With "Foo" in the file c.out and the module

\begin{code} module Main (main) where

import IO (openFile, hGetContents, hClose, IOMode(ReadMode)) import System (getArgs) import Monad (when)

main :: IO () main = do [x] <- getArgs let you_want_it_to_work = read x cout <- openFile "c.out" ReadMode s <- hGetContents cout putStrLn "" when you_want_it_to_work $ putStrLn $ "Got this: " ++ s putStrLn "" hClose cout putStrLn $ "The answer is: " ++ s \end{code}

I expected "The answer is: Foo" to be printed whether the argument was True or False. When it is False, however, GHC (5.02.2, 5.04 and recent CVS HEAD) think s is the empty string and nhc98 (1.10 and a 1.11 from about a year ago) produces a binary that segfaults. At first I thought it was a GHC bug, but now nhc98 also exhibits it I am wondering if it is a bug in my understanding?

Lazy I/O strikes again :-) When the argument is True, you should get Got this: Foo The answer is: Foo and when the argument is False, one possible correct output is: The answer is: which GHC (5.04.2 and CVS HEAD) does indeed produce. Another correct output would be The answer is: Foo in fact, any prefix of "Foo", including the empty string, would be correct. See section 21.2.2 in the (revised) Haskell 98 report. Cheers, Simon