Question

I was reading Joel On Software today and ran across this quote:

Without understanding functional
programming, you can’t invent
MapReduce, the algorithm that makes
Google so massively scalable. The
terms Map and Reduce come from Lisp
and functional programming. MapReduce
is, in retrospect, obvious to anyone
who remembers from their
6.001-equivalent programming class that purely functional programs have
no side effects and are thus trivially
parallelizable.

What does he mean when he says functional programs have no side effects? And how does this make parallelizing trivial?

Answer 1

Let me wikipedia it for you

In brief, a pure function is one that calculate things based only on its given arguments and returns a result.

Writing something to the screen or changing a global variable (or a data member) is a side effect. Relying on data other than that given in an argument also makes your function non-pure although it is not a side effect.

Writing a “pure function” makes it easier to invoke many instances of it in parallel. That’s mainly because being pure, you can be sure it doesn’t effect the outside world and doesn’t rely on outside information.