macro.md

A little Clojure challenge inspired by Let over Lambda.

Write a macro (you can try a function, but it's impossible) that accepts four arguments:

1. an expression that returns a number
2. something to return if that number's negative
3. something to return if that number's zero
4. something to return if that number's positive

Here's the signature: (defmacro nif [expr neg zero pos] ...)

To pass my test, the following form should print "pos!" and return "pos.", with no other side effects:

(nif 10
      (do (println "Negative!") "negative.")
      (do (Thread/sleep 1000000) "zero.")
      (do (println "pos!") "pos."))

May the best lisper win!

Ah, right. Because the do special form evaluates the expressions - I had missed that and wrapped each of the (do ...) in a function.

@mdiin yeah, the trick with the macro here is that a function evaluates all of it's arguments, so you have to scramble a bit to prevent that evaluation. The nif macro lets you think of the inputs as unevaluated lists (or numbers, or whatever they happen to be), and shuffle them into the proper form before they hit the compiler.

@samaaron Thanks! The next quiz will be,

Write a series of macros that wraps clojure.core in a badass, composable, music-generating fiesta of code.

@sritchie :-) I'll work on a solution...