[Haskell-cafe] Confusing change causing space leak

[Denis Bueno]

Hi all,

I discussed this with a few people in #haskell-in-depth, and we
thought I should send this to the list.

The following paste shows the only changes required to remove a space
leak: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=4185#a4185.
Summary: if I reduce a tuple-from-IO to whnf in the "allocate"
function passed to "bracket", no leak.  For example, this also doesn't
leak: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=4185#a4189

That paste page also contains the code for "externalSort".  The
"externalSort" function used in the example corresponds to the "sort"
function provided by the Algorithms.ExternalSort module.

Can anyone explain why the leak happens at all, or why it goes away?
                              Denis