[Haskell-cafe] Possible Improvements

[Derek Elkins]

On Sun, 2007-12-02 at 21:54 -0800, Don Stewart wrote:
> catamorphism:
> > On 12/2/07, Don Stewart <dons at galois.com> wrote:
> > > prstanley:
> > > > Hi
> > > > data Tree = Leaf Int | Node Tree Int Tree
> > > >
> > > > occurs :: Int -> Tree -> Bool
> > > > occurs m (Leaf n) = m == n
> > > > occurs m (Node l n r) = m == n || occurs m l || occurs m r
> > > >
> > > > It works but I'd like to know if it can be improved in any way.
> > >
> > > You could probably get away with:
> > >
> > >     data Tree = Leaf !Int | Node Tree !Int Tree
> > >
> > > but that's a minor issue.
> > >
> > 
> > IMO, there's no reason to even think about putting in the strictness
> > annotations unless you've identified this datatype as part of a
> > performance bottleneck in production code. Otherwise, there's no need
> > to clutter your code and your mind with them :-)
> 
> Very true ("a minor issue").

I agree that (in this context, beginning learning Haskell) it is a
somewhat minor issue.  But I disagree that this is something you should
ignore until it becomes a problem and I do think that it should be part
of learning Haskell.  Properly using strictness is an important part of
using Haskell.  It makes the difference between code that stack
overflows and code that doesn't, code that takes 100 seconds and code
that takes 10, code that uses 3MB of RAM and code that uses 600.  At
least the first of these is not, in my mind, the difference between
"optimized" and "unoptimized", but rather the difference between correct
and incorrect.  Writing better code at the beginning is much easier than
trying to figure out what the problem is later.  Furthermore, writing
better code is not more difficult.  In this case it merely means adding
two characters.  Of late, the "rules of thumb" for this sort of thing
are becoming more widely known.  Such things need to be "instinctively"
part of how you write code, much like writing code tail-recursively or
not using (++) left associatively.  It's not that you should immediately
know that this is better, but (more strongly) that you should not even
think of the worse ways to begin with in many cases.