Forward references

You should know by now that parsing is only the first stage when implementing a DSL and that it cannot detect all the errors from the programs. We need to implement additional checks in a validator.

One important thing we need to check in our Expressions DSL is that an expression does not refer to a variable defined after the very expression. Using an identifier before its declaration is usually called a forward reference.

Therefore, this program should not be considered valid:

var i = j + 1
var j = 0

Since the initialization expression of i refers to j, which is defined after. Of course, this is a design choice. Since we want to interpret the expressions, it makes sense to interpret them in the order they are defined.

This strategy also avoids possible mutual dependency problems:

var i = j + 1
var j = i + 1

A variable which is initialized referring to itself is a special case of the preceding:

var i = i + 1

We want to avoid this because our interpreter would enter an endless...