In SICP, there's a common concept recurrent all throughout the book, when talking about the design of (powerful) programming languages. It first appeared in Section 1.1.

Every powerful language has three mechanisms (...):

• primitive expressions, which represent the simplest entities the language is concerned with

• means of combination, by which compound elements are built from simpler ones

• means of abstraction, by which compound elements can be named and manipulated as units

This categorization is omnipresent all throughout the book and I'm not sure I get it entirely; I am hoping members here with strong CS background can help me out with this.

Primitives

I understand the first point about primitives. I have a vague idea of the subtleties it introduces, and what it may imply. In the words of an unsung WWW hero:

Gilles wroteWhen you get to languages with more features, the modeling can become more ambiguous. Generally speaking, a primitive expression is one that you can't or won't break down into primitive components. But it's like the atom: it's primitive until science marches on. For example, there is a variant of the lambda-calculus in which variables use numbers rather than names (de Bruijn indices), which is particularly convenient when modeling lambda-terms for computer proofs; and in computer proofs, integers are broken down into constituent parts. So in these models, variables aren't primitive expressions after all.

The part about Atoms getting broken down in smaller parts must particularly ring true to scientists here.

Combination vs abstraction

This is where my head explodes. I can understand the distinction between the abstract concepts, but it doesn't tell me, for instance, if the class mechanism of Java is a mean of combination or a mean of abstraction. After all, isn't combination done for abstraction purposes? And, isn't abstraction achieved (almost) exclusively through combination of other elements?

To me, it just feels that the two overlap so much that making the distinction at a definition level is more confusing than anything else.

Can anyone please help me make sense of all this maddness?