[Haskell-beginners] Default values for Data Types?

[Daniel Fischer]

Am Donnerstag, 25. September 2008 03:38 schrieb Mike Sullivan:
> Hi Brent and Brandon,
>
> Thank you for responding! Okay, so from the responses it seems that it's
> simple to define a "datatype" (partially applied function, really) which
> has "default" parameters for all fields. Or, using the same method, for the
> first N fields (N is less than or equal to the number of fields):
>
> defaultCust = Customer 0 "" -- for instance, building upon Brent's example
>
> However, it's not quite so easy to pick and choose (define only the name,
> for instance). I guess this really isn't too much of a limitation, since a
> simple function could set any defaults you want:
>
> defaultCust2 id addr = Customer id "Bill" addr -- function which simulates
> a default value for "name"
>
> So despite the lack of syntactic sugar, the simplicity and power of
> functions can make do. One down side, however, is that you lose the
> flexibility of record syntax (unless there is an analogue for functions
> that I don't know about).

Why would you lose the record syntax?
With

type CustomerID = Int
type Address = Maybe String

data Customer = Customer
    { customerID :: CustomerID
    , customerName :: String
    , customerAddress :: Address
    } deriving (Show)

you define your default customer

cust = Customer{ customerID=0, customerName="", customerAddress=Nothing }

and then just create your customers like

a = cust{customerID=12, customerName="Bill" }
-- a === Customer 12 "Bill" Nothing
b = cust{ customerID=13, customerName="Gordon",customerAddress=Just "10 
Downing Street" }
-- b === Customer 13 "Gordon" (Just "10 Downing Street")

using cust as a smart constructor (to make sure no customers with 
uninitialized fields are created, don't export the data constructor Customer, 
only cust and the fields)


>
> Mike
>
>
>
>
> On Tue, Sep 23, 2008 at 11:25 PM, Brandon S. Allbery KF8NH <
>
> allbery at ece.cmu.edu> wrote:
> > 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 at kf8nh.com
> > system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
> > electrical and computer engineering, carnegie mellon university    KF8NH