[Haskell-cafe] how can I do this the best
Richard A. O'Keefe
ok at cs.otago.ac.nz
Tue Feb 24 01:07:56 UTC 2015
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"
> 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"
errorNumber = ???
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
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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