//this implements pratt parsing(top down recursive parsing)
//DaBeaz's course implemented this with loop instead of recursion (so although top down, it is not recursive). Look in "Solution Items/stuff" folder
//for snapshots showing his approach. Not only is it not recursive(he uses loops), but he has also created a func for each precendence level.
//his approach is more readable than what we have here.
private AST_Helper.Expression parseExpression(Parser_Helper.precedence precedence)
{
if (!this.prefixParseFns.ContainsKey(this.curToken.Type))
{
noPrefixParseFnError(this.curToken.Type);
return(null);
}
Parser_Helper.prefixParserFn prefixParserFn = this.prefixParseFns[this.curToken.Type];
AST_Helper.Expression leftExpr = prefixParserFn();
while (!this.peekTokenIs(TokenHelper.TokenType.SEMICOLON) && precedence < this.peekPrecedence())
{
if (!this.infixParseFns.ContainsKey(this.peekToken.Type))
{
return(leftExpr);
}
Parser_Helper.infixParseFn infixParseFn = this.infixParseFns[this.peekToken.Type];
this.nextToken();
leftExpr = infixParseFn(leftExpr);//construct a new leftExpr expression using current leftExpr and next expr as rightExpr
}
return(leftExpr);
}
private AST_Helper.Expression parseInfixExpression(AST_Helper.Expression left)
{
InfixExpression infixExpression = new InfixExpression {
Token = this.curToken, Operator = this.curToken.Literal, Left = left
};
Parser_Helper.precedence precedence = this.curPrecedence();//precedence level of current token
this.nextToken();
//recursice call back to parseExpression. Control arrived here from parseExpression which we are now calling back
infixExpression.Right = this.parseExpression(precedence);//it is here we pass precedence using the current context
return(infixExpression);
}
