FWIW, Ed Kmett has this already in his semigroupoids package as Data.Functor.Alt: https://hackage.haskell.org/package/semigroupoids On 05/04/15 18:05, Mario Blažević wrote:
On 03/29/2015 08:20 AM, Jeremy wrote:
The proposal to make Semigroup a superclass of Monoid was discussed a while ago [1], and the conclusion was to "put this off until the dust has settled from the AMP and FT changes".
It has occurred to me that the Alternative class, which is pretty much a constructor version of Monoid, would benefit from the same treatment. The greatest difficulty seems to be in finding the name for the Semigroup-equivalent superclass of Alternative. For lack of imagination,I'll just call it Semigroup1:
class Applicative f => Semigroup1 f where (<|>) :: f a -> f a -> f a some :: f a -> f [a] many :: f a -> f [a]
class Semigroup1 f => Alternative f where empty :: f a
The rationale for this change:
1. All the theoretical arguments for introducing Semigroup carry over with no change.
2. There are existing data types that could be made instances of Semigroup1, and not of Alternative. I have not performed any comprehensive survey, but QuickCheck's Gen type can provide one example:
instance Semigroup1 Gen where f <|> g = oneof [f, g]
3. Most interesting instances of Alternative are various parser types. A parser type that doesn't define empty is a parser that cannot fail by construction. That makes intuitive sense. I can imagine some benefits from a set of parser combinators that generated two types, only one of which could fail. Fill in the blanks:
instance Semigroup1 IrrefutableParser instance Alternative Parser
token :: a -> Parser a optional :: Parser a -> IrrefutableParser (Maybe a) many0 :: Parser a -> IrrefutableParser [a]
There are, of course, two important differences between the Monoid and Alternative situation. First, the Semigroup class already exists, if not in base. That only means the proposal outlined above requires more critique and bikeshedding.
The other difference is that the Semigroup operator (<>) is different from the Monoid operator mappend. This is mostly an unfortunate backward compatibility issue, but it does have a benefit of allowing the two classes to exist independently for a while before they are brought together. If we really, *really* desire the same doubtful benefit for Alternative, we can use a new operator name for the Semigroup1 class (I refuse to provide one in this proposal) and then eventually stipulate (<|>) = i-refuse-to-provide-one-in-this-proposal.
My preference, if available, would be to just move the <|> operator into the superclass in 7.12. It's such a perfect operator name. Without a new GHC extension, however, this would mean that every existing instance of Alternative would be broken in 7.12. Furthermore, there would be no way to write an Alternative instance valid in both 7.10 and 7.12 without CPP directives. The only saving grace here is that the Alternative instances are relatively few, and it may just be possible to coordinate all the changes.
So, here's a question to all authors of parser libraries and everybody else that has defined Alternative instances: would you be able and willing to add CPP directives to your code for this purpose?
For everybody else: this is a proposal, so vote on it.
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