Hacker News Reader

Comment by dllthomas | Hacker News Reader

"Turing-complete macro system - I've heard C++ is moving in this direction (not sure to be honest) but Lisp is still ahead on this"

1) Turing-complete is a bug, not a feature.

2) C++ template metaprogramming has been Turing Complete for ages.

3) There are nonetheless significant limitations on what you can do with templates in C++, compared to what you can do with macros in Lisp.

"Turing Complete" is a theoretical construct that doesn't really mean much other than giving you easy ways to prove that certain questions about the system can't always be answered. One significant reason for this is that "Turing Complete" merely means that you can embed any computation in the system in some encoding - with something like a macro system, the encoding you generate is the whole point.

View on HN

I understand what you're saying. What I mean is not the trivial "nand is turing complete" sense, which I agree is harmful and silly, but full eval capabilities. Lisp macros can fully evaluate Lisp code, and I consider this a great feature, but I don't know how else to describe them. "Full evaluation macros"?

I think "powerful" is probably sufficient :-P

To my mind, having eval available is a cheap way to make sure you are sufficiently powerful. In principle, it's not the best way. In practice, it's arguably the best way out of options at hand, though some alternatives compete.

> What I mean is not the trivial "nand is turing complete" sense, which I agree is harmful and silly, but full eval capabilities.

The two are equivalent (provided that the target language that can be evaluated is itself turing complete) -- that's rather the nature of turing completeness. If you have the former, you can make the latter with it and nothing else; if you have the latter, you necessarily also have the former.

Yes, it was a poor choice of terms. To be clear, eval isn't equivalent except within the narrow theoretical notion of turing completeness: it can do I/O, for instance file I/O. And it accepts high-level language code directly which is very important in a practical sense compared to, say, accepting nand instructions only.

It's hard to describe because even "full access to the language at compile time" would be useless if that language were C++. The advantage of macros is that you have access specifically to Lisp at compile time, not just any old blub language. And that just circles back to "Lisp is great". :)

Reply on news.ycombinator.com