Bernd Brassel wrote:
Consider the following program:
module Stack where
import System.IO.Unsafe
main = print (sim (replicate 1299959 ()))
sim [] = True
sim (_:xs) = goodStack (sim xs)
goodStack x = fromJust (Just x) --no stack overflow
badStack x = unsafePerformIO (return x) --stack overflow
fromJust (Just x) = x
goodStack == id, and GHC even with no optimisation will transform it into
id, and inline it into sim. So with goodStack, sim ends up being
tail-recursive. With badStack, sim is no longer tail recursive
(unsafePerformIO is not inlined), so it runs out of stack. Simple!
Is this behaviour necessary? Is there any work around, e.g., employing
the foreign function interface?
There's unsafeInlinePerformIO (sometimes called inlinePerformIO), which is
usable in certain cases, but be very careful. From Data.ByteString.Internal:
{-# INLINE inlinePerformIO #-}
inlinePerformIO :: IO a -> a
#if defined(__GLASGOW_HASKELL__)
inlinePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
#else
inlinePerformIO = unsafePerformIO
#endif
But even this might not give you tail recursion, depending on the context.
Cheers,
Simon
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