On 24/02/2015, at 5:19 am, Roelof Wobben <r.wobben@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?).

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe