On 19-Dec-1997, Simon L Peyton Jones <[EMAIL PROTECTED]> wrote:
>
> I thought you might find the following bug I've just
> found in GHC entertaining.
>
> In the strictness analyser we need to compare abstract values
> so that the fixpoint finder knows when to stop. In the
> middle of this code was the following:
>
> sameVal :: AbsVal -> AbsVal -> Bool
>
> ...equations for sameVal...
>
> sameVal (AbsApproxFun str1 v1) (AbsApproxFun str2 v2)
> = str1 == str2 && sameVal v1 v1
>
> ...more equations...
>
> That "sameVal v1 v1" should of course be "sameVal v1 v2".
...
> this bug has gone un-noticed for I dread to think how long!
>
> It showed up in a nofib test -- sing ho for regression tests.
This bug could have been caught by a very simple static analysis that
is very popular in the logic programming community: singleton variable
warnings. In the code above, the variable `v2' occurs only once.
Singleton variables such as this are often bugs. For cases where the
programmer really does want a singleton variable, the warning can be
suppressed by using a different naming convention (in Prolog, it
would be `_v2'; since Haskell doesn't allow leading underscores,
I suppose you could use `v2_').
So, sing ho for regression tests, but sing louder for static analysis ;-)
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
