[Haskell-cafe] Why functional programming matters

Paul Johnson paul at cogito.org.uk
Thu Jan 24 16:11:40 EST 2008

Simon Peyton-Jones wrote:
> 1. Small examples of actual code. The goal here is (a) to convey a visceral idea of what functional programming *is*, rather than just assume the audience knows (they don't), and (b) to convey an idea of why it might be good.  
Here is one I came across in the last few days.  I was reviewing some 
code in Java, and it contained a function that looked through a list of 
Foo instances for the latest update.  I don't actually speak Java, but 
it went something like this:

   // Get the Foo that was most recently updated.
   Foo latestUpdate (Iterator <Foo> iterator) {
      long latestTimeSoFar = 0;
      Foo latestFooSoFar = Null;

      while (iterator.hasNext()) {
         item = iterator.getNext();
         if (item.updateTime > latestTimeSoFar) {
            latestTimeSoFar = item.updateTime;
            latestFooSoFar = item;
      return latestFooSoFar;

This takes an iterator over some collection of Foos and finds the one 
with the highest value of updateTime.  9 lines of code, or 12 with the 
closing curly brackets.

In Haskell this is so short and obvious you probably wouldn't bother 
declaring it as a function, but if you did, here it is:

   -- Find the Foo that was most recently updated.
   latestUpdate :: [Foo] -> Foo
   latestUpdate foos = maximumBy (comparing updateTime) foos

Of course you could always write it in point-free format, but I think 
that would be over-egging things.


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