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
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.
- 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 (
- 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