# Elt

Posted on 2017-05-20 by nbloomf

This page is part of a series on Arithmetic Made Difficult.

This post is literate Haskell; you can load the source into GHCi and play along.

{-# LANGUAGE NoImplicitPrelude #-}
module Elt
( elt, _test_elt, main_elt
) where

import Testing
import Booleans
import Not
import And
import Or
import Tuples
import NaturalNumbers
import LessThanOrEqualTo
import Lists
import Snoc
import Reverse
import Cat
import Map
import Range
import PrefixAndSuffix
import AllAndAny
import TailsAndInits
import Filter

Today we’ll define a boolean function, $$\elt$$, which detects whether or not a given $$a$$ is an item in a list. As usual we want to define $$\elt$$ as a fold: should we use a right fold or a left fold? Recall that the tradeoff between the two is that foldl is tail recursive, but foldr does not necessarily have to process the entire list. In this case, $$\elt(a)(x)$$ should be able to return early once it sees an $$a$$. So lets say $\elt(a)(x) = \foldr(\varepsilon)(\varphi)(x).$ Now what should $$\varepsilon$$ and $$\varphi$$ be? Well, we want $\begin{eqnarray*} & & \bfalse \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \elt(a)(\nil) \\ & = & \foldr(\varepsilon)(\varphi)(x) \\ & = & \varepsilon, \end{eqnarray*}$ and if $$a = b$$ we want $\begin{eqnarray*} & & \btrue \\ & = & \elt(a)(\cons(b,x)) \\ & = & \foldr(\varepsilon)(\varphi)(\cons(b,x)) \\ & \href{/posts/arithmetic-made-difficult/Lists.html#def-foldr-cons} = & \varphi(b,\foldr(\varepsilon)(\varphi)(x)) \\ & = & \varphi(b,\elt(a)(x)) \end{eqnarray*}$ while if $$a \neq b$$ we want $\begin{eqnarray*} & & \elt(a)(x) \\ & = & \elt(a)(\cons(b,x)) \\ & = & \foldr(\varepsilon)(\varphi)(\cons(b,x)) \\ & \href{/posts/arithmetic-made-difficult/Lists.html#def-foldr-cons} = & \varphi(b,\foldr(\varepsilon)(\varphi)(x)) \\ & = & \varphi(b,\elt(a)(x)). \end{eqnarray*}$ With this in mind we define $$\elt$$ like so.

Let $$A$$ be a set and let $$a \in A$$. Define $$\varphi : A \rightarrow \bool \rightarrow \bool$$ by $\varphi(b,p) = \bif{\beq(a,b)}{\btrue}{p}.$ Now define $$\elt : A \rightarrow \bool^{\lists{A}}$$ by $\elt(a)(x) = \foldr(\bfalse)(\varphi)(x).$

In Haskell:

elt :: (List t, Equal a) => a -> t a -> Bool
elt a = foldr false phi
where
phi b p = if eq a b then true else p

Since $$\elt$$ is defined as a $$\foldr(\ast)(\ast)$$, it can be characterized as the unique solution to a system of functional equations.

Let $$A$$ be a set with $$a \in A$$. $$\elt$$ is the unique map $$f : \lists{A} \rightarrow \bool$$ satisfying the following equations for all $$b \in A$$ and $$x \in \lists{A}$$. $\left\{\begin{array}{l} f(\nil) = \bfalse \\ f(\cons(b,x)) = \bif{\beq(a,b)}{\btrue}{f(x)} \end{array}\right.$

_test_elt_nil :: (List t, Equal a, Equal (t a))
=> t a -> Test (a -> Bool)
_test_elt_nil t =
testName "elt(a)(nil) == false" $\a -> eq (elt a (nil withTypeOf t)) false _test_elt_cons :: (List t, Equal a, Equal (t a)) => t a -> Test (a -> a -> t a -> Bool) _test_elt_cons _ = testName "elt(a)(cons(b,x)) == if(eq(a,b),true,elt(a)(x))"$
\a b x -> eq
(elt a (cons b x))
(if eq a b then true else elt a x)

Special cases.

Let $$A$$ be a set. For all $$a,b \in A$$, we have $\elt(a,\cons(b,\nil)) = \beq(a,b).$

Note that $\begin{eqnarray*} & & \elt(a,\cons(b,\nil)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bif{\beq(a,b)}{\btrue}{\elt(a,\nil)} \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bif{\beq(a,b)}{\btrue}{\bfalse} \\ & = & \beq(a,b) \end{eqnarray*}$ as claimed.

