On 15/12/2010 04:01, Jonathan Geddes wrote:
Fail can't just be removed. That would just break too much code. For example, I find myself writing code like the following:
[a,b,c]<- Just someList
in place of
let [a,b,c] = someList
so that pattern match failure is "lifted" into the maybe monad (as long as I'm already in the maybe monad).
I would like to see a MonadFail class, with one method 'fail', such that it is a compile-time error to try 'failable' pattern matches in a monad that is not an instance of MonadFail.
--Jonathan
Perhaps pattern match failures in a MonadPlus should bind to mzero - I believe that this is what your example and similar wish to achieve. I've updated http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal to mention this. Does anyone know why fail and Pointed (in the paragraph "This would eliminate...") aren't beginning a new paragraph? They are both preceded by a blank line in the wiki source.