eclipse plugin. When I parse the expression (8 + a45) using the interpreter, only part of the parse tree is generated:
Why does the second term (a45) not get parsed? The same happens if both terms are numbers.
You'll want to create a parser rule that has an EOF (end of file) token in it so that the parser will be forced to go through the entire token stream.
Add this rule to your grammar:
parse
: arith EOF
;
and let the interpreter start at that rule instead of the arith rule:
Related
The grammar below is failing to generate a parser with this error:
error(119): Sable.g4::: The following sets of rules are mutually left-recursive [expression]
1 error(s)
The error disappears when I comment out the embedded actions, but comes back when I make them effective.
Why are the actions bringing about such left-recursion that antlr4 cannot handle it?
grammar Sable;
options {}
#header {
package org.sable.parser;
import org.sable.parser.internal.*;
}
sourceFile : statement* EOF;
statement : expressionStatement (SEMICOLON | NEWLINE);
expressionStatement : expression;
expression:
{System.out.println("w");} expression '+' expression {System.out.println("tf?");}
| IDENTIFIER
;
SEMICOLON : ';';
RPARENTH : '(';
LPARENTH : ')';
NEWLINE:
('\u000D' '\u000A')
| '\u000A'
;
WS : WhiteSpaceNotNewline -> channel(HIDDEN);
IDENTIFIER:
(IdentifierHead IdentifierCharacter*)
| ('`'(IdentifierHead IdentifierCharacter*)'`')
;
fragment DecimalDigit :'0'..'9';
fragment IdentifierHead:
'a'..'z'
| 'A'..'Z'
;
fragment IdentifierCharacter:
DecimalDigit
| IdentifierHead
;
// Non-newline whitespaces are defined apart because they carry meaning in
// certain contexts, e.g. within space-aware operators.
fragment WhiteSpaceNotNewline : [\u0020\u000C\u0009u000B\u000C];
UPDATE: the following workaround kind of solves this specific situation, but not the case when init/after-like actions are needed on a lesser scope - per choice, not per rule.
expression
#init {
enable(Token.HIDDEN_CHANNEL);
}
#after {
disable(Token.HIDDEN_CHANNEL);
}:
expression '+' expression
| IDENTIFIER
;
I have a grammar for arithmetic expression which solves number of expression (one per line) in a text file. While compiling YACC I am getting message 2 shift reduce conflicts. But my calculations are proper. If parser is giving proper output how does it resolves the shift/reduce conflict. And In my case is there any way to solve it in YACC Grammar.
YACC GRAMMAR
Calc : Expr {printf(" = %d\n",$1);}
| Calc Expr {printf(" = %d\n",$2);}
| error {yyerror("\nBad Expression\n ");}
;
Expr : Term { $$ = $1; }
| Expr '+' Term { $$ = $1 + $3; }
| Expr '-' Term { $$ = $1 - $3; }
;
Term : Fact { $$ = $1; }
| Term '*' Fact { $$ = $1 * $3; }
| Term '/' Fact { if($3==0){
yyerror("Divide by Zero Encountered.");
break;}
else
$$ = $1 / $3;
}
;
Fact : Prim { $$ = $1; }
| '-' Prim { $$ = -$2; }
;
Prim : '(' Expr ')' { $$ = $2; }
| Id { $$ = $1; }
;
Id :NUM { $$ = yylval; }
;
What change should I do to remove such conflicts in my grammar ?
Bison/yacc resolves shift-reduce conflicts by choosing to shift. This is explained in the bison manual in the section on Shift-Reduce conflicts.
Your problem is that your input is just a series of Exprs, run together without any delimiter between them. That means that:
4 - 2
could be one expression (4-2) or it could be two expressions (4, -2). Since bison-generated parsers always prefer to shift, the parser will choose to parse it as one expression, even if it were typed on two lines:
4
-2
If you want to allow users to type their expressions like that, without any separator, then you could either live with the conflict (since it is relatively benign) or you could codify it into your grammar, but that's quite a bit more work. To put it into the grammar, you need to define two different types of Expr: one (which is the one you use at the top level) cannot start with an unary minus, and the other one (which you can use anywhere else) is allowed to start with a unary minus.
I suspect that what you really want to do is use newlines or some other kind of expression separator. That's as simple as passing the newline through to your parser and changing Calc to Calc: | Calc '\n' | Calc Expr '\n'.
I'm sure that this appears somewhere else on SO, but I can't find it. So here is how you disallow the use of unary minus at the beginning of an expression, so that you can run expressions together without delimiters. The non-terminals starting n_ cannot start with a unary minus:
input: %empty | input n_expr { /* print $2 */ }
expr: term | expr '+' term | expr '-' term
n_expr: n_term | n_expr '+' term | n_expr '-' term
term: factor | term '*' factor | term '/' factor
n_term: value | n_term '+' factor | n_term '/' factor
factor: value | '-' factor
value: NUM | '(' expr ')'
That parses the same language as your grammar, but without generating the shift-reduce conflict. Since it parses the same language, the input
4
-2
will still be parsed as a single expression; to get the expected result you would need to type
4
(-2)
Background
I have been using ANTLRWorks (V 1.4.3) for a few days now and trying to write a simple Boolean parser. The combined lexer/parser grammar below works well for most of the requirements including support for quoted white-spaced text as operands for a Boolean expression.
