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Gentle Introduction to Haskell 98, Online Supplement
Part 15
Covers Section 7.3, 7.3, 7.5
> module Part15() where
> import IO
Section 7.3
To experiment with I/O errors, we need to get a bit creative. Generating
an error is generally OS specific so instead we will use errors that
are directly under user control. The function
userError :: String -> IOError
generates an error value and the function
ioeGetErrorString :: IOError -> String
gets the string out of an error.
This version of getChar that raises an error when an @ or ? is entered:
> getCharE :: IO Char
> getCharE = do c <- getChar
> case c of
> '@' -> ioError (userError "@")
> '?' -> ioError (userError "?")
> _ -> return c
Using this extended getChar we can build getChar' to catch only @
There is currently a bug in Hugs - if this gets fixed change the
definition is isAtError.
> isAtError :: IOError -> Bool
> isAtError e = ioeGetErrorString e == "User error: @" -- for bug in hugs
> -- isAtError e = ioeGetErrorString e == "@" -- Should be this
> getChar' :: IO Char
> getChar' = getCharE `catch` atHandler where
> atHandler e = if isAtError e then return '\n' else ioError e
> getLine' :: IO String
> getLine' = catch getLine'' (\err -> return ("Error: " ++ show err))
> where
> getLine'' = do c <- getChar'
> if c == '\n' then return ""
> else do l <- getLine'
> return (c:l)
Observe the behavior when you enter @ or ?
> e1 = getCharE
Now try lines with @ and ? in them.
> e2 = do l <- getLine'
> putStrLn l
Section 7.4
You will have to set up some files to play with if you want to try
this one.
> e3 = do fromHandle <- getAndOpenFile "Copy from: " ReadMode
> toHandle <- getAndOpenFile "Copy to: " WriteMode
> contents <- hGetContents fromHandle
> hPutStr toHandle contents
> hClose toHandle
> putStr "Done."
> getAndOpenFile :: String -> IOMode -> IO Handle
> getAndOpenFile prompt mode =
> do putStr prompt
> name <- getLine
> catch (openFile name mode)
> (\_ -> do putStrLn ("Cannot open "++ name ++ "\n")
> getAndOpenFile prompt mode)
>
Continued in part16.lhs
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