Chapter 4, 2nd page (in my edition)

"Metalinguistic abstraction -- establishing new languages -- plays an important role in all branches of engineering design. It is particularly important to computer programming, because in programming not only can we formulate new languages but we can also implement these languages by constructing evaluators. An evaluator (or interpreter) for a programming language is a procedure that, when applied to an expression of the language, performs the actions required to evaluate that expression.

It is no exaggeration to regard this as the most fundamental idea in programming:

The evaluator, which determines the meaning of expressions in a programming language, is just another program.

To appreciate this point is to change our images of ourselves as programmers. We come to see ourselves as designers of languages, rather than only users of languages designed by others.

In fact, we can regard almost any program as the evaluator for some language."

I didn't understand that last sentence at all. The authors give some examples(a constraint propagation framework etc) from earlier chapters, which they say are languages with their own primitives etc.

But the claim seems to be more generic (emphasis mine) "we can regard almost any program as the evaluator for some language"

I am having some trouble wrapping my head around that. What "language" would a typical web app be viewed as? What are its semantics?

Let us say I have an inventory tracking web app program which deals with, say users, warehouses, orders etc. How is this "viewable as a language"? How do I go about writing an evaluator for this "language"?

Where does a program stop being "viewable as a language" If I write a small program (say to calculate Fibonacci numbers, can that be "viewed as a language"?

I am missing something very profound here. If anyone has any insights, I would be very grateful.

Help?