On Tuesday 15 February 2011 23:29:39, Andrew Coppin wrote:
On 15/02/2011 08:35 PM, Daniel Fischer wrote:
The result is that the list
[1 .. 10*1024*1024*k]
from the penultimate line of random_file is shared between the four iterations of the inner loop in file_batch (for k = 1 .. 4). Oops.
Ouch! That's gotta sting in the morning... o_O
I think it has already stung, or we wouldn't have this thread, would we?
I suppose what we could really do with is a combinator that runs a monadic action N times, without actually constructing a list N elements long in order to do so.
True enough. But I guess nobody¹ bothered yet because there are so many possible designs and most of them are trivial to implement in a line or two (so the pain of writing them repeatedly isn't bad enough). ¹ Not quite true, there's the monad-loops package on hackage which provides a handful of loops. But not the trivial nTimesDo.
Then it becomes blatently obvious that there's nothing to share, and the problem goes away.
They say that in Haskell, "a list *is* a loop". Apparently, not always.
No, not always. Some lists are just, you know, lists.