Thanks for the further explanation.
On reflection there probably shouldn't be a default declaration for mzero.
Any compiler for Haskell must do *something* if a method is
called for which there is neither an explicit declaration in
the instance, nor a default method. ... Leaving out the default
method would let a compiler halt execution reporting
"Pattern match failure in `do' expression at line 39 of Foo.hs"
which is what we want.
Well, you can't always give such a message, because the monad might be
unresolved at the point the `do' appears (as in all your examples
where MonadZero had to appear in the scope). And you message looks
odd, since the problem is that mzero is undefined. But these are
implementation issues.
I see the attraction of adding `mzero' to Monad. The chief
disadvantage is that it makes the class hierarchy less expressive. Is
there anyone out there who values the separation of Monad and
MonadZero for purposes other than `do' notation? If not, then I
don't oppose this option.
I like Ralph's suggestion to use `fail' instead of `mzero', and
to change `fail' in IO to `raise', if that doesn't break too many
existing programs.
-- P
