This is the sixth part of the YAUL series. For your convenience you can find other parts in the table of contents in Part 1 — Introduction

We continue developing custom language on .NET platform using LINQ expressions. Today we are going to implement conditional operator.


We start with PLY grammar. Code for if is pretty simple:

We define if as keyword followed by condition in parenthesis, and statements. We need to handle matching else block. This part is a common source of ambiguity in parsers — imagine the following code:

Code is poorly formatted deliberately. We have two ifs and only one else — the question is: is the else clause matching first or second if? This problem occurs in most of today’s languages and is usually solved by assumption that the else matches closes if. So we should parse this code as:

and not as:

In grammar’s terms this is known as reduce-reduce conflict.

Let’s move on. We need to parse conditions:

This part is pretty easy. We handle common relational operators making a binary operation.

C# part

Binary operation was described in previous part. Now code for if:

Nothing difficult here. We have only two cases to handle, if with matching else or without. We also need to recursively find labels in blocks.


We are now able to handle conditions in YAUL. Next time we will examine basic loop concept.