Why is there a space leak here?

[Tom Pledger]

David Bakin writes:
 :
 | I have been puzzling over this for nearly a full day (getting this
 | reduced version from my own code which wasn't working).  In
 | general, how can I either a) analyze code looking for a space leak
 | or b) experiment (e.g., using Hugs) to find a space leak?  Thanks!
 | -- Dave

a) Look at how much of the list needs to exist at any one time.

 | -- This has a space leak, e.g., when reducing (length (foo1 1000000))
 | foo1 m 
 |   = take m v
 |     where
 |       v = 1 : flatten (map triple v)
 |       triple x = [x,x,x]

When you consume the (3N)th cell of v, you can't yet garbage collect
the Nth cell because it will be needed for generating the (3N+1)th,
(3N+2)th and (3N+3)th.

So, as you proceed along the list, about two thirds of it must be
retained in memory.

 | -- This has no space leak, e.g., when reducing (length (foo2 1000000))
 | foo2 m 
 |   = take m v
 |     where
 |       v = 1 : flatten (map single v)
 |       single x = [x]

By contrast, when you consume the (N+1)th cell of this v, you free up
the Nth, so foo2 runs in constant space.

 | -- flatten a list-of-lists
 | flatten :: [[a]] -> [a]
 :

Rather like concat?

Regards,
Tom