On 2008 Oct 3, at 16:12, Andrew Coppin wrote:
Brandon S. Allbery KF8NH wrote:
On Oct 3, 2008, at 15:10 , Andrew Coppin wrote:
Again, it looks like MonadPlus == Monad + Monoid, except all the method names are different. Why do we have this confusing duplication?
Because typeclasses aren't like OO classes. Specifically: while you can specify what looks like class inheritance (e.g. "this Monad is also a Monoid" you can't override inherited methods (because it's a Monad, you can't specify as part of the Monad instance the definition of a Monoid class function). So if you want to define MonadPlus to look like a Monad and a Monoid, you have to pick one and *duplicate* the other (without using the same names, since they're already taken by the typeclass you *don't* choose).
I was thinking more, why not just delete MonadPlus completely, and have any function that needs a monad that's also a monoid say so in its context? (Obviously one of the answers to that is "because it would break vast amounts of existing code".)
It also touches on some Haskell98 braindamage. (Look up "MonadZero".) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH