# Default method implementation

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− | It's always a design question, whether to provide default implementions of methods of [[Type class]]es and how to | + | It's always a design question, whether to provide default implementions of methods of [[Type class]]es and how to design them. |

Here are some suggestions: | Here are some suggestions: | ||

* Make only [[slim instance declaration|trivial]] default implementations. | * Make only [[slim instance declaration|trivial]] default implementations. | ||

− | * Check | + | * Check that default implementations do not introduce type class dependencies that are not necessary otherwise |

: E.g. if you have a <hask>Monad</hask> constraint, but no <hask>Functor</hask> constraint, then use <hask>liftM</hask> instead of <hask>fmap</hask>. | : E.g. if you have a <hask>Monad</hask> constraint, but no <hask>Functor</hask> constraint, then use <hask>liftM</hask> instead of <hask>fmap</hask>. | ||

− | * If you have | + | * If you have a choice of 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 <hask>divMod</hask>, <hask>div</hask>, and <hask>mod</hask> should implement the defaults, where <hask>divMod</hask> calls <hask>div</hask> and <hask>mod</hask> and vice versa. | : E.g. the default methods of the class providing <hask>divMod</hask>, <hask>div</hask>, and <hask>mod</hask> should implement the defaults, where <hask>divMod</hask> calls <hask>div</hask> and <hask>mod</hask> and vice versa. | ||

: Do not try to implement <hask>mod</hask> by repeated subtraction or so. | : Do not try to implement <hask>mod</hask> by repeated subtraction or so. |

## Revision as of 16:04, 7 June 2007

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 constraint, but noMonadconstraint, then useFunctorinstead ofliftM.fmap

- If you have a choice of 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, anddivshould implement the defaults, wheremodcallsdivModanddivand vice versa.mod
- Do not try to implement by repeated subtraction or so.mod

- Document which methods must be implemented at least.

- E.g. "instances must implement or (divModanddiv)"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