I don't really understand what you want to achieve : constructors are functions, thus first class objects ... I suppose you want a destructor, ie a function extracting the first name from a property for example. You may want to have a look there : http://haskell.org/hawiki/DecoratingStructures Pierre Ralph Hodgson a écrit :
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 __________
A Frames test
module Frames where Define frame slots:
type FirstName = String -- first name type LastName = String -- last name type Organization = String type Email = String type WorkPhone = String type CellPhone = String type TelephoneNumber = String
Define slots for a contact
data ContactProperty = FN FirstName | LN LastName | OR Organization | EM Email | WP TelephoneNumber | MP TelephoneNumber deriving (Show, Eq)
data Contact = Contact [ContactProperty] deriving (Show, Eq)
type Contacts = [ Contact]
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.
getProperty:: [ContactProperty] -> FirstName getProperty ((FN fn):_) = fn getProperty (_:xs) = getProperty xs getProperty [] = "unknown"
firstName:: Contact -> FirstNamen firstName (Contact cpl) = getProperty cpl
Define Contacts
c1::Contacts c1 = [ ( Contact [(FN "Ralph"),(LN "Hodgson"),(OR "TopQuadrant"),(EM "rhodgson@topquadrant.com")]), ( Contact [(FN "Mills"),(LN "Davis"),(EM "mdavis@project10x.com")])]
Tests
t1=firstName $ head c1 -- should be "Ralph" t2=firstName $ last c1 -- should be "Mills"
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-- Pierre Barbier de Reuille INRA - UMR Cirad/Inra/Cnrs/Univ.MontpellierII AMAP Botanique et Bio-informatique de l'Architecture des Plantes TA40/PSII, Boulevard de la Lironde 34398 MONTPELLIER CEDEX 5, France tel : (33) 4 67 61 65 77 fax : (33) 4 67 61 56 68