I have a grammar with some ambuguities I need to resolve.
One of the rules takes the following form:
TArg:
anys=Anys
| rnumb1=PNumb ".." (rnumb2=PNumb)?
;
Or this image, if you prefer
The rule Anys has the potential to start with a PNumb. I can see where the ambiguity is, but how to I tell XText to take the second path if it sees a PNumb followed by the double dot?
Presumably, if I use
TArg:
(=> rnumb1=PNumb ".." (rnumb2=PNumb)?)
|anys=Anys
;
Then it will always choose the first if it sees a number, regargless of if it sees the "..", and I will run into problems.
What is the correct usage/placement of the syntactic predicate here to allow Antlr to look ahead to see if the ".." is present?
Cheers in advance.
You need to also include the '..'
TArg:
=>(rnumb1=PNumb "..") (rnumb2=PNumb)?
| anys=Anys
;
Related
I dont have any prior knowledge about ANTLR(I recently learned a little bit about ANTLR4), but I have to translate an old grammar to a newer version and eclipse is telling me, that their are no viable alternatives for those characters and shows the syntax error " '!' came as a complete surprise to me".
I already deleted those characters and it does not seam to be a problem, but maybe it had a special function in ANTLR3.
Thanks in advance.
global_block:
DATABASE! IDENTIFIER!
| GLOBALS! define_section!+ END! GLOBALS!
| GLOBALS! STRING!
;
main_block: MAIN sequence? END em=MAIN
-> ^(MAIN MAIN '(' ')' sequence? $em)
;
^ and -> are related to tree rewriting: https://theantlrguy.atlassian.net/wiki/spaces/ANTLR3/pages/2687090/Tree+construction
ANTLR4 does not support it (v4 has listeners and visitors for tree traversal, but no rewriting anymore). Just remove all of these ! and -> ... in parser rules (do not remove the -> ... inside lexer rules like -> channel(...), which is still supported in v4).
So in your case, these rules would be valid in ANTLR4:
global_block
: DATABASE IDENTIFIER
| GLOBALS define_section+ END GLOBALS
| GLOBALS STRING
;
main_block
: MAIN sequence? END MAIN
;
The $ can still be used in ANTLR4: they are used to reference sub-rules or tokens:
expression
: lhs=expression operator=(PLUS | MINUS) rhs=expression
| NUMBER
;
so that in embedded code block, you can do: $lhs.someField.someMethod(). In your case, you can also just remove them because they are probably only used in the tree rewrite rules.
EDIT
kaby76 has a Github page with some instructions for converting grammars to ANTLR4: https://github.com/kaby76/AntlrVSIX/blob/master/doc/Import.md#antlr3
I'm trying to match the following phrase:
<svg/onload="alert(1);">
And I need the tokens to be like:
'<svg', 'onload="alert(1);", '>'
So basically I need to skip the / in the <svg/onload part. But the skip phrase is not allowed here:
Attribute
: ('/' -> skip) Identifier '=' StringLiteral?
;
The error was
error(133): HTML.g4:35:11: ->command in lexer rule Attribute must be last element of single outermost alt
Any ideas?
The error message pretty much tells you what the problem is. The skip command has to be at the end of the rule. You cannot skip intermediate tokens, but only entire rules.
However, I wonder why you want to skip the slash. Why not just let the lexer scan everything (it has to anyway) and then ignore the tokens you don't need? Also I wouldn't use a lexer rule, but a parser rule, to allow arbitrary spaces between elements.
Try lexer's setText(getText().replace("/", "")) or any other matched string manipulation
Currently working with ANTLR and found out something interesting that's not working as I intended.
I try to run something along the lines of "test 10 cm" through my grammar and it fails, however "test 10 c m" works as the previous should. The "cm" portion of the code is what I call "wholeunit" in my grammar and it is as follows:
wholeunit :
siunit
| unitmod siunit
| wholeunit NUM
| wholeunit '/' wholeunit
| wholeunit '.' wholeunit
;
What it's doing right now is the "unitmod siunit" portion of the rule where unitmod = c and siunit = m .
What I'd like to know is how would I make it so the grammar would still follow the rule "unitmod siunit" without the need for a space in the middle, I might be missing something huge. (Yes, I have spaces and tabs marked to be skipped)
Probable cause is "cm" being considered another token together (possibly same token type as "test"), rather than "c" and "m" as separate tokens.
Remember that in ANTLR lexer, the rule matching the longest input wins.
One solution would possibly be to make the wholeunit a lexer rule rather than parser rule, and make sure it's above the rule that matches any word (like "test") - if same input can be matched by multiple rules, ANTLR selects the first rule in order they're defined in.
So i am writing a small language and i am using antlrv4 as my tool. Antlr autogenerates lexer and parser files when u compile your grammar file(.g4). I am using javac btw. I want my language to have no semicolons and the way i want to do this is: if there is an identifier or ")" as the last token in a line, the lexer will automatically put the semicolon(Similar to what "go" language does). How would i approach something like this? There are other things like ATN(which i think is augmented transition network) and dfa(which i think is deterministic finite automaton) in the lexer file which i don't understand or how they relate to the lexing process?. Any help is appreciated. (btw i am still working on the grammar file so i don't have that fully completed).
Several points here: the ATN and the DFA are internal structures for parser + lexer and not something you would touch to change parsing behavior. Also, it's not clear to me why you want to have the lexer insert a semicolon at some point. What exactly do you want to accomplish by that (don't say: to make semicolons optional in the parser, I mean the underlying reason).
If you want to accept a command without a trailing semicolon you can make that optional:
assignment: simpleAssignment | complexAssignment SEMI?;
The parser will give you the content of the assignment rule regardless whether there is a trailing semicolon or not. Is that what you want?
I am upgrading my Antlr grammar file to latest Antlr4.
I have converted most of the file but stuck in syntax difference that I can't figure out. The 3 such difference is:
equationset: equation* EOF!;
equation: variable ASSIGN expression -> ^(EQUATION variable expression)
;
orExpression
: andExpression ( OR^ andExpression )*
;
In first one, the error is due to !. I am not sure whether EOF and EOF! is same or not. Removing ! resolves the error, but I want to be sure that is the correct fix.
In 2nd rule, -> and ^ is giving error. I am not sure what is Antlr4 equivalent.
In 3rd rule, ^ is giving error. Removing it fixes the error, but I can't find any migration guide that explains what should be equivalent for this.
Can you please give me the Antrl4 equivalent of these 3 rules and give some brief explanation what is the difference? If you can refer to any other resource where I can find the answer is OK as well.
Thanks in advance.
Many of the ANTLR3 grammars contain syntax tree manipulations which are no longer supported with ANTLR4 (now we get a parse tree instead of a syntax tree). What you see here is exactly that.
EOF! means EOF should be matched but not appear in the AST. Since there is no AST anymore you cannot change that, so remove the exclamation mark.
The construct -> ^(EQUATION variable expression) rewrites the AST created by the equation rule. Since there is no AST anymore you cannot change that, so remove that part.
OR^ finally determines that the OR operator should become the root of the generated AST. Since there is no AST anymore ..., you got the point now :-)