Friday, April 11, 2008

The unintentional infinite list

The other day I was happily hacking away at some Haskell code. Same thing as always: think hard at about how to represent my problem in Haskell, then type the code quickly enough. Then launch my HUnit test. Tests doesn't return, just seem to be looping for ever!! Uh oh.
So I check my code, all the foldr and foldl and such, to make sure nowhere I create an infinite sequence. Nothing. So I took a deep breath and started the ghci debugger. I'm used to graphical debugger (like Java in Eclipse) but hey I managed to pretty quickly locate the offending function. Only a typo that couldn't be caught by the compiler. Since it's not the first time this has happened to me, here it is:
I tend to not be too comfortable with big one liner functions, so I revert often to several lines after a let instruction to cleanly separate each step of the computation. So when I repeatedly change some structure I tend to get:

fn a =
a1=fn1 a
a2=fn2 a1
in fn3 a2

and the typo there was a line like:
    a2 = fn2 a2

So I had typed in a2 instead of a1, and the compiler was perfectly happy, I wanted to create an infinite list, right? Er... That wasn't exactly my intention.
So I fixed the typo. Then ran my test. Which passed. Of course.

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