It's not so much that it's *necessary* as that it's *possible*. The
existence of two functions in Data.Traversable explains both of the
superclasses of Traversable:
fmapDefault :: Traversable t => (a -> b) -> t a -> t b
foldMapDefault :: (Traversable t, Monoid m) => (a -> m) -> t a -> m
Each of these is written using only traverse, and they can be used to
define fmap and foldMap for types when you've written traverse.
Hint: Consider traversing using the following applicative functors:
newtype Const a b = Const a
instance Monoid a => Applicative (Const a)
newtype Identity a = Identity a
instance Applicative Identity
On Feb 5, 2016 1:45 PM, "David Banas"
Hi all,
I don't understand why Foldable is a necessary super-class of Traversable, and I suspect that the Applicative/Monoid duality, which I've just begun discovering in the literature, has something to do with why that is so.
Can anyone give me a hint, without giving me the answer?
Thanks! -db
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