Data.Default.Generic

Deprecated: Use DefaultSignatures or DeriveAnyClass language extension instead.

DEPRECATED because package data-default-class now supports GHC Generics.

Create Default instances using GHC Generics. For more information see:

• GHC User's Guide
• HaskellWiki: GHC.Generics

Package data-default-class supports GHC Generics since version 0.1.0, but it is still useful to have GDefault and genericDef exposed for more complex cases.

Documentation

Rule of thumb, if generic instance definition, contains more code then the explicit definition, then use the explicit definition.

Note, that sum types aren't supported, but even if they were, it is always better to explicitly specify Default instance for sum types.

Usage example:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic)


data MyType = MyType Int (Maybe String)
  deriving (Generic, Show)

instance Default MyType where
    def = genericDef

>>> def :: MyType
MyType 0 Nothing

genericDef :: (Generic a, GDefault (Rep a)) => a

Derive implementation of def by using GHC Generics.

class GDefault f where

Simple derivation of def definition that handles only product types, but not sum types. In case of sum types it is better to provide hand written instances for Default.

Minimal complete definition

gdef

Methods

gdef :: f a

Instances

GDefault U1

Methods

gdef :: U1 a

Default a => GDefault (K1 i a)

Methods

gdef :: K1 i a a

(GDefault a, GDefault b) => GDefault ((:*:) a b)

Methods

gdef :: (a :*: b) a

GDefault a => GDefault (M1 i c a)

Methods

gdef :: M1 i c a a