MonadFail proposal (MFP): Moving fail out of Monad

[Dan Doel]

This is all well defined in the Haskell 1.4 report. Back then Haskell had
"unfailable" patterns for desugaring do notation, because there was no fail
(it was introduced in H98).

I believe a pattern is classified as unfailable there if it is irrefutable
or refutable only by bottom. Which of course is the distinction here, (x,y)
is unfailable, but not irrefutable.

And of course, it seems that GHC never actually stopped implementing
unfailable patterns, even though they were removed from the report (or
someone added it back at some point). You just have to know how to observe
this fact.

-- Dan

On Thu, Jun 11, 2015 at 12:11 PM, David Turner <
dct25-561bs at mythic-beasts.com> wrote:

> Quoting the Haskell 2010 Report section 3.17.2: Attempting to match a
> pattern can have one of three results: it may fail; it may succeed
> ...; or it may diverge. Then in paragraph 5:
>
> Matching the pattern con pat1 ... patn against a value, where con is a
> constructor defined by data, depends on the value:
> - If the value is of the form con v1 ... vn, sub-patterns are matched
> left-to-right against the components of the data value; if all matches
> succeed, the overall match succeeds; the first to fail or diverge
> causes the overall match to fail or diverge, respectively.
> - If the value is of the form con' v1 ... vm, where con is a different
> constructor to con', the match fails.
> - If the value is ⊥, the match diverges.
>
> In particular, matching (_,_) can only succeed or diverge: failure is
> not an option! Desugaring 'do' handles match failure with a catch-all
> case that calls 'fail' but doesn't handle ⊥.
>
>
>
> On 11 June 2015 at 16:28, Wolfgang Jeltsch <g9ks157k at acme.softbase.org>
> wrote:
> > Are you sure that desugaring works this way? If yes, this should be
> > considered a bug and be fixed, I would say. It is very illogical.
> >
> > All the best,
> > Wolfgang
> >
> > Am Donnerstag, den 11.06.2015, 16:23 +0100 schrieb David Turner:
> >> AIUI the point about ⊥ and (⊥, ⊥) being different doesn't matter here:
> >> a bind for a single-constructor datatype never desugars in a way that
> >> uses fail (which isn't to say that it can't be undefined)
> >>
> >> For instance:
> >>
> >> runErrorT (do { (_,_) <- return undefined; return () } :: ErrorT String
> IO ())
> >>
> >> throws an exception, even though the bind is in ErrorT where fail just
> >> returns left:
> >>
> >> runErrorT (do { fail "oops"; return () } :: ErrorT String IO ())
> >>
> >> => Left "oops"
> >>
> >> Hope that helps, and hope I understand correctly!
> >>
> >> David
> >>
> >>
> >> On 11 June 2015 at 16:08, Wolfgang Jeltsch <g9ks157k at acme.softbase.org>
> wrote:
> >> > Hi David,
> >> >
> >> > thank you very much for this proposal. I think having fail in Monad is
> >> > just plain wrong, and I am therefore very happy to see it being moved
> >> > out.
> >> >
> >> > I have some remarks, though:
> >> >
> >> >> A class of patterns that are conditionally failable are `newtype`s,
> >> >> and single constructor `data` types, which are unfailable by
> >> >> themselves, but may fail if matching on their fields is done with
> >> >> failable paterns.
> >> >
> >> > The part about single-constructor data types is not true. A
> >> > single-constructor data type has a value ⊥ that is different from
> >> > applying the data constructor to ⊥’s. For example, ⊥ and (⊥, ⊥) are
> two
> >> > different values. Matching ⊥ against the pattern (_, _) fails,
> matching
> >> > (⊥, ⊥) against (_, _) succeeds. So single-constructor data types are
> not
> >> > different from all other data types in this respect. The dividing line
> >> > really runs between data types and newtypes. So only matches against
> >> > patterns C p where C is a newtype constructor and p is unfailable
> should
> >> > be considered unfailable.
> >> >
> >> >>   - Applicative `do` notation is coming sooner or later, `fail` might
> >> >>     be useful in this more general scenario. Due to the AMP, it is
> >> >>     trivial to change the `MonadFail` superclass to `Applicative`
> >> >>     later. (The name will be a bit misleading, but it's a very small
> >> >>     price to pay.)
> >> >
> >> > I think it would be very misleading having a MonadFail class that
> might
> >> > have instances that are not monads, and that this is a price we should
> >> > not pay. So we should not name the class MonadFail. Maybe, Fail would
> be
> >> > a good name.
> >> >
> >> >> I think we should keep the `Monad` superclass for three main reasons:
> >> >>
> >> >>   - We don't want to see `(Monad m, MonadFail m) =>` all over the
> place.
> >> >
> >> > But exactly this will happen if we change the superclass of
> (Monad)Fail
> >> > from Monad to Applicative. So it might be better to impose a more
> >> > light-weight constraint in the first place. Functor m might be a good
> >> > choice.
> >> >
> >> > All the best,
> >> > Wolfgang
> >> >
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> >
> >
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