Implementing Domain-Specific Languages with Xtext and Xtend

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 (including the initialization expression of a variable) 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:

i = j + 1
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 ...