| > 1.Fix up the current version.
| > use MonadZero for do expressions with *irrefutable* patterns
| > (instead of *unfailable* patterns as now)
| > 2.Nuke MonadZero altogether.
| > add mfail :: m a to Monad instead
There is another variation on 2 that you don't seem to be considering:
- Retain MonadZero
(So you don't break existing code using MonadZero)
- Add mfail :: m a to Monad, with a default definition like
mfail = error "fail", and prelude definitions of mfail = [] and
mfail = Nothing, for lists and Maybe, respectively.
(So everything works as you'd expect without changing user code)
- Define the semantics of do notation solely in terms of the operators
of the (revised) Monad class, >>=, return, and mfail.
(So types don't change when you use tuples, add extra constructors, etc.)
A happy compromise between the alternatives you give?
All the best,
Mark
