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Question 2.3 - The naturality of the diagonal

\(\require{begingroup}\begingroup\require{AMScd}\newcommand{\tlap}[1]{\raisebox{0pt}[0pt][0pt]{#1}}\newcommand{\blap}[1]{\vbox to 0pt{\hbox{#1}\vss}}\) The naturality of the diagonal.

Let \(C\) and \(D\) be categories, and let \(F,G: C \rightarrow D\) be functors. Recall that a natural transformation \(\alpha: F \implies G\) consists of a morphism \(\alpha_c: F(c) \rightarrow G(c)\) for each \(c \in Ob(C)\), such that for each \(f: c_1 \rightarrow c_2\) in \(C\) the square:

\begin{CD} F(c_1) @>F(f)>> F(c_2) \\@V{\alpha_{c_1}}VV {}@VV{\alpha_{c_2}}V \\ G(c_1) @>>G(f)> G(c_2) \end{CD}

commutes in \(D\).

Write \(id_{Set}: Set \rightarrow Set\) for the identity functor on \(Set\), and Double: \(Set \rightarrow Set\) for the functor that sends a set \(X\) to the set \(X \times X\), and a function \(f: X \rightarrow Y\) to the function Double(\(f\)): \(X \times X \rightarrow Y \times Y\) that maps \((x_1,x_2)\) to \((f(x_1),f(x_2))\).

Part A. For each set \(X\), define the diagonal function \(\delta_x: X \rightarrow X \times X\) to map \(x\) to the pair \((x,x)\). Prove that \(\delta: id_{Set} \implies\) Double defines a natural transformation.

Part B. Using the universal property of the product, write a polymorphic Haskell function

diag :: a -> (a,a)

implementing this natural transformation.


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