_test_elt_single :: (List t, Equal a, Equal (t a))
=> t a -> Test (a -> a -> Bool)
_test_elt_single t =
testName "elt(a)(cons(b,nil)) == eq(a,b)" $\a b -> eq (elt a (cons b (nil withTypeOf t))) (eq a b) $$\elt$$ interacts with $$\cat$$. Let $$A$$ be a set. For all $$a \in A$$ and $$x,y \in \lists{A}$$ we have $\elt(a)(\cat(x,y)) = \bor(\elt(a)(x),\elt(a)(y)).$ We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a)(\cat(x,y)) \\ & = & \elt(a)(\cat(\nil,y)) \\ & \href{/posts/arithmetic-made-difficult/Cat.html#cor-cat-nil} = & \elt(a)(y) \\ & \href{/posts/arithmetic-made-difficult/Or.html#thm-or-false-left} = & \bor(\bfalse,\elt(a)(y)) \\ & = & \bor(\elt(a)(\nil),\elt(a)(y)) \\ & \hyp{x = \nil} = & \bor(\elt(a)(x),\elt(a)(y)) \end{eqnarray*}$ as claimed. For the inductive step, suppose the equality holds for all $$y$$ some $$x$$, and let $$b \in A$$. If $$b = a$$ we have $\begin{eqnarray*} & & \elt(a)(\cat(\cons(b,x),y)) \\ & \href{/posts/arithmetic-made-difficult/Cat.html#cor-cat-cons} = & \elt(a)(\cons(b,\cat(x,y))) \\ & = & \bif{\beq(a,b)}{\btrue}{\elt(a)(\cat(x,y))} \\ & = & \btrue \\ & \href{/posts/arithmetic-made-difficult/Or.html#thm-or-true-left} = & \bor(\btrue,\elt(a)(y)) \\ & = & \bor(\bif{\beq(a,b)}{\btrue}{\elt(a)(x)},\elt(a)(y)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bor(\elt(a)(\cons(b,x)),\elt(a)(y)) \end{eqnarray*}$ as claimed. If $$b \neq a$$, we have $\begin{eqnarray*} & & \elt(a)(\cat(\cons(b,x),y)) \\ & \href{/posts/arithmetic-made-difficult/Cat.html#cor-cat-cons} = & \elt(a)(\cons(b,\cat(x,y))) \\ & = & \bif{\beq(a,b)}{\btrue}{\elt(a)(\cat(x,y))} \\ & = & \elt(a)(\cat(x,y)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#thm-elt-cat} = & \bor(\elt(a)(x),\elt(a)(y)) \\ & = & \bor(\bif{\beq(a,b)}{\btrue}{\elt(a)(x)},\elt(a)(y)) \\' & = & \bor(\elt(a)(\cons(b,x)),\elt(a)(y)) \end{eqnarray*}$ as claimed. _test_elt_cat :: (List t, Equal a, Equal (t a)) => t a -> Test (a -> t a -> t a -> Bool) _test_elt_cat _ = testName "elt(a)(cat(x,y)) == or(elt(a)(x),elt(a)(y))"$
\a x y -> eq (elt a (cat x y)) (or (elt a x) (elt a y))

$$\elt$$ interacts with $$\snoc$$.

Let $$A$$ be a set, with $$a \in A$$. For all $$b \in A$$ and $$x \in \lists{A}$$ we have $\elt(a)(\snoc(b,x)) = \bif{\beq(a,b)}{\btrue}{\elt(a)(x)}.$

