Am Mittwoch, 13. April 2005 15:43 schrieb Ralph Hodgson:
I am learning Haskell and have set a small exercise for myself on a frames and slots program. Would appreciate some help on how to pass constructors of data structures as arguments to a function.
Thanks,
-- Ralph <snip> Now I need a way to extract properties from the frame. Start by testing pattern matching without using parameters. Then I need to find out how to pass a constructor as a parameter.
Your code works fine. I'm not sure, what your problem is. Given type-correctness, data constructors can be passed as arguments to functions like any other function. Probably that's not your question, however. As a wild guess, maybe you should use labelled records, data Contact = Contact { firstName :: FirstName , lastName :: LastName , ... } and you have your selector-functions. And it's possible to define partial contacts as me = Contact{firstName="Daniel", lastName="Fischer"} -- just don't ask for my phone-number or anything else which is undefined. If I'm far off, state your problem more precisely.
getProperty:: [ContactProperty] -> FirstName getProperty ((FN fn):_) = fn getProperty (_:xs) = getProperty xs getProperty [] = "unknown"
firstName:: Contact -> FirstName firstName (Contact cpl) = getProperty cpl
Cheers, Daniel