[piano]

> - Pure functions: these allow you to statically guarantee that a piece of code doesn't perform any IO and is a completely deterministic function of its inputs.

Is this actually true? As far as I know D's pure isn't the same as eg. Haskell pure, it only prevents modifications of global state, but you can still touch data referenced by arguments. I haven't done much D though so I'm not sure what the semantics really are...

[WalterBright]

If the parameters to a pure function are marked immutable, then you cannot change data referenced by those arguments. It's functionally pure.

Ironically, sometimes people complain that the purity checks are too stringent :-)

One thing interesting about pure functions in D is the purity restrictions can be relaxed by using a `debug` prefix:

    pure int test(int i) {
      printf("%d\n", i); // error, printf isn't pure
      debug printf("%d\n", i); // ok
      return i;
    }

This makes it very, very handy to debug your functional code.