I have the following grammar rules defined to cover tuples of the form: (a), (a,), (a,b), (a,b,) and so on. However, antlr3 gives the warning:
"Decision can match input such as "COMMA" using multiple alternatives: 1, 2
I believe this means that my grammar is not LL(1). This caught me by surprise as, based on my extremely limited understanding of this topic, the parser would only need to look one token ahead from (COMMA)? to ')' in order to know which comma it was on.
Also based on the discussion I found here I am further confused: Amend JSON - based grammar to allow for trailing comma
And their source code here: https://github.com/doctrine/annotations/blob/1.13.x/lib/Doctrine/Common/Annotations/DocParser.php#L1307
Is this because of the kind of parser that antlr is trying to generate and not because my grammar isn't LL(1)? Any insight would be appreciated.
options {k=1; backtrack=no;}
tuple : '(' IDENT (COMMA IDENT)* (COMMA)? ')';
DIGIT : '0'..'9' ;
LOWER : 'a'..'z' ;
UPPER : 'A'..'Z' ;
IDENT : (LOWER | UPPER | '_') (LOWER | UPPER | '_' | DIGIT)* ;
edit: changed typo in tuple: ... from (IDENT)? to (COMMA)?
Note:
The question has been edited since this answer was written. In the original, the grammar had the line:
tuple : '(' IDENT (COMMA IDENT)* (IDENT)? ')';
and that's what this answer is referring to.
That grammar works without warnings, but it doesn't describe the language you intend to parse. It accepts, for example, (a, b c) but fails to accept (a, b,).
My best guess is that you actually used something like the grammars in the links you provide, in which the final optional element is a comma, not an identifier:
tuple : '(' IDENT (COMMA IDENT)* (COMMA)? ')';
That does give the warning you indicate, and it won't match (a,) (for example), because, as the warning says, the second alternative has been disabled.
LL(1) as a property of formal grammars only applies to grammars with fixed right-hand sides, as opposed to the "Extended" BNF used by many top-down parser generators, including Antlr, in which a right-hand side can be a set of possibilities. It's possible to expand EBNF using additional non-terminals for each subrule (although there is not necessarily a canonical expansion, and expansions might differ in their parsing category). But, informally, we could extend the concept of LL(k) by saying that in every EBNF right-hand side, at every point where there is more than one alternative, the parser must be able to predict the appropriate alternative looking only at the next k tokens.
You're right that the grammar you provide is LL(1) in that sense. When the parser has just seen IDENT, it has three clear alternatives, each marked by a different lookahead token:
COMMA ↠ predict another repetition of (COMMA IDENT).
IDENT ↠ predict (IDENT).
')' ↠ predict an empty (IDENT)?.
But in the correct grammar (with my modification above), IDENT is a syntax error and COMMA could be either another repetition of ( COMMA IDENT ), or it could be the COMMA in ( COMMA )?.
You could change k=1 to k=2, thereby allowing the parser to examine the next two tokens, and if you did so it would compile with no warnings. In effect, that grammar is LL(2).
You could make an LL(1) grammar by left-factoring the expansion of the EBNF, but it's not going to be as pretty (or as easy for a reader to understand). So if you have a parser generator which can cope with the grammar as written, you might as well not worry about it.
But, for what it's worth, here's a possible solution:
tuple : '(' idents ')' ;
idents : IDENT ( COMMA ( idents )? )? ;
Untested because I don't have a working Antlr3 installation, but it at least compiles the grammar without warnings. Sorry if there is a problem.
It would probably be better to use tuple : '(' (idents)? ')'; in order to allow empty tuples. Also, there's no obvious reason to insist on COMMA instead of just using ',', assuming that '(' and ')' work as expected on Antlr3.
