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Consider the following definitions

LIST:

$\overline{nil} \ \ \ \ \ \frac{l}{a \ l}$ $a \in A$

$A^* = \mu \widehat{LIST}, \ A^{\infty} = v \widehat{LIST}$

NAT:

$\overline{0} \ \ \ \ \ \frac{x}{s(x)}$

$ N = \mu \widehat{NAT}, \ N^{\infty} = v \widehat{NAT}$

Given those definitions I have to define a length function over $A^{*} \leftarrow{N}$ such that $length(l)$ outputs the length of $l$.

I have tried to do this problem using structural induction on a given $l$ but I don't seem to get any relevant result.

In general, how would you approach this kind of problem?

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    $\begingroup$ The idea is that $\operatorname{length}(\mathrm{nil}) = 0$ whereas $\operatorname{length}(a\ell) = s(\operatorname{length}(\ell))$. $\endgroup$ – Yuval Filmus Oct 17 '18 at 4:05
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Let F denote a “shape”, as your LIST and NAT shapes for example.

Declaring $I := μF$ is tantamount to saying that $I$ is recursively defined by some $α : F(I) ≅ I$ and moreover for any $β : F(K) → K$ there is a unique $f : I → K$ that respects the “F”-structure.

Since you define $A^* = μ(LIST)$, where the “LIST” structure on a set $X$ consists of a constant $c : X$ and a function $n : A × X → X$ --- c.f., your $nil, -·-$ --- we automatically obtain an operation $A^* → ℕ$ if we can endow $ℕ$ with a “LIST” structure; namely a constant such as zero $0 : ℕ$ and an operation on ℕ such as $A \times ℕ → ℕ : (a, n) ↦ s\, n$.

So by the μ-definition of lists, since we furnished ℕ with a LIST structure, we have that there is a unique function, call it $length$, that respects the LIST structure in that it takes the shape constants-to-constants and the shape functions-to-functions: $$ \mathsf{length}(nil) \;=\; 0 $$ $$ \mathsf{length}(a \cdot l) \;=\; s(\mathsf{length}\, l) $$

Alternatively, these properties also serve to “define length by structural induction on the shape of lists”.

Hope that helps a bit :-)

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