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

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

On 24 February 2015 at 13:22, Roelof Wobben <r.wobben at home.nl> wrote:

> 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 code looks just fine. Great work.
The trouble is with the third case. The error message is fairly instructive.

log.hs:19:36:
    Couldn't match expected type ‘String -> LogMessage’
                with actual type ‘LogMessage’
    The function ‘LogMessage’ is applied to four arguments,
    but its type ‘MessageType -> TimeStamp -> String -> LogMessage’
    has only three
    In the expression:
      LogMessage Error (read errorcode) (read time) (unwords text)
    In a case alternative:
        ("E" : errorcode : time : text)
          -> LogMessage Error (read errorcode) (read time) (unwords text)

log.hs:19:47:
    Couldn't match expected type ‘MessageType’
                with actual type ‘Int -> MessageType’
    Probable cause: ‘Error’ is applied to too few arguments
    In the first argument of ‘LogMessage’, namely ‘Error’
    In the expression:
      LogMessage Error (read errorcode) (read time) (unwords text)

Long story short, you meant to write

    LogMessage (Error (read errorcode)) ...

instead of

    LogMessage Error (read errorcode) ...        -- Too many arguments to
LogMessage (error message 1)
                                                 -- Too few arguments to
Error (error message 2)

which is a very common error.
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