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Comment by Slackwise | Hacker News Reader

Whenever I want someone to understand Lisp being a homoiconic metaprogramming language, I link them this:

http://www.defmacro.org/ramblings/lisp.html

maybe someone can help me understand. I now get homoiconisity but the example he uses doesn't seem to explain why the macro is needed. He explains a problem that can be solved by writing a function. Instead he presents what seems to be in effect an unevaluated function... so what's the point?

I agree that the macro example is not chosen well. Macros are just functions that take their arguments unevaluated and output code, so each macro call could be replaced by a function call where the arguments are wrapped in a list and the function executes the code directly instead of generating it.

Then the only reason you'd have to use macros is if you want to do something at compile time. Like generating specialized code so the compiler can optimize it, instead of reinterpreting the describing data on each invocation. Or like reading the database schema to generate accessors.

But as the article states, compile time is (or can be) at runtime.. so it's a bit difficult for me to wrap my head around its practical effect :)

Lisp has several time phases that may or may not interleave, so yeah it can be confusing.. but keeping things simple and mostly only thinking about macros at compile time or read time can still go a long way. Ultimately an important and practical high level effect is macros let you create new syntax, not just computation at compile time to save some function calls at runtime. Want infix math? You can have infix math: https://github.com/rigetticomputing/cmu-infix Going crazy with lexical nesting (similar to callback hell, but imagine each callback needs to close over the environment it's called from, not just defined in)? A one-liner might help: https://fare.livejournal.com/189741.html Want powerful looping constructs, powerful OOP, pattern matching, etc? Of course you can stick with simple macros that are little more than function wrappers, but you can do a lot more.

Macro examples are often poor, and my advice is to always favour functions, since then you are working with actual values. For instance, with-open-file should be a higher order function, not a macro as it is in CL.

That leaves a few other uses: macros as compiler extensions for generating fast code (see "Paradigms in Common LISP" for a lovely example of a parser generator). But heed warnings about premature optimisation.

It leaves macros for novel binding strategies. List comprehensions, do-notation, destructuring-bind, pattern matching, CL's LOOP and macros that anonymously bind a pronoun like "it".

I have no shame in using macros for really succinct control structures where delaying evaluation with a lambda would just look gross (see "or" and "and" macros), but maybe that's just a cry for lazy evaluation :P

That's a great link!

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