On Wed, Feb 18, 2015 at 7:16 PM, Dudley Brooks <dbrooks@runforyourlife.org> wrote:

Hmm. Well, I'd say that that's a feature of, specifically, Haskell's pattern-matching strategy and list-description syntax, rather than of recursion in general or the structure of this particular problem. In other languages with recursion you might have no choice except to start with the base case, even for this problem, or else you'd get the same kind of error you mention below (depending on the language). I think it's good when you're *learning* recursion to always start with the base case(s).

I disagree that this is a Haskell-specific feature. Any else-if like structure will have this property, no matter what language it's in. That Haskell provides a syntax as part of the function declaration is special, but that doesn't let you avoid the else-if construct when the problem requires it.

It may be my fondness for proof by induction, but I think doing the base case first is a good idea for another reason. The code for the recursive cases assumes that you can correctly handle all the "smaller" cases. If that's wrong because some assumption about the base case turns out to be false when you actually write it, then you have to rewrite the recursive cases for the correct base case. So it's better to make sure your base case is going to work before you start writing the code that's going to use it.