There is no need to have both `mzero' and `mfail' in every monad.
Just have `mfail'. Leave `zero' and `plus' to MonadPlus. This should
make Eric partially happy. It also means one can simply write
instance Monad [] where
...return, >>=, >> as before...
mfail s = []
rather than
instance Monad [] where
...return, >>=, >> as before...
mfail s = mzero
mzero = []
The names `mzero' and `mfail' are horrible. I like Ralph's suggestion
to change `fail' to `raise' in the IO monad, and use `fail' for
`mfail'. If that doesn't work, try something else, but please
pick names that have a simple meaning in English (as with `return')
not monsters like `mzero' and `mfail'. -- P