I'm trying to parse 'for loop' according to this (partial) grammar:
grammar GaleugParserNew;
/*
* PARSER RULES
*/
relational
: '>'
| '<'
;
varChange
: '++'
| '--'
;
values
: ID
| DIGIT
;
for_stat
: FOR '(' ID '=' values ';' values relational values ';' ID varChange ')' '{' '}'
;
/*
* LEXER RULES
*/
FOR : 'for' ;
ID : [a-zA-Z_] [a-zA-Z_0-9]* ;
DIGIT : [0-9]+ ;
SPACE : [ \t\r\n] -> skip ;
When I try to generate the gui of how it's parsed, it's not following the grammar I provided above. This is what it produces:
I've encountered this problem before, what I did then was simply exit cmd, open it again and compile everything and somehow that worked then. It's not working now though.
I'm not really very knowledgeable about antlr4 so I'm not sure where to look to solve this problem.
Must be a problem of the IDE you are using. The grammar is fine and produces this parse tree in Visual Studio Code:
I guess the IDE is using the wrong parser or lexer (maybe from a different work file?). Print the lexer tokens to see if they are what you expect. Hint: avoid defining implicit lexer tokens (like '(', '}' etc.), which will allow to give the tokens good names.
I am learning how to use the "more" lexer command. I typed in the lexer grammar shown in the ANTLR book, page 281:
lexer grammar Lexer_To_Test_More_Command ;
LQUOTE : '"' -> more, mode(STR) ;
WS : [ \t\r\n]+ -> skip ;
mode STR ;
STRING : '"' -> mode(DEFAULT_MODE) ;
TEXT : . -> more ;
Then I created this simple parser to use the lexer:
grammar Parser_To_Test_More_Command ;
import Lexer_To_Test_More_Command ;
test: STRING EOF ;
Then I opened a DOS window and entered this command:
antlr4 Parser_To_Test_More_Command.g4
That generated this warning message:
warning(155): Parser_To_Test_More_Command.g4:3:29: rule LQUOTE
contains a lexer command with an unrecognized constant value; lexer
interpreters may produce incorrect output
Am I doing something wrong in the lexer or parser?
Combined grammars (which are grammars that start with just grammar, instead of parser grammar or lexer grammar) cannot use lexer modes. Instead of using the import feature¹, you should use the tokenVocab feature like this:
Lexer_To_Test_More_Command.g4:
lexer grammar Lexer_To_Test_More_Command;
// lexer rules and modes here
Parser_To_Test_More_Command.g4:
parser grammar Parser_To_Test_More_Command;
options {
tokenVocab = Lexer_To_Test_More_Command;
}
// parser rules here
¹ I actually recommend avoiding the import statement altogether in ANTLR. The method I described above is almost always preferable.
I recently started studying ANTLR. Below is the grammar for the arithmetic expression.
The problem is that when I am putting (calling) expression rule in the term rule then it is parsing incorrectly even for (9+8). It is somehow ignoring the right parenthesis.
While when I put add rule instead of calling expression rule from the rule term, it is working fine.
As in:
term:
INTEGER
| '(' add ')'
;
Can anyone tell me why it is happening because more or les they both are the same.
Grammer for which it is giving incorrect results
term
:
INTEGER
| '(' expression ')'
;
mult
:
term ('*' term)*
;
add
:
mult ('+' mult)*
;
expression
:
add
;
When I parse "(8+9)" with a parser generated from your grammar, starting with the expression rule, I get the following parse tree:
In other words: it works just fine.
Perhaps you're using ANTLRWorks' (or ANTLR IDE's) interpreter to test your grammar? In thta case: don't use the interpreter, it's buggy. Use ANTLRWorks' debugger instead (the image is exported from ANTLRWorks' debugger).
Inconvenience in specifying grammars - we cannot factor out bin_op in following example (Bison):
expr : expr bin_op expr ;
bin_op : Add
| Mul
;
because of shift/reduce conflicts. Is there parsing technique or parser generator which allows such thing?
Yes
Yacc has been employed to parse almost every real programming language and some other types. I'm not nearly its biggest fan but it can certainly parse that grammar or perhaps an equivalent that is written with yacc in mind.
And shift/reduce conflicts are not errors.