Default method implementation

It's always a design question, whether to provide default implementions of methods of Type classes and how to design them.

Here are some suggestions:

• Make only trivial default implementations.
• Check, that default implementations do not introduce type class dependencies, that are not necessary otherwise

E.g. if you have a Monad constraint, but no Functor constraint, then use liftM instead of fmap.

• If you have the choice, whether to implement the method default by custom code or by calling other methods of the same class, call methods of the same class. This reduces the amount of implementation work for class instances. It also makes it more probable, that no extra super-classes are needed.

E.g. the default methods of the class providing divMod, div, and mod should implement the defaults, where divMod calls div and mod and vice versa.

Do not try to implement mod by repeated subtraction or so.

• Document which methods must be implemented at least.

E.g. "instances must implement divMod or (div and mod)"

• Instance implementations should not call other methods of the same class with respect to instantiated type.

See also

http://www.haskell.org/pipermail/haskell-cafe/2006-November/019329.html