On Sun, Dec 11, 2005 at 08:37:06PM +0000, Joel Reymont wrote:
Would you care to elaborate? This has not caused any problems for me so far but this is probably due to my usage.
This is a know danger of using unsafePerformIO and one reason for "unsafe" in its name.
From http://www.cse.unsw.edu.au/~chak/haskell/ffi/ffi/ffise5.html: Great care should be exercised in the use of this primitive. Not only because of the danger of introducing side effects, but also because unsafePerformIO may compromise typing; to avoid this, the programmer should ensure that the result of unsafePerformIO has a monomorphic type.
If you wonder why, analyze this code and see what happens when you run it. import Control.Concurrent import System.IO.Unsafe {-# NOINLINE children #-} children :: MVar [a] children = unsafePerformIO $ newEmptyMVar main = do putMVar children (["foo", "bar", "baz"]) l <- takeMVar children print (l :: [Integer]) It would be nice if GHC could warn about such situations, at least in the most simple, recognizable cases. Seems like a good task for a new GHC developer. Best regards Tomasz -- I am searching for a programmer who is good at least in some of [Haskell, ML, C++, Linux, FreeBSD, math] for work in Warsaw, Poland