On Wed, Jun 30, 2010 at 12:13:07AM +0200, Claus Reinke wrote:
The proposal is to move the Monad and MonadFix instances for Either (currently in the mtl package) to Control.Monad.Instances and Control.Monad.Fix respectively (both in the base package). The Monad instance is still an orphan, to retain Haskell 98 compatibility, but the MonadFix instance is together with its class. The Error constraint is removed from both instances, and the default definition of fail is used.
-1, because the default definition of fail is error, which would render the Monad instance useless (unless I'm missing something?).
You would just use Left instead of 'fail' if you want to show failure in an Either value.
a) you are suggesting to bypass a method of the proposed Monad instance because it isn't useful, I am suggesting not to define a Monad instance with unusable methods b) desugaring of pattern-match failure in Monads calls fail, not Left
However, there is no particular reason to associate 'Left' with failure. Not having fail be 'Left' is a signifigant feature, not a limitation of an Either instance. A notable example would be a constraint solver, an algorithm might attempt various solution strategies, even recursively calling itself on subproblems and return the first solution found via 'Left', happily jumping out of the whole computation with its prize. Strangely enough, this means 'Left' indicates success and 'Right' indicates failure in some sense. Defining 'fail' to be Left anything is clearly not correct. 'Either' is a monad with a non-local return, _not_ necessarily an error monad. The fact we can't come up with a sensible 'fail' for it is not an indication of a problem with fail, or with either, but with simply assuming incorrectly that a non-local return must mean failure, we should have accepted what the type system was telling us and realized that we shouldnt be giving it a fail :). I do think we should make a dedicated failure monad though. It is also a useful thing. John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/