Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?

15 Jun 2006

      On Thu, 2006-06-15 at 12:43 +0100, Vladimir Portnykh wrote:
...
Suppose there is a data definition in Haskell:
data MyType = MyType { date	:: Double,
         weight 	:: Double,
        height	:: Double
      } deriving (Eq, Ord, Show)
Is it possible to check if the field height, for example, is filled 
in(defined)? Can we give default values in Haskell?
There is no implicit null, standard technique is to use an explicit null
using the Maybe type:

data MyType = MyType {
  date	 :: Maybe Double,
  weight :: Maybe Double,
  height :: Maybe Double
} deriving (Eq, Ord, Show)

The Maybe type is defined like this:

data Maybe a = Nothing | Just a

so a value of type 'Maybe a' can be either Just x or Nothing.

So you can represent your lack of a value using Nothing. You can get
something similar to default values in a record by using the ordinary
record update syntax. Suppose we start with a default record value:

default :: MyType
default = MyType { date = Nothing, weight = Nothing, height = Nothing }

then you can construct your records using:

foo = default { weight = 3.2 }

so foo will get all the default values except for the ones you
explicitly set.

Duncan

Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?

Duncan Coutts