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

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

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