[Haskell-cafe] Strange results when trying to create large Bool
bos at serpentine.com
Wed Jul 11 13:55:28 EDT 2007
Richard Kelsall wrote:
> main = do
> n <- getArgs >>= readIO . head :: IO Int
> a <- newArray (1,n) True :: IO (IOUArray Int Bool)
> printf "Created array 1 .. %8d \n" (n::Int) :: IO ()
> It appears to work up to quite large numbers, but then gets strange.
> When I give it an array size of 1,000,000,000,000 it returns this
> Created array 1 .. -727379968
> Presumably the Int has overflowed without warning when read from the
Yes, that's right.
> I guess there must be a switch to make it produce a nice
> error message rather than overflowing without warning.
Actually, there isn't. Int is a bit of an odd fish that way; it's a
window onto the underlying machine's behaviour, not a tidy, well-behaved
mathematical ring. In a similar vein, I was initially perplexed when I
found that an expression like this produces garbage instead of an error:
read "11111111111111111111111111111111111" :: Int
I have not seen a lot of interest expressed in fixing this sort of
misbehaviour, which jars a little with the usual emphasis on stringency
> It seems to randomly claim to have successfully created huge sizes
> of array.
This may be outside of Haskell's control, as you're not actually
touching the memory you allocate. I wouldn't be surprised if the
underlying page allocation is succeeding by virtue of the OS being
willing to overcommit resources that may not actually be used. (This
would be normal behaviour on Linux, for example.) In such a case, the
Haskell runtime may not receive an error until you try to actually touch
You can get GHC to fix an upper limit on the heap size it will try to
use, by passing "+RTS -M768m -RTS" to your compiled program on the
command line. That should cause your program to crash more reliably.
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