Add Extend and Semigroup instances.

[joneshf]

No description provided.

[garyb]

Nice, thanks.

[paf31]

Is this the only way of extending a Semigroup? Seems like this would also be valid:

instance semigroupMaybe :: (Semigroup a) => Semigroup (Maybe a) where
  (<>) (Just x) (Just y) = Just (x <> y)
  (<>) _ _ = Nothing

Seems like your case is already handled by First in purescript-monoids.

Also, just for my own curiosity, what is the Extend instance used for?

[paf31]

Sorry, ignore the part about First. Obviously incorrect.

Still, does it make sense to newtype this?

[garyb]

The version I just merged is the way Haskell implements Monoid Maybe, so I figure it's right.

Extend is the superclass of Comonad, it's bind with the arrows reversed.

[paf31]

I was just wondering what it's used for. I mean, I know what the Maybe monad is used for, but Maybe isn't a Comonad, so I don't see an obvious meaning of the Extend instance. Trying to interpret it in terms of tree redecoration but having trouble.

[paf31]

Why not just (<<=) f x = Just $ f x for example? Does that violate one of the laws?

[garyb]

Ah, I see what you mean. @joneshf?

[joneshf]

Right, I think they're both lawful instances and it comes down to semantics. Though, one instance might play nicer with CoFree whereas the other might not. Not entirely sure there.

I think short cutting the Nothing keeps the semantics of it being optional. Always passing it to the function means we'd have to join . If we try to duplicate Nothing, it seems sensible we get Nothing. Maybe it makes more sense to duplicate Nothing and get Just Nothing. Maybe since they're both lawful, we should newtype them and call it a day.

Thoughts?

[paf31]

I don't mind about Extend, I was just curious more than anything else. Let's wait and see if anyone has any comments I guess.

[joneshf]

Sounds good to me.

Honestly, I don't have an immediate use for this. I just saw it could be done, so I made a PR :D

[garyb]

Should I do a release of this with Extend as it is then? Since it's already merged...

[paf31]

Sounds good to me.