We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a)(\snoc(b,\nil)) \\ & \href{/posts/arithmetic-made-difficult/Snoc.html#cor-snoc-nil} = & \elt(a)(\cons(b,\nil)) \\ & = & \bif{\beq(a,b)}{\btrue}{\elt(a)(\nil)} \end{eqnarray*}$ as needed. For the inductive step, suppose the equality holds for all $$b$$ for some $$x$$, and let $$c \in A$$. Now $\begin{eqnarray*} & & \elt(a)(\snoc(b,\cons(c,x))) \\ & \href{/posts/arithmetic-made-difficult/Snoc.html#cor-snoc-cons} = & \elt(a)(\cons(c,\snoc(b,x))) \\ & = & \bif{\beq(a,c)}{\btrue}{\elt(a)(\snoc(b,x))} \\ & \href{/posts/arithmetic-made-difficult/Elt.html#thm-elt-snoc} = & \bif{\beq(a,c)}{\btrue}{\bif{\beq(a,b)}{\btrue}{\elt(a)(x)}} \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#thm-if-commute-false} = & \bif{\beq(a,b)}{\btrue}{\bif{\beq(a,c)}{\btrue}{\elt(a)(x)}} \\ & = & \bif{\beq(a,b)}{\btrue}{\elt(a)(\cons(c,x))} \end{eqnarray*}$ as needed.

_test_elt_snoc :: (List t, Equal a, Equal (t a))
=> t a -> Test (a -> a -> t a -> Bool)
_test_elt_snoc _ =
testName "elt(a)(snoc(b,x)) == if(eq(a,b),true,elt(a)(x))" $\a b x -> eq (elt a (snoc b x)) (if eq a b then true else elt a x) $$\elt$$ interacts with $$\rev$$. Let $$A$$ be a set. For all $$a \in A$$ and $$x \in \lists{A}$$ we have $\elt(a)(x) = \elt(a)(\rev(x)).$ We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a)(\rev(x)) \\ & = & \elt(a)(\rev(\nil)) \\ & \href{/posts/arithmetic-made-difficult/Reverse.html#cor-rev-nil} = & \elt(a)(\nil) \\ & = & \elt(a)(x) \end{eqnarray*}$ as claimed. For the inductive step, suppose the equality holds for some $$x$$ and let $$b \in A$$. Now $\begin{eqnarray*} & & \elt(a)(\rev(\cons(b,x))) \\ & \href{/posts/arithmetic-made-difficult/Reverse.html#cor-rev-cons} = & \elt(a)(\snoc(b,\rev(x))) \\ & = & \bif{\beq(a,b)}{\btrue}{\elt(a)(\rev(x))} \\ & \href{/posts/arithmetic-made-difficult/Elt.html#thm-elt-rev} = & \bif{\beq(a,b)}{\btrue}{\elt(a)(x)} \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \elt(a)(\cons(b,x)) \end{eqnarray*}$ as claimed. _test_elt_rev :: (List t, Equal a, Equal (t a)) => t a -> Test (a -> t a -> Bool) _test_elt_rev _ = testName "elt(a)(x) == elt(a)(rev(x))"$
\a x -> eq (elt a x) (elt a (rev x))

$$\elt$$ interacts with $$\map(f)$$ when $$f$$ is injective.

Let $$A$$ and $$B$$ be sets and suppose $$f : A \rightarrow B$$ is injective. Then for all $$a \in A$$ and $$x \in \lists{A}$$ we have $\elt(a)(x) = \elt(f(a),\map(f)(x)).$

We proceed by list induction on $$x$$. For the base case $$x = \nil$$ we have $\begin{eqnarray*} & & \elt(a)(\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \elt(f(a),\nil) \\ & \href{/posts/arithmetic-made-difficult/Map.html#cor-map-nil} = & \elt(f(a),\map(f)(\nil)) \end{eqnarray*}$ as claimed. For the inductive step, suppose the equality holds for all $$x$$ and let $$b \in A$$. Note that $$\beq(a,b) = \beq(f(a),f(b))$$. Then we have $\begin{eqnarray*} & & \elt(a)(\cons(b,x)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bif{\beq(a,b)}{\btrue}{\elt(a)(x)} \\ & = & \bif{\beq(f(a),f(b))}{\btrue}{\elt(f(a),\map(f)(x))} \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \elt(f(a),\cons(f(b),\map(f)(x))) \\ & \href{/posts/arithmetic-made-difficult/Map.html#cor-map-cons} = & \elt(f(a),\map(f)(\cons(b,x))) \end{eqnarray*}$ as claimed.

As a special case, the items in $$\tails(x)$$ and $$\inits(x)$$ are precisely the suffixes and prefixes of $$x$$, respectively.

Let $$A$$ be a set with $$x,y \in \lists{A}$$. We have the following.

