6.6. Deriving mechanism

Haskell 98 allows the programmer to add a deriving clause to a data type declaration, to generate a standard instance declaration for specified class. GHC extends this mechanism along several axes:

• The derivation mechanism can be used separately from the data type declaration, using the standalone deriving mechanism.

• In Haskell 98, the only derivable classes are Eq, Ord, Enum, Ix, Bounded, Read, and Show. Various language extensions extend this list.

• Besides the stock approach to deriving instances by generating all method definitions, GHC supports two additional deriving strategies, which can derive arbitrary classes:

  • Generalised newtype deriving for newtypes and

  • deriving any class using an empty instance declaration.

  The user can optionally declare the desired deriving strategy, especially if the compiler chooses the wrong one by default.

• 6.6.1. Deriving instances for empty data types
• 6.6.2. Inferred context for deriving clauses
• 6.6.3. Stand-alone deriving declarations
• 6.6.4. Deriving instances of extra classes (Data, etc.)
• 6.6.5. Generalised derived instances for newtypes
• 6.6.6. Deriving any other class
• 6.6.7. Deriving strategies
• 6.6.8. Deriving via