Hi, readFile is lazy IO, so the unsafePerformIO or equivalents are already there. It should be enough to invent a strict process function, but which ghc 6.12.3 isn't able to figure out that it is strict. Does it still work with : writeFile "output" $! process inp This raises another question : the strictness analysis is undecidable, but do we know of an explicit sequence of functions, strict in their argument, which ghc will always fail in recognizing that they are strict ? (we need an infinite sequence, because ghc could be programed to recognize a specific function each time, of course). PE El 12/08/2010, a las 10:09, Yitzchak Gale escribió:
The file "error_puzzle.hs" begins like this:
main = do inp <- readFile "input" writeFile "output" $ process inp
process :: String -> String
When compiled with GHC 6.12.3 and run, it gives the following result:
$ ./error_puzzle error_puzzle: output: hClose: illegal operation (handle is finalized)
What is the cause of the error? Give a definition for process and an input file that reproduce this result. No "unsafe" functions like unsafePerformIO are allowed.
Have fun! -Yitz _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe