I need a Xtext grammar rule (or multiple) working similar to the following:
1: CollectionGetElement:
2: val=[VariableReference] '='
3: (ref=[List] | ref=[Bytefield] | ref=[Map])
4: '[' keys+=GetElementKeyType ']' ('[' keys+=GetElementKeyType ']')* ';';
5: GetElementKeyType:
6: key=[VariableReference] | INT | STRING;
Like this unfortuantely it doesn't work, because of the 3 line!
I also tried 3 seperated rules (for: map, list and bytefield), but then It's difficult (impossible) for the parser to recognize the correct rule.
ListGetElement:
val=[VariableReference] '='
ref=[List]
'[' key+=GetElementKeyType ']' ('[' key+=GetElementKeyType ']')* ';';
... same for the others
Error then is:
Decision can match input such as "RULE_ID '=' RULE_ID '[' RULE_ID ']' '[' RULE_ID ']' ';'" using multiple alternatives: 5, 6
The following alternatives can never be matched: 6
What's the best way to achive this?
there are two problems in your grammar,
assigning 3 different types to attribute 'ref'
generating 3 different types by parsing some ID
I am not sure what do you want to do. But, I can give you an example. Hope it can help you.
e.g.
List:
'list' '(' elements += Element * ')';
Map:
'map' '(' pairs += Pair * ')';
GeneralDataType:
List | Map
CollectionGetElement:
val=[VariableReference] '='
ref = GeneralDataType
;
Related
I'm making a Lisp parser and I wanted to accept the following line:
(defvar a [+ 2 1] )
But not this one:
(defvar a [+ 2 1) ]
The solution i came up with was something like this:
define_var: ('(' KW_DEFVAR ID expression ')' ) | ('[' KW_DEFVAR ID expression ']');
It works, but for longer expressions this is not viable.
You could move the tokens/rules in common in a sub rule like this:
define_var
: '(' define_var_subrule ')'
| '[' define_var_subrule ']'
;
define_var_subrule
: KW_DEFVAR ID expression
;
I m trying to create a list in xtext, If anybody can help me to create a grammar for that, it will be really helpful. I tried writing this but its not the xtext format so i getting errors on that.
List:
'List' name=ID type = Nlist;
Nlist:
Array | Object
;
Array:
{Array} "[" values*=Value[','] "]"
;
Value:
STRING | FLOAT | BOOL | Object | Array | "null"
;
Object:
"{" members*=Member[','] "}"
;
Member:
key=STRING ':' value=Value
I m new to this one, Any help will be appreciated.
Thank you.
the default syntax for comma separated lists is e.g.
MyList: '#[' (elements+=Element (',' elements+=Element )*)? ']';
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 using bison to write a parser to a C-like grammar, but I'm having problem in variable declaration.
My variables can be simples variable, arrays or structs, so I can have a variable like a.b[3].c.
I have the following rule:
var : NAME /* NAME is a string */
| var '[' expr ']'
| var '.' var
;
which are giving me a shift/reduce conflict, but I can't think of a way to solve this.
How can I rewrite the grammar or use Bison precedence to resolve the problem?
I don't know why you want the . and [] operators to be associative, however something like this might inspire you.
y
%union{}
%token NAME
%start var
%left '.'
%left '['
%%
var:
NAME
| var '[' expr ']'
| var '.' var
Also, as var probably appears in expr, there may be other problems with the grammar.
It's certainly possible to resolve this shift-reduce conflict with a precedence declaration, but in my opinion that just makes the grammar harder to read. It's better to make the grammar reflect the semantics of the language.
A var (what some might call an lvalue or a reference) is either the name of a simple scalar variable or an expression built up iteratively from a var naming a complex value and a selector, either a member name or an array index. In other words:
var : NAME
| var selector
;
selector: '.' NAME
| '[' expression ']'
;
This makes it clear what the meaning of a.b[3].c is, for example; the semantics are described by the parse tree:
a.b[3].c
+----+----+
| |
a.b[3] .c
+---+--+
| |
a.b [3]
+-+-+
| |
a .b
It's not necessary to create the selector rule; it would be trivial to wrap the two rules together. If you don't feel that it is clearer as written above, the following will work just as well:
var: NAME
| var '.' NAME
| var '[' expression ']'
;
I have a YACC grammar for parsing expressions in C++. Here is lite version:
// yacc.y
%token IDENT
%%
expr:
call_expr
| expr '<' call_expr
| expr '>' call_expr
;
call_expr:
IDENT
| '(' expr ')'
| IDENT '<' args '>' '(' args ')'
;
args:
IDENT
| args ',' IDENT
;
%%
When I want to support function call with template arguments, I got a shift/reduce conflict.
When we got input IDENT '<' IDENT, yacc doesn't know whether we should shift or reduce.
I want IDENT '<' args '>' '(' args ')' got higher precedence level than expr '<' call_expr, so I can parse the following exprs.
x < y
f<x>(a,b)
f<x,y>(a,b) < g<x,y>(c,d)
I see C++/C# both support this syntax. Is there any way to solve this problem with yacc?
How do I modify the .y file?
Thank you!
You want the -v option to yacc/bison. It will give you a .output file with all the info about the generated shift/reduce parser. With your grammar, bison gives you:
State 1 conflicts: 1 shift/reduce
:
state 1
4 call_expr: IDENT .
6 | IDENT . '<' args '>' '(' args ')'
'<' shift, and go to state 5
'<' [reduce using rule 4 (call_expr)]
$default reduce using rule 4 (call_expr)
which shows you where the problem is. After seeing an IDENT, when the next token is <, it doesn't know if it should reduce that call_expr (to ultimately match the rule expr: expr '<' call_expr) or if it should shift to match rule 6.
Parsing this with only 1 token lookahead is hard, as you have two distinct meanings of the token < (less-than or open angle bracket), and which is meant depends on the later tokens.
This case is actually even worse, as it is ambiguous, as an input like
a < b > ( c )
is probably a template call with both args lists being singletons, but it could also be
( a < b ) > ( c )
so just unfactoring the grammar won't help. Your best bet is to use a more powerful parsing method, like bison's %glr-parser option or btyacc