Problem
I would like the grammar to work for white-spaced operands without the need of quotes.
Example
For example, expression-
"left right" AND center
should have the same parse tree even after dropping the quotes-
left right AND center.
I have been learning about backtracking, predicates etc but can't seem to find a solution.
Code
Below is the grammar I have got so far. Any feedback on the foolish mistakes is appreciated :).
Lexer/Parser Grammar
grammar boolean_expr;
options {
TokenLabelType=CommonToken;
output=AST;
ASTLabelType=CommonTree;
}
#modifier{public}
#ctorModifier{public}
#lexer::namespace{Org.CSharp.Parsers}
#parser::namespace{Org.CSharp.Parsers}
public
evaluator
: expr EOF
;
public
expr
: orexpr
;
public
orexpr
: andexpr (OR^ andexpr)*
;
public
andexpr
: notexpr (AND^ notexpr)*
;
public
notexpr
: (NOT^)? atom
;
public
atom
: word | LPAREN! expr RPAREN!
;
public
word
: QUOTED_TEXT | TEXT
;
/*
* Lexer Rules
*/
LPAREN
: '('
;
RPAREN
: ')'
;
AND
: 'AND'
;
OR
: 'OR'
;
NOT
: 'NOT'
;
WS
: ( ' ' | '\t' | '\r' | '\n') {$channel=HIDDEN;}
;
QUOTED_TEXT
: '"' (LETTER | DIGIT | ' ' | ',' | '-')+ '"'
;
TEXT
: (LETTER | DIGIT)+
;
/*
Fragment lexer rules can be used by other lexer rules, but do not return tokens by themselves
*/
fragment DIGIT
: ('0'..'9')
;
fragment LOWER
: ('a'..'z')
;
fragment UPPER
: ('A'..'Z')
;
fragment LETTER
: LOWER | UPPER
;
Simply let TEXT in your atom rule match once or more: TEXT+. When it matches a TEXT token more than once, you'll also want to create a custom root node for these TEXT tokens (I added an imaginary token called WORD in the grammar below).
grammar boolean_expr;
options {
output=AST;
}
tokens {
WORD;
}
evaluator
: expr EOF
;
...
word
: QUOTED_TEXT
| TEXT+ -> ^(WORD TEXT+)
;
...
Your input "left right AND center" would now be parsed as follows:
Below is a cut down version of a grammar that is parsing an input assembly file. Everything in my grammar is fine until i use labels that have 3 characters (i.e. same length as an OPCODE in my grammar), so I'm assuming Antlr is matching it as an OPCODE rather than a LABEL, but how do I say "in this position, it should be a LABEL, not an OPCODE"?
Trial input:
set a, label1
set b, abc
Output from a standard rig gives:
line 2:5 missing EOF at ','
(OP_BAS set a (REF label1)) (OP_SPE set b)
When I step debug through ANTLRWorks, I see it start down instruction rule 2, but at the reference to "abc" jumps to rule 3 and then fail at the ",".
I can solve this with massive left factoring, but it makes the grammar incredibly unreadable. I'm trying to find a compromise (there isn't so much input that the global backtrack is a hit on performance) between readability and functionality.
grammar TestLabel;
options {
language = Java;
output = AST;
ASTLabelType = CommonTree;
backtrack = true;
}
tokens {
NEGATION;
OP_BAS;
OP_SPE;
OP_CMD;
REF;
DEF;
}
program
: instruction* EOF!
;
instruction
: LABELDEF -> ^(DEF LABELDEF)
| OPCODE dst_op ',' src_op -> ^(OP_BAS OPCODE dst_op src_op)
| OPCODE src_op -> ^(OP_SPE OPCODE src_op)
| OPCODE -> ^(OP_CMD OPCODE)
;
operand
: REG
| LABEL -> ^(REF LABEL)
| expr
;
dst_op
: PUSH
| operand
;
src_op
: POP
| operand
;
term
: '('! expr ')'!
