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

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