CODEWITHSUNDEEP
parsinggrammarrecursive-descent
mtk358
mtk358

Reputation: 565

Converting a grammar to LL(1)

I have this grammar:

program ::= expr_list

expr_list ::= {LF} [expr {LF {LF} expr}] {LF}

lvalue ::= [expr DOT] NAME

call_param ::= [[NAME COLON] expr {COMMA [NAME COLON] expr}]

func_param ::= [NAME [COLON expr] {COMMA NAME [COLON expr]}]

expr ::= lvalue
       | lvalue ASSIGN expr
       | expr OPAREN call_param CPAREN
       | FUNC func_param LF expr_list END
       | IF expr LF expr_list {ELSEIF expr LF expr_list} [ELSE expr_list] ENDIF
       | WHILE expr LF expr_list LOOP
       | DO expr_list LOOP WHILE expr LF
       | INTEGER

I partially wrote a recursive descent parser:

void Parser::ntProgram()
{
    ntExprList();
}

void Parser::ntExprList()
{
    // ???
}

void Parser::ntLvalue()
{
    // ???
}

void Parser::ntCallParam()
{
    // ???
}

void Parser::ntFuncParam()
{
    if (accept(Lexer::NameTok)) {
        if (accept(Lexer::ColonTok)) {
            ntExpr();
        }
    }
    while (accept(Lexer::CommaTok)) {
        expect(Lexer::NameTok);
        if (accept(Lexer::ColonTok)) {
            ntExpr();
        }
    }
}

void Parser::ntExpr()
{
    if (accept(Lexer::FuncTok))
    {
        ntFuncParam();
        expect(Lexer::LfTok);
        ntExprList();
        expect(Lexer::EndTok);
    }
    else if (accept(Lexer::WhileTok))
    {
        ntExpr();
        expect(Lexer::LfTok);
        ntExprList();
        expect(Lexer::LoopTok);
    }
    else if (accept(Lexer::DoTok))
    {
        ntExprList();
        expect(Lexer::WhileTok);
        expect(Lexer::LoopTok);
        ntExpr();
        expect(Lexer::LfTok);
    }
    else if (accept(Lexer::IfTok))
    {
        ntExpr();
        expect(Lexer::LfTok);
        ntExprList();
        while (accept(Lexer::ElseifTok))
        {
            ntExpr();
            expect(Lexer::LfTok);
            ntExprList();
        }
        if (accept(Lexer::ElseTok))
        {
            ntExprList();
        }
        expect(Lexer::EndifTok);
    }
    else if (accept(Lexer::IntegerTok))
    {
    }
}

But I don't know what to do in some parts, for example the way that an expr can be an lvalue, whose first item could be an expr.

Upvotes: 0

Views: 2120

Answers (1)

ollb
ollb

Reputation: 1464

In order to be able to parse the expr rule, you have to eliminate left recursion first. This is well explained on Wikipedia:

http://en.wikipedia.org/wiki/Left_recursion

Upvotes: 1

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