Happstack.Data.Default

Provides default values for Haskell datatypes.

The interface

class Data DefaultD a => Default a where

The Default class provides a defaultValue value, which is the default value for that type.

There is no instance for arbitrary types by default, but if you declare an instance without providing the value then one will be built using the first constructor. defaultValue is used to provide values for any arguments of the constructor.

If you want an instance for all types then import Happstack.Data.Default.Generic.

Methods

defaultValue :: a

Instances

Default Bool

Default Char

Default Double

Default Float

Default Int

Default Int8

Default Int16

Default Int32

Default Int64

Default Integer

Default Word

Default Word8

Default Word16

Default Word32

Default Word64

Default ()

Data DefaultD a => Default a

Default Element

Default User

Default Pass

Default UserInfo

Default ByteString

Default Text

Default Text

Data DefaultD a => Default [a]

Default t0 => Default (Proxy t0)

Default a => Default (ForeignPtr a)

Default a => Default (Maybe a)

(Data DefaultD a, Ord a) => Default (Set a)

(Default a, Default b) => Default (Either a b)

(Default a, Default b) => Default (a, b)

(Data DefaultD a, Data DefaultD b, Ord a) => Default (Map a b)

(Default a, Default b, Default c) => Default (a, b, c)

(Default a, Default b, Default c, Default d) => Default (a, b, c, d)

Writing your own instances

defaultDefaultValue :: (Data DefaultD a, Default a) => a

This is the defaultValue that is used in an instance if you don't specify one. It may be a useful building block when writing your own instances.

Advanced usage

data DefaultD a

When writing your own generic functions for Default you may need to access the class method through this datatype rather than directly.

Constructors

DefaultD

Fields

defaultValueD :: a

Instances

Default t => Sat (DefaultD t)

defaultProxy :: Proxy DefaultD

When writing your own generic functions for Default you may need this, the proxy value.