ProgrammingLanguageNr1.Parser.ifThenElse C# (CSharp) Method

        private AST ifThenElse()
        {
            #if WRITE_DEBUG_INFO
            Console.WriteLine("if block");
            #endif
            AST ifThenElseTree;
            AST trueChild;
            AST falseChild = null;
            AST expr;

            try {
                Token ifToken = match(Token.TokenType.IF);
                expr = expression(); // child 0

                if (expr == null) {
                    throw new Error("The if statement is missing an expression after the 'if'", Error.ErrorType.SYNTAX, ifToken.LineNr, ifToken.LinePosition);
                }

                if(lookAheadType(1) == Token.TokenType.NEW_LINE) {
                    match(Token.TokenType.NEW_LINE);
                }
                else {
                    throw new Error("Found assignment (=) in if statement. Use == instead?", Error.ErrorType.SYNTAX, ifToken.LineNr, ifToken.LinePosition);
                }

                trueChild = statementList(false); // child 1

                if ((lookAheadType(1) == Token.TokenType.ELSE) && (lookAheadType(2) == Token.TokenType.IF)) {
            #if WRITE_DEBUG_INFO
            Console.WriteLine("if else block");
            #endif
                    match(Token.TokenType.ELSE);
                    var ifElseBranch = statement(); // ifThenElse();
                    // have to put it into a statement list to make scopes work
                    falseChild = new AST(new Token(Token.TokenType.STATEMENT_LIST, "<STATEMENT_LIST>"));
                    if(ifElseBranch != null) {
                        falseChild.addChild(ifElseBranch);
                    }
            #if WRITE_DEBUG_INFO
            Console.WriteLine("Popping out from ifElse branch");
            #endif
                }
                else if (lookAheadType(1) == Token.TokenType.ELSE) {
            #if WRITE_DEBUG_INFO
                    Console.WriteLine("else block");
            #endif
                    match(Token.TokenType.ELSE);

                    if(lookAhead(1).getTokenType() == Token.TokenType.NEW_LINE) {
                        match(Token.TokenType.NEW_LINE);
                    } else {
                        throw new Error("The else statement is missing a line break after it", Error.ErrorType.SYNTAX, ifToken.LineNr, ifToken.LinePosition);
                    }

                    falseChild = statementList(false); // child 2

                    if(lookAhead(1).getTokenType() == Token.TokenType.BLOCK_END) {
                        match(Token.TokenType.BLOCK_END);
                    } else {
                        throw new Error("The if statement is missing a following 'end'", Error.ErrorType.SYNTAX, ifToken.LineNr, ifToken.LinePosition);
                    }
                }
                else {
            #if WRITE_DEBUG_INFO
                    Console.WriteLine("no else block");
            #endif
                    if(lookAhead(1).getTokenType() == Token.TokenType.BLOCK_END) {
                        match(Token.TokenType.BLOCK_END);
                    } else {
                        throw new Error("The if statement is missing a following 'end'", Error.ErrorType.SYNTAX, ifToken.LineNr, ifToken.LinePosition);
                    }
                }
            }
            catch(Error e) {
                // The error caught here will probably be from the match() function.
                // Since that means we're missing some part of the if-statement we can give a better
                // error message by throwing a new one.
                throw e; // new Error("Something is wrong with the IF-statement", Error.ErrorType.SYNTAX, e.getLineNr(), e.getLinePosition());
            }

            ifThenElseTree = new AST_IfNode(new Token(Token.TokenType.IF, "if", lookAhead(1).LineNr, lookAhead(1).LinePosition));
            ifThenElseTree.addChild (expr);

            ifThenElseTree.addChild(trueChild);
            if (falseChild != null)
            {
                ifThenElseTree.addChild(falseChild);
            }

            return ifThenElseTree;
        }