16 August 2011

Loops and Side-effects

TeX has a better looping construct.

I designed a small Java-like language for some experiments. I think it is a good idea to be able to say "this piece of code has no side-effects." (If you saw how I write C/++, you'd probably be surprised by what I just said.) There are various ways of isolating side-effects. My requirements were that (1) it must look much like Java and (2) there should be some way to contain side-effects. So I simply banned side-effects from expressions.

Then I proceeded to translate the following Java code.

while (i.hasNext()) {
  Object o = i.next();
  // bla
}

Method calls, however, possibly have side-effects so I could not use them in expressions. A method call is a statement in my toy language.

while (true) {
  boolean c = i.hasNext();
  if (!c) break;
  Object o = i.next();
  // bla
}

This loop reminded me of \loop ... \if ... \repeat in $\TeX$. The keywords loop and repeat, however, are rather un-Java-like-esque. So I decided to say do {...} while (...) {...}. This way, the normal do-loop and the while-loop are special cases obtained by omitting one of the bodies.

do boolean c = i.hasNext()
while (c) {
  Object o = i.next();
  // bla
}

Note that in Java (and C/++) one has the illusion that loops test their condition at the beginning (while) or at the end (do) but never in the middle. This is not true, because expressions may have side-effects.

If languages would have do-while-loops, then I'd probably break far less often.

I decided to write this post after seeing another post about how to write a similar kind of loop in OCaml.

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