[Haskell-cafe] Re: space leak with 'concat' ?

[Svein Ove Aas]

On Thu, Feb 12, 2009 at 10:36 AM, Simon Marlow <marlowsd at gmail.com> wrote:
> Peter Verswyvelen wrote:
>>
>> Yes, I was really surprised that this was the case. I while ago I did a
>> little FRP experiment. I made a top level binding to a list of timer event
>> occurrences. The list was generated on another thread. To my surprise, I did
>> not have space leak, which is amazingly cool, but it felt odd :) Is it
>> documented when GHC will garbage collect CAFs?
>
> CAFs are garbage collected when they are unreachable by traversing the code
> that is reachable from the currently running program.  In practice we don't
> actually traverse the code, instead we have these "static reference tables"
> that list the top-level closures referenced by each code block, and traverse
> those instead.
>
> Unfortunately we didn't get around to documenting the details of how this
> works...
>
Using this as a guide, I tested these two programs:

====
str = concat $ repeat "foo "

main1 = print foo
main2 = print foo >> print foo
=====

As I'm sure you realize, the first ran in constant memory; the second,
not so much. Very interesting.