Can someone clarify what this Joel On Software quote means: (functional programs have no side effects)
functional-programming
Solution
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.
Problem
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?