On 9/14/07, L.Guo
Hi MailList Haskell-Cafe:
I am tring to solve Project Euler problem 70. And write some code. (will at the end of this mail) And, I run the code in GHCi.
The problem is that, when the input is 1,000,000, it works fine, when the input is up to 10,000,000, the memory GHCi used increase very fast and did not stop.
Is this a memory leak ? or, is there some mis-understand about array ?
Haskell's boxed arrays are non-strict, which can cause space leaks when you don't actually need the laziness. The usual quick-&-dirty solution is to switch to unboxed arrays (e.g. IOUArray), which are inherently strict, but they're only available for certain primitive element types. The solution, then, is to make sure you adequately force your thunks before storing them in an array.
update arr p i = do (_,o) <- readArray arr i writeArray arr i (True,o*(p-1)/p)
Here's a possible fix, though it's untested, and I'm sure there are more elegant ways to write it: update arr p i = do (_, o) <- readArray arr i let val = o * (p-1) / p val `seq` return () -- force the thunk writeArray arr i (True, val) This ensures that the second component of the pair is a value, rather than a thunk. Strictifying data such as numbers and booleans is relatively easy, but things get tricky when you try to force more complicated structures (deepSeq, anyone?). Stuart