Exercise 4-30

  1. Justify all steps the proof of the unitality of unit profunctors.

  2. In the case of \(\mathcal{V}=\mathbf{Bool}\) show each step in the forward direction is actually an equality. NOCARD

Solution(1)
  1. Done, see the proof

    • \(\forall p: P(p,p)=true\) for a Bool-enriched category, because that is the only option for \(I=true\leq P(p,p)\)

      • \(true \land x = x\)

    • If \(\phi(p,q)\):

      • then at least one element of the set being joined over is true (where \(p=p'\) such that \(P(p,p')\land \phi(p',q) = true\)), and the least upper bound of such a set is \(true\)

    • Else:

      • Then every element of that set is \(false\) such that the join is also \(false\).

        • If \(p\leq p'\), it fails because of the second conjunct (consider the constraint on profunctors: we are demanding equal or more resources than something we know fails)

        • If \(p \not \leq p'\), it fails because of the first conjunct. In a Bool-category \(P\), \(P(a,b)\) iff \(a \leq b\).