| literal
;
unary
: ('+'! | negation^ )* term
;
negation
: '-' -> NEGATION
;
mult
: unary ( ( '*'^ | '/'^ ) unary )*
;
expr
: mult ( ( '+'^ | '-'^ ) mult )*
;
literal
: number
| CHAR
;
number
: HEX
| BIN
| DECIMAL
;
REG: ('A'..'C'|'I'..'J'|'X'..'Z'|'a'..'c'|'i'..'j'|'x'..'z') ;
OPCODE: LETTER LETTER LETTER;
HEX: '0x' ( 'a'..'f' | 'A'..'F' | DIGIT )+ ;
BIN: '0b' ('0'|'1')+;
DECIMAL: DIGIT+ ;
LABEL: ( '.' | LETTER | DIGIT | '_' )+ ;
LABELDEF: ':' ( '.' | LETTER | DIGIT | '_' )+ {setText(getText().substring(1));} ;
STRING: '\"' .* '\"' {setText(getText().substring(1, getText().length()-1));} ;
CHAR: '\'' . '\'' {setText(getText().substring(1, 2));} ;
WS: (' ' | '\n' | '\r' | '\t' | '\f')+ { $channel = HIDDEN; } ;
fragment LETTER: ('a'..'z'|'A'..'Z') ;
fragment DIGIT: '0'..'9' ;
fragment PUSH: ('P'|'p')('U'|'u')('S'|'s')('H'|'h');
fragment POP: ('P'|'p')('O'|'o')('P'|'p');
The parser has no influence on what tokens the lexer produces. So, the input "abc" will always be tokenized as a OPCODE, no matter what the parser tries to match.
What you can do is create a label parser rules that matches either a LABEL or OPCODE and then use this label rule in your operand rule:
label
: LABEL
| OPCODE
;
operand
: REG
| label -> ^(REF label)
| expr
;
resulting in the following AST for your example input:
This will only match OPCODE, but will not change the type of the token. If you want the type to be changed as well, add a bit of custom code to the rule that changes it to type LABEL:
label
: LABEL
| t=OPCODE {$t.setType(LABEL);}
;
I am using ANTLR to create an and/or parser+evaluator. Expressions will have the format like:
x eq 1 && y eq 10
(x lt 10 && x gt 1) OR x eq -1
I was reading this post on logic expressions in ANTLR Looking for advice on project. Parsing logical expression and I found the grammar posted there a good start:
grammar Logic;
parse
: expression EOF
;
expression
: implication
;
implication
: or ('->' or)*
;
or
: and ('&&' and)*
;
and
: not ('||' not)*
;
not
: '~' atom
| atom
;
atom
: ID
| '(' expression ')'
;
ID : ('a'..'z' | 'A'..'Z')+;
Space : (' ' | '\t' | '\r' | '\n')+ {$channel=HIDDEN;};
However, while getting a tree from the parser works for expressions where the variables are just one character (ie, "(A || B) AND C", I am having a hard time adapting this to my case (in the example "x eq 1 && y eq 10" I'd expect one "AND" parent and two children, "x eq 1" and "y eq 10", see the test case below).
#Test
public void simpleAndEvaluation() throws RecognitionException{
String src = "1 eq 1 && B";
LogicLexer lexer = new LogicLexer(new ANTLRStringStream(src));
LogicParser parser = new LogicParser(new CommonTokenStream(lexer));
CommonTree tree = (CommonTree)parser.parse().getTree();
assertEquals("&&",tree.getText());
assertEquals("1 eq 1",tree.getChild(0).getText());
assertEquals("a neq a",tree.getChild(1).getText());
}
I believe this is related with the "ID". What would the correct syntax be?
For those interested, I made some improvements in my grammar file (see bellow)
Current limitations:
only works with &&/||, not AND/OR (not very problematic)
you can't have spaces between the parenthesis and the &&/|| (I solve that by replacing " (" with ")" and ") " with ")" in the source String before feeding the lexer)
grammar Logic;
options {
output = AST;
}
tokens {
AND = '&&';
OR = '||';
NOT = '~';
}
// parser/production rules start with a lower case letter
parse
: expression EOF! // omit the EOF token
;
expression
: or
;
or
: and (OR^ and)* // make `||` the root
;
and
: not (AND^ not)* // make `&&` the root
;
not
: NOT^ atom // make `~` the root
| atom
;
atom
: ID
| '('! expression ')'! // omit both `(` and `)`
;
// lexer/terminal rules start with an upper case letter
ID
:
(
'a'..'z'
| 'A'..'Z'
| '0'..'9' | ' '
| SYMBOL
)+
;
SYMBOL
:
('+'|'-'|'*'|'/'|'_')
;
ID : ('a'..'z' | 'A'..'Z')+;
states that an identifier is a sequence of one or more letters, but does not allow any digits. Try
ID : ('a'..'z' | 'A'..'Z' | '0'..'9')+;
which will allow e.g. abc, 123, 12ab, and ab12. If you don't want the latter types, you'll have to restructure the rule a little bit (left as a challenge...)
In order to accept arbitrarily many identifiers, you could define atom as ID+ instead of ID.
Also, you will likely need to specify AND, OR, -> and ~ as tokens so that, as #Bart Kiers says, the first two won't get classified as ID, and so that the latter two will get recognized at all.