Re: [Haskell-cafe] Using catch and errors

22 Apr 2005

      On 4/22/05, Alexandre Weffort Thenorio  wrote:
...
I am trying to get catch to work but I guess I am total newbie on this one.
main :: IO()
main =  do
  removeFile "./newint.hex"
  hexFile <- catch (readFile int.hex)  (\_ -> do putStrLn "Cannot find
int.hex"
        getLine --Simply stop program so user can read error message
        return() --Quit)
 hexFile <- readFile "int.hex"
--res of program goes here
getLine
return()
But it keeps giving me error on getLine. How can I quit a program in case of
an error giving an error message?
Try:

main :: IO ()
main =
  do removeFile "./newint.hex"
     hexFile <- catch (readFile "int.hex")  (\_ -> error "Cannot find int.hex")
     getLine
     return ()

Notice that 'return' in Haskell is very different from 'return' in C.
...
Also I tried something like
main :: IO()
main =  do
            ex <- doesFileExist "./newint.hex"
             if ex then removeFile "./newint.hex"
But that won't work either. Any ideas? I'd like to catch different errors
and quit program giving different error messages
'if/then/else' expressions _must_ have an 'else' part. In this case
you can use 'Control.Monad.when': 'when ex (removeFile "newint.hex")'.
...
Last but not least I have the function
outputLine keyno key orgFile = do
    let part1 = getLeft keyno (orgFile!!1)
    let part2 = getRight keyno (orgFile!!1)
    let total = part1 ++ (map toUpper key) ++ part2
    let checks = checksum (drop 1 total)
    let final = total ++ checks ++ "\n"
    newHexFile <- openFile "newint.hex" WriteMode
     hPutStrLn newHexFile (orgFile!!0 ++ "\n" ++ final ++ unlines (drop 2
orgFile))
I'd like to be able to check for stuff such as size of key and wheter keyno
is only 1 or 0 and quit program with error message otherwise but again
couldn't quite get catch to work.
You can use pattern matches and guards for that.

outputLine keyno key (orgFile1:orgFile2:restOfOrgFile)
  | length key == someKeyLength
      = your code here
  | otherwise
      = error "Invalid key length"
outputLine keyno key orgFile
      = error "invalid orgFile"

I'm not sure what you use 'keyno' for but perhaps you could define a
'data Keyno = Zero | One' data type and catch such errors at
compile-time?
...
Any reading tips about subject would be appreciated.
I've heard "Yet Another Haskell Tutorial" should be good but I've
never read it myself. You can download it from
http://www.isi.edu/~hdaume/htut/

-- 
Friendly,
  Lemmih