Personal tools

Default method implementation

From HaskellWiki

(Difference between revisions)
Jump to: navigation, search
m (typo and grammar fixes)
Line 3: Line 3:
 
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, that default implementations do not introduce type class dependencies, that are not necessary otherwise
+
* 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 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.
+
* 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 16:03, 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
Monad
constraint, but no
Functor
constraint, then use
liftM
instead of
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
,
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