[Haskell-cafe] Passing Constructors as arguments

13 Apr 2005


      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 -> FirstName
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"

[Haskell-cafe] Passing Constructors as arguments

Ralph Hodgson