1. $$\elt(y,\tails(x)) = \suffix(y,x)$$.
2. $$\elt(y,\inits(x)) = \prefix(y,x)$$.
1. We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we consider two cases. If $$y = \nil$$ we have $\begin{eqnarray*} & & \elt(y,\tails(x)) \\ & = & \elt(\nil,\tails(\nil)) \\ & \href{/posts/arithmetic-made-difficult/TailsAndInits.html#cor-tails-nil} = & \elt(\nil,\cons(\nil,\nil)) \\ & = & \bif{\beq(\nil,\nil)}{\btrue}{\elt(\nil,\nil)} \\ & = & \btrue \\ & \href{/posts/arithmetic-made-difficult/PrefixAndSuffix.html#cor-prefix-nil-left} = & \prefix(\nil,\nil) \\ & = & \prefix(\rev(\nil),\rev(\nil)) \\ & = & \suffix(\nil,\nil) \\ & = & \suffix(y,x) \end{eqnarray*}$ as claimed. Suppose $$y \neq \nil$$; then we have $$y = \snoc(b,u)$$ for some $$b \in A$$ and $$u \in \lists{A}$$. Now $\begin{eqnarray*} & & \elt(y,\tails(x)) \\ & = & \elt(y,\tails(\nil)) \\ & \href{/posts/arithmetic-made-difficult/TailsAndInits.html#cor-tails-nil} = & \elt(y,\cons(\nil,\nil)) \\ & = & \bif{\beq(y,\nil)}{\btrue}{\elt(y,\nil)} \\ & = & \elt(y,\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \\ & \href{/posts/arithmetic-made-difficult/PrefixAndSuffix.html#thm-suffix-snoc-nil} = & \suffix(\snoc(b,u),\nil) \\ & = & \suffix(y,x) \end{eqnarray*}$ as claimed. For the inductive step, suppose the equality holds for all $$y$$ for some $$x$$ and let $$a \in A$$. Again we consider two possibilities for $$y$$. If $$y = \cons(a,x)$$, we have $\begin{eqnarray*} & & \elt(y,\tails(\cons(a,x))) \\ & \href{/posts/arithmetic-made-difficult/TailsAndInits.html#cor-tails-cons} = & \elt(y,\cons(\cons(a,x),\tails(x))) \\ & = & \bif{\beq(y,\cons(a,x))}{\btrue}{\elt(y,\tails(x))} \\ & = & \btrue \\ & \href{/posts/arithmetic-made-difficult/PrefixAndSuffix.html#thm-suffix-reflexive} = & \suffix(\cons(a,x),\cons(a,x)) \\ & = & \suffix(y,\cons(a,x)) \end{eqnarray*}$ as claimed. Suppose $$y \neq \cons(a,x)$$. Now $\begin{eqnarray*} & & \elt(y,\tails(\cons(a,x))) \\ & \href{/posts/arithmetic-made-difficult/TailsAndInits.html#cor-tails-cons} = & \elt(y,\cons(\cons(a,x),\tails(x))) \\ & = & \bif{\beq(y,\cons(a,x))}{\btrue}{\elt(y,\tails(x))} \\ & = & \elt(y,tails(x)) \\ & = & \suffix(y,x) \\ & = & \suffix(y,\cons(a,x)) \end{eqnarray*}$ as claimed.
2. We have $\begin{eqnarray*} & & \elt(y,\inits(x)) \\ & = & \elt(y,\rev(\map(\rev)(\tails(\rev(x))))) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#thm-elt-rev} = & \elt(y,\map(\rev)(\tails(\rev(x)))) \\ & = & \elt(\rev(y),\map(\rev)(\map(\rev)(\tails(\rev(x))))) \\ & = & \elt(\rev(y),\map(\rev \circ \rev)(\tails(\rev(x)))) \\ & = & \elt(\rev(y),\tails(\rev(x))) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#thm-elt-tails} = & \suffix(\rev(y),\rev(x)) \\ & \href{/posts/arithmetic-made-difficult/PrefixAndSuffix.html#thm-prefix-rev} = & \prefix(y,x) \end{eqnarray*}$ as claimed.
_test_elt_tails :: (List t, Equal a, Equal (t a))
=> t a -> Test (t a -> t a -> Bool)
_test_elt_tails _ =
testName "elt(y,tails(x)) == suffix(y,x)" $\x y -> eq (elt y (tails x)) (suffix y x) _test_elt_inits :: (List t, Equal a, Equal (t a)) => t a -> Test (t a -> t a -> Bool) _test_elt_inits _ = testName "elt(y,inits(x)) == prefix(y,x)"$
\x y -> eq (elt y (inits x)) (prefix y x)

$$\elt$$ interacts with $$\filter$$.

