## Category Theory for Programmers Chapter 15: The Yoneda Lemma

1. Show that the two functions phi and psi that form the Yoneda isomorphism in Haskell are inverses of each other.

phi :: (forall x . (a -> x) -> F x) -> F a
phi alpha = alpha id

psi :: F a -> (forall x . (a -> x) -> F x)
psi fa h = fmap h fa


However, since we know this is a bijection, all that remains to show the structure is preserved and that it is an isomorphism. Proving they are identities might be easier.

Need to verify phi . psi = id and psi . phi = id.

• Want to show: (phi . psi) fa = fa
• phi (psi fa)
• phi (\h -> psi fa h)
• phi (\h -> fmap h fa)
• (\h -> fmap h fa) id
• fmap id fa
• fa

Done.

• Want to show: (psi . phi) nt = nt
• psi (phi nt)
• psi (nt id)
• (\fa h -> fmap h fa) (nt id)
• (\h -> fmap h (nt id))
• …👐 something about natural transformations and functors…
• (\h -> nt (h . id)
• (\h -> nt h) = nt

Feels like to go this way, we have to look inside the natural transformation or fmap somehow to un-pack it. I can’t think of what’s needed.

2. How does the Yoneda lemma work for functors from a discrete category?

Discrete categories only contain objects and identity morphisms. So for the Yoneda lemma which states

$[\mathscr{C}, \mathbf{Set}](H^A, F) \cong F(A)$

we have that the set of natural transformations from $H^A$ to $F$ is the same as $F(A)$, then we only have that identity morphism on the RHS, and so on the LHS, we only have the identity natural transformation, or specifically, a singleton set.

3. Construct another representation of the data type ([()], list of units encoding $\mathbb{N}$) using the Yoneda lemma for the list functor.