MonadFail proposal (MFP): Moving fail out of Monad

Edward Kmett ekmett at gmail.com
Wed Jun 10 19:48:07 UTC 2015 

I mentioned in another thread on this topic that it may be perfectly
reasonable to extend the class with another method that handles just the
pattern match failure case with the details necessary to reproduce the
current errors.

class Monad m => MonadFail m where
  fail :: String -> m a
  patternMatchFailure :: Location -> CallStack -> Whatever Other
Information You Like -> String -> m a
  patternMatchFailure l w ... = fail (code to generate the string we
produce now using the inputs given)

Then a particular concrete MonadFail instance could choose to throw a GHC
style extensible exception, it could format the string, it could default to
mzero, etc.

instance MonadFail IO where
  patternMatchFailure a b c .. = throwIO $ PatternMatchFailure a b c ..

But if we don't capture the location information / string / whatever
_somehow_ then we lose information relative to the status quo, just by
going down to mzero on a failure. Users use this in their debugging today
to find where code went wrong that they weren't expecting to go wrong.

Beyond handling these two "traditional" error cases, I think everything
else should be left to something that doesn't infect as central a place as
Prelude.

Doing a "general" MonadError with fundeps or without fundeps and just MPTCs
still requires you to extend the language of the standard to support
language features it doesn't currently incorporate.

Trying to upgrade 'fail' itself to take an argument that isn't just a
String breaks all the code that uses -XOverloadedStrings, so if you want
more information it is going to have to be in a different method than fail,
but it could live in the same class.

Finally fail has different semantics than mzero for important monads like
STM that exist today.

-Edward

On Wed, Jun 10, 2015 at 6:07 PM, David Feuer <david.feuer at gmail.com> wrote:

> But why does a string actually make sense in the context of handling
> pattern match failures? Sticking to a Haskell 98 solution, I would
> think MonadZero would be the way to go for those, rather than
> MonadFail. What, after all, can you really do with the string
> generated by a pattern match failure? For everything other than
> pattern match failures, I would think the user should use MonadError,
> a non-fundep MonadError, or just work directly without classes.
>
> On Wed, Jun 10, 2015 at 11:53 AM, Edward Kmett <ekmett at gmail.com> wrote:
> > 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.
> >> >> >
> >> >> >
> >> >> > _______________________________________________
> >> >> > Libraries mailing list
> >> >> > Libraries at haskell.org
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> >> >> _______________________________________________
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> >> >
> >> >
> >
> >
>
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