MonadFail proposal (MFP): Moving fail out of Monad

[Edward Kmett]

You could handle that case explicitly by giving a class that converted a
string into e and putting that constraint on the MonadFail instance for
Either:

class Error a where
  strMsg :: String -> a

instance Error e => MonadFail (Either e) where
  fail = Left . strMsg

We used to do this in the mtl, with the Error class, but it then had to
encumber the entire Monad, so even folks who didn't want it needed to
supply a garbage instance.

Right now, fail for Either is necessarily _error_ because we can't put it
in the left side without incurring a constraint on every user of the monad.

At least here the ad hoc construction can be offloaded to the particular
MonadFail instance, or to whatever monad someone makes up for working with
their Either-like construction.

-Edward

On Wed, Jun 10, 2015 at 5:44 PM, David Feuer <david.feuer at gmail.com> wrote:

> My main concern, I suppose, is that I don't see a way (without
> extensions) to deal with even the most basic interesting failure
> monad: Either e. It therefore seems really only to be suitable for
> pattern match failure and user-generated IOErrors, which don't really
> strike me as terribly natural bedfellows.
>
> On Wed, Jun 10, 2015 at 10:55 AM, Edward Kmett <ekmett at gmail.com> wrote:
> > This would require you to add MPTCs to the language standard, which means
> > standardizing how they work.
> >
> > Any solution involving SomeException or any of its variants is going to
> drag
> > in GADTs, Typeable, higher rank types.
> >
> > ... and it would drag them inexorably into the Prelude, not just base.
> >
> > Compared to a simple
> >
> > class Monad m => MonadFail m where
> >   fail :: String -> m a
> >
> > that is a very hard sell!
> >
> > On the other hand, I do think what we could do is add more information
> about
> > pattern match failures by adding another member to the class
> >
> > class Monad m => MonadFail m where
> >   patternMatchFailure :: Location -> String -> whatever else you like ->
> m a
> >   patternMatchFailure l s ... = fail (code to generate the string we
> > generate in the compiler using just the parts we're passed)
> >
> >   fail :: String -> m a
> >
> > Then the existing 'fail' desugaring could be done in terms of this
> > additional member and its default implementation.
> >
> > This remains entirely in the "small" subset of Haskell that is well
> behaved.
> > It doesn't change if we go and radically redefine the way the exception
> > hierarchy works, and it doesn't require a ton of standardization effort.
> >
> > Now if we want to make the fail instance for IO or other MonadThrow
> > instances package up the patternMatchFailure and throw it in an
> exception we
> > have the freedom, but we're avoid locking ourselves in to actually
> trying to
> > figure out how to standardize all of the particulars of the exception
> > machinery into the language standard.
> >
> > -Edward
> >
> > On Wed, Jun 10, 2015 at 4:19 PM, David Feuer <david.feuer at gmail.com>
> wrote:
> >>
> >> Here's a crazy question: would a version of MonadError without the
> >> fundep do the trick?
> >>
> >> class Monad m => MonadFail e m where
> >>   fail :: e -> m a
> >>
> >> instance MonadFail a [] where
> >>   fail = const []
> >>
> >> instance (a ~ e) => MonadFail e (Either a) where
> >>   fail = Left
> >>
> >> instance MonadFail SomeException IO where
> >>   fail = throwIO
> >> instance MonadFail IOException IO where
> >>   fail = throwIO
> >> ...
> >> instance MonadFail String IO where
> >>   fail = throwIO . userError
> >>
> >> On Wed, Jun 10, 2015 at 8:32 AM, Mario Blažević <blamario at ciktel.net>
> >> wrote:
> >> > +1 from me.
> >> >
> >> >     A minor nitpick: the proposal should clarify which of the existing
> >> > instances of Monad from base get a MonadFail instance. My
> understanding
> >> > is
> >> > that none of them would define fail = error, but that has not been
> made
> >> > explicit.
> >> >
> >> >
> >> > _______________________________________________
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> >
> >
>
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