On Thu, Jan 15, 2009 at 11:46 AM, Ross Mellgren <rmm-hask...@z.odi.ac> wrote: > > Usually when encountering something like "Monoid" (if I didn't already know > it), I'd look it up in the library docs. The problem I've had with this > tactic is twofold: > > First, the docs for the typeclass usually don't give any practical examples, > so sometimes it's hard to be sure that the "append" in "mappend" means what > you think it means.
The documentation for Monoid is embarrassingly brief. "The monoid class. A minimal complete definition must supply mempty and mappend, and these should satisfy the monoid laws." It doesn't even list the monoid laws! > Second is that there appears to be no way to document an _instance_. It > would be really handy if there were even a single line under "Instances > > Monoid ([] a)" that explained how the type class was implemented for the > list type. As it is, if you know what a Monoid is already, it's easy to > figure out how it would be implemented. Not necessarily. Any instance of MonadPlus (or Alternative) has at least two reasonable Monoid instances: (mplus, mzero) and (liftM2 mappend, return mempty). [] uses the first and Maybe uses the second. I recommend not creating direct instances of Monoid for this reason. If you want to use Monoid with Int, you have to use Sum Int or Product Int. -- Dave Menendez <d...@zednenem.com> <http://www.eyrie.org/~zednenem/> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