Let $$A$$ be a set and $$p : A \rightarrow \bool$$ a predicate. For all $$a \in A$$ and $$x \in \lists{A}$$, we have $\elt(a,\filter(p)(x)) = \band(p(a),\elt(a,x)).$

We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a,\filter(p)(\nil)) \\ & \href{/posts/arithmetic-made-difficult/Filter.html#cor-filter-nil} = & \elt(a,\nil) \\ & = & \bfalse \\ & \href{/posts/arithmetic-made-difficult/And.html#thm-and-false-right} = & \band(p(a),\bfalse) \\ & = & \band(p(a),\elt(a,\nil)) \end{eqnarray*}$ as needed. For the inductive step, suppose the equality holds for all $$a$$ for some $$x$$ and let $$b \in A$$. Note that if $$a = b$$, then $$p(a) = p(b)$$. Now $\begin{eqnarray*} & & \elt(a,\filter(p)(\cons(b,x))) \\ & \href{/posts/arithmetic-made-difficult/Filter.html#cor-filter-cons} = & \elt(a,\bif{p(b)}{\cons(b,\filter(p)(x))}{\filter(p)(x)}) \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#thm-iffunc} = & \bif{p(b)}{\elt(a,\cons(b,\filter(p)(x)))}{\elt(a,\filter(p)(x))} \\ & = & \bif{p(b)}{\bif{\beq(a,b)}{\btrue}{\elt(a,\filter(p)(x))}}{\elt(a,\filter(p)(x))} \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#thm-if-commute-true} = & \bif{\beq(a,b)}{\bif{p(b)}{\btrue}{\elt(a,\filter(p)(x))}}{\elt(a,\filter(p)(x))} \\ & = & \bif{\beq(a,b)}{\bif{p(b)}{\btrue}{\band(p(a),\elt(a,x))}}{\band(p(a),\elt(a,x))} \\ & = & \bif{\beq(a,b)}{\bif{p(a)}{\btrue}{\band(p(a),\elt(a,x))}}{\band(p(a),\elt(a,x))} \\ & = & \bif{\beq(a,b)}{\bif{p(a)}{\band(p(a),\btrue)}{\band(p(a),\elt(a,x))}}{\band(p(a),\elt(a,x))} \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#thm-iffunc} = & \bif{\beq(a,b)}{\band(p(a),\bif{p(a)}{\btrue}{\elt(a,x)})}{\band(p(a),\elt(a,x))} \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#thm-iffunc} = & \band(p(a),\bif{\beq(a,b)}{\bif{p(a)}{\btrue}{\elt(a,x)}}{\elt(a,x)}) \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#thm-if-commute-true} = & \band(p(a),\bif{p(a)}{\bif{\beq(a,b)}{\btrue}{\elt(a,x)}}{\elt(a,x)}) \\ & = & \band(p(a),\bif{p(a)}{\elt(a,\cons(b,x))}{\elt(a,x)}) \\ & \href{/posts/arithmetic-made-difficult/And.html#thm-and-if-right} = & \band(p(a),\elt(a,\cons(b,x))) \end{eqnarray*}$ as needed.

_test_elt_filter :: (List t, Equal a, Equal (t a))
=> t a -> Test ((a -> Bool) -> a -> t a -> Bool)
_test_elt_filter _ =
testName "elt(a)(filter(p,x)) == and(p(a),elt(a,x))" $\p a x -> eq (elt a (filter p x)) (and (p a) (elt a x)) As a special case, if we filter $$a$$ out of a list, it is no longer an item there. Let $$A$$ be a set, with $$a \in A$$ and $$x \in \lists{A}$$. Then $\elt(a)(\filter(\bnot(\beq(a)),x)) = \bfalse.$ We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a)(\filter(\bnot(\beq(a,-)),\nil)) \\ & = & \elt(a)(\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \end{eqnarray*}$ as claimed. For the inductive step, suppose the equality holds for $$x$$ and let $$b \in A$$. We have two possibilities. If $$b = a$$ (that is, $$\bnot(\beq(a,b)) = \bfalse$$), we have $\begin{eqnarray*} & & \elt(a)(\filter(\bnot(\beq(a,-)),\cons(b,x))) \\ & = & \elt(a)(\filter(\bnot(\beq(a,-)),x)) \\ & = & \bfalse \end{eqnarray*}$ as claimed. If $$b \neq a$$ (that is, $$\bnot(\beq(a,b)) = \btrue$$), we have $\begin{eqnarray*} & & \elt(a)(\filter(\bnot(\beq(a,-)),\cons(b,x))) \\ & = & \elt(a)(\cons(b,\filter(\bnot(\beq(a,-)))(x))) \\ & = & \filter(\bnot(\beq(a,-)))(x) \\ & = & \bfalse \end{eqnarray*}$ as claimed. _test_elt_filter_eq :: (List t, Equal a, Equal (t a)) => t a -> Test (a -> t a -> Bool) _test_elt_filter_eq _ = testName "elt(a)(filter(eq(a,-),x)) == false"$
\a x -> eq (elt a (filter (not . eq a) x)) false

