[Haskell-beginners] Maybe, Either

[Brent Yorgey]

On Wed, Sep 16, 2009 at 01:19:53AM +0300, Michael Snoyman wrote:
> On Tue, Sep 15, 2009 at 11:49 PM, Brandon S. Allbery KF8NH <
> allbery at ece.cmu.edu> wrote:
> 
> > I'm looking at the Control.Monad documentation (
> > http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Monad.html),
> > and it doesn't seem that IO is an instance of MonadPlus. I get the same
> > results with a simple code check. Can you clarify? Calling lookup from IO is
> > a common use case for me.
> >
> >
> > That's a bit of stupidity I wish would be fixed; for some bizarre reason,
> > the instance is in Control.Monad.Error.
> >
> 
> And according to the documentation, that instance has a broken version of
> mzero. So it seems the argument of use MonadPlus because fail is not always
> defined properly doesn't exactly hold much water here. Can you tell me why
> you would still recommend representing lookup failure with the mzero
> function instead of the seemingly more aptly named and more available fail
> function?

The IO instance of mzero is only broken in the sense that it does not
satisfy the law

  m >> mzero === mzero

since the effects of m will happen on the left but not on the right.
But this is to be expected with the IO monad: if you have to perform
some I/O effects in order to find out whether the computation fails or
not, you can't expect to roll back the effects once you do fail.  In
practice I don't think this would be much of an issue.

But anyway, the reason I recommend not using fail is because it is an
inelegant, unprincipled hack.  If you have no problem with inelegant,
unprincipled hacks then I guess I won't be able to convince you. =)

Also, if you haven't already, you should read the email from Conor
McBride in this same thread: after reading his email I'm now of the
opinion that you shouldn't even use MonadPlus, but simply use Maybe.  

-Brent