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Comment by zem | Hacker News Reader

i think it's deeper than that. the progression is from object (every one hand-crafted) to template-based object-maker (hand-crafted per kind of object, but can produce a bunch of objects of that kind), to metatemplate-based template maker.

however, once you hit that second level of abstraction, you have almost invariably added enough flexibility that the range of templates your template-maker can make includes templates for other template makers. that is, it's not just a quirk of linguistics that a meta-meta-template-maker is called a meta-template-maker, they really do tend to be objects on the same level of abstraction and flexibility.

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We seem to be saying the same thing; I just don’t see any inherent meaning in it. A literal interpretation of meta- would have us define it like this:

    Instance ::= 0
    Meta(X)  ::= Succ(X)

But it’s much more convenient to define meta- recursively:

    Instance ::= 0
    Meta(X)  ::= Succ(X)
               | Meta(Succ(X))

In both cases, we have an arbitrary designation:

    Class    ::= Succ(Instance)

That kind of redundancy is a linguistic artifact, not a deeply meaningful one.

It’s not the case that reaching the second level of abstraction necessarily results in enough flexibility to make the inductive step. As a counterexample, consider C macros, which only add one meta- because they are only expanded once. Lisp macros, on the other hand, are expanded until a fixed point is found. That’s why Lisp macros are Turing-complete while the C preprocessor is only a pushdown automaton.

A bit of disagreement with your last paragraph: The (Common) Lisp macro processor is Turing-complete trivially because it invokes a Turing-complete language along the way, not because of power inherent in its expansion strategy.

Common Lisp macro bodies are written in Common Lisp, i.e. written in a Turing-complete language — they would still be Turing-complete even if we wrote them in the subset Lisp-without-macros. The C preprocessor, on the other hand, does not invoke a Turing-complete language to compute a macro's expansion.

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