$$\elt$$ is an $$\any$$.

Let $$A$$ be a set, with $$a \in A$$ and $$x \in \lists{A}$$. Then $\elt(a)(x) = \any(\beq(a,-),x).$

We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a)(x) \\ & = & \elt(a)(\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \\ & = & \any(\beq(a,-),\nil) \\ & = & \any(\beq(a,-),x) \end{eqnarray*}$ as needed. For the inductive step, suppose the equality holds for some $$x$$ and let $$b \in A$$. We consider two possibilities. If $$a = b$$, we have $\begin{eqnarray*} & & \elt(a)(\cons(b,x)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bif{\beq(a,b)}{\btrue}{\elt(a)(x)} \\ & = & \bif{\btrue}{\btrue}{\elt(a)(x)} \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#cor-if-true} = & \btrue \\ & = & \bor(\btrue,\any(\beq(a,-),x)) \\ & = & \bor(\beq(a,b),\any(\beq(a,-),x)) \\ & = & \any(\beq(a,-),\cons(b,x)) \end{eqnarray*}$ as needed. If $$a \neq b$$, using the inductive hypothesis we have $\begin{eqnarray*} & & \elt(a)(\cons(b,x)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bif{\beq(a,b)}{\btrue}{\elt(a)(x)} \\ & = & \bif{\bfalse}{\btrue}{\elt(a)(x)} \\ & \href{/posts/arithmetic-made-difficult/Booleans.html#cor-if-false} = & \elt(a)(x) \\ & = & \any(\beq(a,-),x) \\ & = & \bor(\bfalse,\any(\beq(a,-),x)) \\ & = & \bor(\beq(a,b),\any(\beq(a,-),x)) \\ & = & \any(\beq(a,-),\cons(b,x)) \end{eqnarray*}$ as needed.

_test_elt_any :: (List t, Equal a, Equal (t a))
=> t a -> Test (a -> t a -> Bool)
_test_elt_any _ =
testName "elt(a)(x) == any(eq(a,-))(x)" $\a x -> eq (elt a x) (any (eq a) x) $$\elt$$ can detect when two lists are distinct. Let $$A$$ be a set, with $$a \in A$$ and $$x,y \in \lists{A}$$. If $$\elt(a)(x) \neq \elt(a)(y)$$, then $$x \neq y$$. The contrapositive of this statement is trivial and kind of silly looking: If $$x = y$$, then $$\elt(a)(x) = \elt(a)(y)$$. But notice that this theorem gives us a simple way to detect when two lists are distinct from each other. _test_elt_distinct :: (List t, Equal a, Equal (t a)) => t a -> Test (a -> t a -> t a -> Bool) _test_elt_distinct _ = testName "if elt(a)(x) /= elt(a)(y) then x /= y"$
\a x y -> if not (eq (elt a x) (elt a y))
then not (eq x y)
else true

$$\elt$$ interacts with $$\map(f)$$ when $$f$$ is injective.

