instance Foldable f => Foldable1 (Cofree f)

[Icelandjack]

I wondered if the constraint on Foldable1 (Cofree f) could be weakened, this is my attempt

instance Foldable f => Foldable1 (Cofree f) where
  foldMap1 :: Semigroup m => (a -> m) -> (Cofree f a -> m)
  foldMap1 f (a :< (!as)) =
    foldMap1 f (a :| toList (foldMap (foldMap1 Single) as))

data FMG a = None | Single a | FMG a :<> FMG a
  deriving Foldable

instance Semigroup (FMG a) where
  (<>) = (:<>)

instance Monoid (FMG a) where
  mempty = None
  mappend = (:<>)

[Icelandjack]

I want a more elegant solution to this, but the instance makes sense: Cofree Maybe is the non-empty list and as such should have a Foldable1 instance even though Maybe is just Foldable. This can probably be nicer using reflection

[ElvishJerricco]

Just freely lift the semigroup to a monoid using Option and use foldMap. I'm on mobile, so forgive me if this doesn't compile or has typos:

instance Foldable f => Foldable1 (Cofree f) where
  foldMap1 f (a :< as) = case getOption $ foldMap (Option . Just . foldMap1 f) as of
    Just b -> f a <> b
    Nothing -> f a

[Icelandjack]

That compiles smoothly and answers my question, I imagine the same can be written for Traversable1.. it always fun when the context differs like this.

Should this be added?

[xuwei-k]

FYI scalaz/scalaz@a05ab2c in scalaz

[andrewthad]

What's the status of this? @ElvishJerricco Would you take a PR for weakening the Foldable1 constraint in the instance head to Foldable?