Re: MonadFail proposal (MFP): Moving fail out of Monad

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 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 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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