On 2008 Sep 22, at 15:38, Mike Sullivan wrote:
I was wondering if there is syntactic sugar in Haskell for defining a default value for fields in a data type. For instance, say I have a type that is defined in record syntax:
type CustomerID = Int
type Address = Maybe String
data Customer = Customer {
customerID :: CustomerID
, customerName :: String
, customerAddress :: Address
} deriving (Show)
Is there any way to define default values for some (or all) fields such that they may be omitted from a declaration, and still have it generate a valid object?

e.g.)
a = Customer{customerID = 12, customerName="Bill"}
-- I would like a{customerAddress} to default to Nothing (for instance).

It seems to me that this would be a nice feature to have, if it does not exist. Am I missing something?

aCustomer = Customer { customerAddress = Nothing }
-- ...
a = aCustomer { customerID = 12, customerName = "Bill" }

This will net you a compile time warning about uninitialized fields in aCustomer, and if you fail to initialize a Customer properly it will produce a runtime error:

    *Main> customerName b
    "*** Exception: fooo.hs:6:12-49: Missing field in record construction Main.customerName

You will *not* get  a warning for missing values in variables initialized this way, only for the special initializer value.  I'm not sure if it's possible to make the type system handle this, but if it is then it'll probably be painful and ugly.  (Now watch someone post an elegant one-liner....)

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH