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, andShow. 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
