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

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

>From the homework:

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

data LogMessage = LogMessage MessageType TimeStamp String
    deriving (Eq, Show)

data MaybeLogMessage = ValidM LogMessage -- A valid msg
                     | InvalidLM String  -- Invalid msg

parseMessage :: String -> MaybeLogMessage
parseMessage = undefined

To implement parseMessage, we consume the string from left to right
word-by-word.
If the first word is E, then we read the second word as in integer
indicating severity and proceed further.
Info and Warning don't require more information, so the next word will be
the timestamp in those cases.
If the pattern fails anywhere, we return the whole string as an InvalidLM.

Hope this helps.


On 24 February 2015 at 06:37, Richard A. O'Keefe <ok at cs.otago.ac.nz> wrote:

>
> On 24/02/2015, at 5:19 am, Roelof Wobben <r.wobben at home.nl> wrote:
>
> > I tried it another way more like explained on this page :
> http://www.seas.upenn.edu/~cis194/spring13/lectures/02-ADTs.html
> >
> > so I tried this :
> >
> > parseMessage :: [Char] -> [Char]
> > parseMessage s
> > case Errornumber of
> >     IsDigit Errornumber  -> "Geldige string"
> >     otherwise            -> "Ongeldige string"
> >   where
> >       Error = s words
> >       Errornumber = Error(ErrorNumber _ _ )
> >       Errorcode = Error(_ Errorcode _ )
> >
> > but now I cannot use where :(
>
> That's not your problem.
>
> IsDigit ErrorNumber is not a pattern.
>
> parseMessage s =
>     if isDigit errorNumber then "Geldige string"
>     else                        "Ongelidige string"
>   where
>     errorNumber = ???
>
> is OK.
>
> Now I cannot make sense of
>     Error = s words
>
>         identifiers beginning with capital letters are used for
>         - module names
>         - type constructors
>         - data constructors
>         You want a variable here, so it must begin with a
>         lower case letter.
>
>         s words treats a string s as a function and applies it
>         to the function words as argument: s(words).  But that
>         does not type check.  You mean words s.
>
>         The result of words s, whatever else it may be, is not
>         an error.
>
>     Errornumber = Error(ErrorNumber _ _)
>
>         In the form "expr where pattern = expr", the thing after
>         the equal sign must be an expression.  But
>         Error(ErrorNumber _ _) is not an expression.  "_" is a
>         PATTERN (= I do not care what goes here) but never an
>         EXPRESSION (because what value would it have?).
>
>
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-- 
Regards

Sumit Sahrawat
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