Question

I’m trying to implement a parser by directly reading a treeWalker and implementing the commands needed for the compiler on the fly. So if I have a command like:

 statement
            :
            ^('WRITE' expression) 
            { 
                //Here is the command that is created by my Tree Parser
                ch.emitRO("OUT",0,0,0,"write out the value of ac");
                //and then I handle it in my other classes
            }
;

I want it to write OUT 0,0,0; to a file. That’s my grammar.

I have a problem though with the loop section in my grammar it is:

'WHILE'^ expression 'DO' stat_seq 'ENDDO' 

and in the tree parser:

doWhileStatement
:
^('WHILE' expression 'DO' stat_seq 'ENDDO')
;

What I want to do is directly parse the code from the while loop into the commands I need. I came up with this solution but it doesn’t work:

doWhileStatement
        :
        ^('WHILE' e=expression head='DO'
            {
                int loopHead =((CommonTree) head).getTokenStartIndex();

            }

            stat_seq

            {
                if ($e.result==1) {
                     input.seek(loopHead);
                     doWhileStatement();
                }
            }
         'ENDDO')
; 

for the record here are some of the other commands I’ve written:
(ignore the code written in brackets, it’s for the generation of the commands in a text file.)

    stat_seq
            :
            (statement)+
            ;
statement
        :
        ^(':=' ID e=expression) { variables.put($ID.text,e); }
        | ^('WRITE' expression) 
        { 
            ch.emitRM("LDC",ac,$expression.result,0,"pass the expression value to the ac reg");
            ch.emitRO("OUT",ac,0,0,"write out the value of ac");
        }
        | ^('READ' ID)
        {
            ch.emitRO("IN",ac,0,0,"read value");
        }
        | ^('IF' expression 'THEN'
        {
            ch.emitRM("LDC",ac1,$expression.result,0,"pass the expression result to the ac reg");
            int savedLoc1 = ch.emitSkip(1);
        }
        sseq1=stat_seq  
        'ELSE'
        {
            int savedLoc2 = ch.emitSkip(1);
            ch.emitBackup(savedLoc1);
            ch.emitRM("JEQ",ac1,savedLoc2+1,0,"skip as many places as needed depending on the expression");
            ch.emitRestore();
        }
         sseq2=stat_seq
         {
            int savedLoc3 = ch.emitSkip(0);
            ch.emitBackup(savedLoc2);
            ch.emitRM("LDC",PC_REG,savedLoc3,0,"skip for the else command");
            ch.emitRestore();
         }

          'ENDIF')
        | doWhileStatement
        ;

Any help would be appreciated, thank you

Answer 1

I found it for everyone who has the same problem I did it like this and it’s working:

^('WHILE'  

        {int c = input.index();}
        expression   

        {int s=input.index();}
        .* )// .* is a sequence of statements

        {
        int next = input.index(); // index of node following WHILE
        input.seek(c);
        match(input, Token.DOWN, null); 
        pushFollow(FOLLOW_expression_in_statement339);
        int condition = expression();

        state._fsp--;            
        //there is a problem here
        //expression() seemed to be reading from the grammar file and I couldn't
        //get it to read from the tree walker rule somehow
        //It printed something like no viable alt at input 'DOWN'
        //I googled it and found this mistake
        // So I copied the code from the normal while statement
        // And pasted it here and it works like a charm
        // Normally there should only be int condition = expression()   

        while ( condition == 1 ) {
            input.seek(s);
            stat_seq();//stat_seq is a sequence of statements: (statement ';')+
            input.seek(c);

            match(input, Token.DOWN, null); //Copied value from EvaluatorWalker.java
            //cause couldn't find another way to do it
            pushFollow(FOLLOW_expression_in_statement339);
            condition = expression();

            state._fsp--;
            System.out.println("condition:"+condition + " i:"+ variables.get("i"));
        }
        input.seek(next);
     }

I wrote the problem at the comments of my code. If anyone can help me out and answer this for me how to do it I would be grateful. It’s so weird that there is nearly no feedback on a correct way to implement loops within a tree grammar on the fly.

Regards,
Alex