Re: [Haskell-cafe] how can I do this the best

24 Feb 2015

      Thanks,

I rewrote it like this :

parseMessage :: String -> LogMessage
parseMessage s =
    case words s of
         ("I":time:text)           -> LogMessage Info (read time) 
(unwords text)
         ("W":time:text)           -> LogMessage Warning (read time) 
(unwords text)
         ("E":errorcode:time:text) -> LogMessage Error (read errorcode) 
(read time) (unwords text)
         _                         ->  Unknown "This is not in the right 
format"

The only thing Im facing now It the Error part.

Messagetype has this signature :

data MessageType = Info
                  | Warning
                  | Error Int
   deriving (Show, Eq)

So Error has a Messagatype which contains 2 things.
but because of the use of words I looks like this I think [ "E", 46, 
23456 , "This", "is", "a", "error". ]
Can I somehow use unwords here to to make the Messagetype work.

Roelof

Richard A. O'Keefe schreef op 24-2-2015 om 2:01:
...
On 24/02/2015, at 3:50 am, Roelof Wobben  wrote:
...
isValid :: [Char] -> Bool
isValid s = go (words s)
     where
       go ([a]:b:_) = isLetter a && all isDigit b
       go _         = False
My attention was caught by the remarkably unhelpful name "go".
What goes?  Where does it go?  What does going mean?
This is a good time to use a 'case':
isValid s =
   case words s of
     [a]:b:_ -> isLetter a && all isDigit b
     _       -> False
...
parseMessage :: [Char] -> [Char]
parseMessage s = isValid s
     where
         isValid = "Geldige string"
         _       = "Ongeldige string"
In the first line of this function, you are calling
isValid with an argument.  But you are defining
isValid to be a string!
parseMessage s =
   if isValid s then "Geldige string" else "Ongeldige string"
"parseMessage: is a bad name because the purpose of the
function is *NOT* to break the message into parts and
deliver those parts but to CHECK or VALIDATE or CLASSIFY
the message.
The code
import Data.Char
isValid s =
   case words s of
     [a]:b:_ -> isLetter a && all isDigit b
     _       -> False
parseMessage s =
   if isValid s then "Geldige string" else "Ongeldige string"
main =
     putStrLn $ parseMessage "I 4764 He trusts to you to set them free,"
compiles without warnings and produces the result I think you want.