Derived Instances

Minor adjustments

Haskell 98 allows derived instances of Bounded only for enumerations and single constructor types. Proposal: allow derived instances of Bounded for all algebraic data types, with

minBound = K1 minBound ... minBound
maxBound = Kn maxBound ... maxBound

Automatic derivation

Automatic derivation of instances is jolly useful. Current problems are:

  • Automatic derivation is only available for certain classes defined in the Prelude and standard libraries.
  • There is no way to specify how to derive a user-defined class in the general case.
  • There is no way to derive an instance of a class for a data type that is defined elsewhere (in another module).
  • There is no easy way to refer to the definitions that would have been derived when defining an explicit instance.
  • Deriving fails for some Haskell-98 data types. Try
    data Rose f a = Fork a (f (Rose f a)) deriving Show
    as an example.
  • Even in the absence of user-defined deriving, it should be easy for a newtype declaration to inherit some of the instances of its parent type, no matter where the class was defined, but this is currently only available as a GHC extension to Haskell 98 (see NewtypeDeriving).

User-specified derived instances

There are various extant proposals to enable the user-specification of the derivation of class instances:

Standalone derived instances

The development version of GHC adds a top-level declaration on the form:

'deriving' qtycls 'for' qtycon

which produces the same instance as would be produced by a deriving clause in a data or newtype declaration, but allows this derivation to be decoupled from the datatype declaration, e.g. in another module. See Stand-alone deriving declarations on the GHC Wiki.

Last modified 12 years ago Last modified on Apr 18, 2007 7:46:51 PM