[antlr-interest] String concatenation expression rule

[franck102]

Hi Bart, thanks very for the quick reply.

I should have made it clear that concatenating is only legal if at least one of the operands is a string literal - that is where I am having an issue. In other terms the input

3 3<EOF>
should cause a syntax error, while
3 "" 3<EOF>
should not (and evaluates to the string "33")


Franck



________________________________
 From: Bart Kiers [via ANTLR] <ml-node+s1301665n7008016h67 at n2.nabble.com>
To: franck102 <franck102 at yahoo.com> 
Sent: Friday, November 18, 2011 1:11 PM
Subject: Re: String concatenation expression rule
 

On Fri, Nov 18, 2011 at 12:39 PM, franck102 <[hidden email]> wrote: 


> I am writing a grammar for a fairly complex expression language, and in 
> particular I need to support string concatenation which is performed simply 
> by separating string literals with a space; and which automatically 
> converts 
> other expressions to a string if needed to concatenate: 
> "a" "b" -> "ab" 
> 2+3 "mm" -> "5mm" 
> 
> I suspect I could use predicates to write a rule like this: 
> 
> concatExpression 
>        :        ( expression | STRING_LITERAL )+ { apply only if at least 
> one of the elements is a string literal }? 
> 
> Is there a way to achieve this? The alternative formulations I can think of 
> are pretty messy... 
> 
> As far as I understand it, you don't need any predicate. I see a 
concat-expression has a lower precedence than addition, in which case this 
could do the trick: 

grammar T; 

options { 
  output=AST; 
} 

tokens { 
  ROOT; 
  CONCAT; 
} 

parse 
  :  (expression ';')* EOF -> ^(ROOT expression*) 
  ; 

expression 
  :  (add -> add) (add+ -> ^(CONCAT add+))? 
  ; 

add 
  :  atom (('+' | '-')^ atom)* 
  ; 

atom 
  :  Number 
  |  String 
  |  '(' expression ')' -> expression 
  ; 

Number : '0'..'9'+ ('.' '0'..'9'+)?; 
String : '"' ~'"'* '"'; 
Space  : ' ' {skip();}; 

You can test it with the following class: 

import org.antlr.runtime.*; 
import org.antlr.runtime.tree.*; 
import org.antlr.stringtemplate.*; 

public class Main { 
  public static void main(String[] args) throws Exception { 
    String src = "42 - 2; 2 + 3 \"mm\"; \"a\" \"b\" 4-3-2 \"c\"; \"pi = \" 
3.14159;"; 
    TLexer lexer = new TLexer(new ANTLRStringStream(src)); 
    TParser parser = new TParser(new CommonTokenStream(lexer)); 
    CommonTree root = (CommonTree)parser.parse().getTree(); ; 
    System.out.println(new DOTTreeGenerator().toDOT(root)); 
  } 
} 

Regards, 

Bart. 

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