[Haskell-beginners] Default values for Data Types?

[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).

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
>
>
>
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