> 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
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