On Mon, Sep 23, 2019 at 08:05:38PM +0100, Juan Casanova wrote:
data SOTermPF fn p f = ConstF fn | Proj Int | CompF p [f] deriving Eq newtype SOTermF fn f = SOF (SOTermPF fn f f) deriving Eq
My Traversable instance (with comments on intermediate types because otherwise it can get pretty obscure):
Everyone else has made good comments, but to add a little and to be a little more explicit: your data type uses [] so it makes sense to directly inherit the Traversable (and hence Foldable) instance from that. Your instance was a little convoluted. Here's what I would suggest: instance Foldable (SOTermF fn) where foldMap = foldMapDefault instance Traversable (SOTermF fn) where traverse f (SOF (ConstF c)) = pure (SOF (ConstF c)) traverse f (SOF (Proj idx)) = pure (SOF (Proj idx)) traverse f (SOF (CompF g sargs)) = SOF <$> (CompF <$> f g <*> traverse f sargs)