On Dec 21, 2011, at 6:52 PM, Fedor Gogolev wrote:
> Hello. I'm trying to get some threads that I can stop and get last
> values that was computed (and that values are IO values, in fact).
> Here is my first approach:
> [...]
> tick :: Int -> IO Int
> tick v = return $ v + 1
> [...]
> The problem is that it looks a little messy and what's worse it leaks
> memory. So I'm wondering if there is a better approach to do so or
> some fix to memory leak.
I don't have any tips for cleaning up the code off the top of my head, but I
suspect that the memory leak is coming from the fact that the expression (v+1)
is not being forced, which means that each iteration of the loop is
constructing a new thunk with a reference to the old thunk resulting in a data
structure that is growing in memory usage over time. In this case the fix is
easy: just replace "return" in "tick" with "evaluate" from Control.Exception,
since "evaluate" guarantees in this case that the expression will be evaluated
before being returned.
(Caveat: What I said about "evaluate" will be true for numeric expressions,
but for non-trivial data structures "evaluate" only ensures that the expression
is evaluated to something called weak-head normal form, which essentially means
that it will only evaluate enough of the expression to figure out what the
outermost constructor of the datatype is; it just so happens that for numeric
expressions the outermost data constructor is enough to give you the numeric
value.)
Cheers,
Greg
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