Let $$A$$ and $$B$$ be sets with $$f : A \rightarrow B$$ injective. For all $$a \in A$$ and $$x \in \lists{A}$$, we have $\elt(a,x) = \elt(f(a),\map(f)(x)).$

We proceed by list induction on $$x$$. For the base case $$x = \nil$$, we have $\begin{eqnarray*} & & \elt(a,\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \elt(f(a),\nil) \\ & \href{/posts/arithmetic-made-difficult/Map.html#cor-map-nil} = & \elt(f(a),\map(f)(\nil)) \end{eqnarray*}$ as needed. For the inductive step, suppose the equality holds for all $$a$$ for some $$x$$, and let $$b \in A$$. Note that $$\beq(a,b) = \beq(f(a),f(b))$$, so we have $\begin{eqnarray*} & & \elt(a,\cons(b,x)) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bif{\beq(a,b)}{\btrue}{\elt(a,x)} \\ & = & \bif{\beq(f(a),f(b))}{\btrue}{\elt(f(a),\map(f)(x))} \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \elt(f(a),\cons(f(b),\map(f)(x))) \\ & \href{/posts/arithmetic-made-difficult/Map.html#cor-map-cons} = & \elt(f(a),\map(f)(\cons(b,x))) \end{eqnarray*}$ as needed.

$$\elt$$ interacts with $$\range$$.

Let $$a,b,c \in \nats$$. If $$\nleq(a,c)$$, then $\elt(a,\range(\next(c),b)) = \bfalse.$

We proceed by induction on $$b$$. If $$b = \zero$$, we have $\begin{eqnarray*} & & \elt(a,\range(\next(c),\zero)) \\ & = & \elt(a,\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \end{eqnarray*}$ as needed. For the inductive step, suppose the equation holds for all $$a$$ and $$c$$ for some $$b$$. Suppose $$\nleq(a,c)$$; then $$\nleq(a,\next(c))$$. Now $\begin{eqnarray*} & & \elt(a,\range(\next(c),\next(b))) \\ & = & \elt(a,\cons(\next(c),\range(\next(\next(c)),b))) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-cons} = & \bif{\beq(a,\next(c))}{\btrue}{\elt(a,\range(\next(\next(c)),b))} \\ & = & \elt(a,\range(\next(\next(c)),b)) \\ & = & \bfalse \end{eqnarray*}$ as needed.

_test_elt_range :: (List t, Natural n, Equal n)
=> t a -> n -> Test (t n -> n -> n -> n -> Bool)
_test_elt_range _ _ =
testName "if leq(a,c) then eq(elt(a,range(next(c),b)),false)" $\t a b c -> if leq a c then eq (elt a ((range (next c) b) withTypeOf t)) false else true $$\elt$$ interacts with $$\cons$$ on lists of lists. Let $$A$$ be a set. For all $$a,b \in A$$, $$x \in \lists{A}$$, and $$z \in \lists{\lists{A}}$$, we have $\elt(\cons(a,x),\map(\cons(b,-))(z)) = \band(\beq(a,b),\elt(x,z)).$ We proceed by list induction on $$z$$. For the base case $$z = \nil$$, we have $\begin{eqnarray*} & & \elt(\cons(a,x),\map(\cons(b,-))(\nil)) \\ & = & \elt(\cons(a,x),\nil) \\ & \href{/posts/arithmetic-made-difficult/Elt.html#cor-elt-nil} = & \bfalse \\ & \href{/posts/arithmetic-made-difficult/And.html#thm-and-false-right} = & \band(\beq(a,b),\bfalse) \\ & = & \band(\beq(a,b),\elt(x,\nil)) \end{eqnarray*}$ as needed. For the inductive step, suppose the equality holds for all $$a$$, $$b$$, and $$x$$ for some $$z$$, and let $$u \in \lists{A}$$. Now $\begin{eqnarray*} & & \elt(\cons(a,x),\map(\cons(b,-))(\cons(u,z))) \\ & = & \elt(\cons(a,x),\cons(\cons(b,u),\map(\cons(b,-))(z))) \\ & = & \bif{\beq(\cons(a,x),\cons(b,u))}{\btrue}{\elt(\cons(a,x),\map(\cons(b,-))(z))} \\ & = & \bif{\beq(\cons(a,x),\cons(b,u))}{\btrue}{\band(\beq(a,b),\elt(x,z))} \\ & \href{/posts/arithmetic-made-difficult/Lists.html#thm-list-eq-cons} = & \bif{\band(\beq(a,b),\beq(x,u))}{\btrue}{\band(\beq(a,b),\elt(x,z))} \\ & \href{/posts/arithmetic-made-difficult/Or.html#thm-or-is-if} = & \bor(\band(\beq(a,b),\beq(x,u)),\band(\beq(a,b),\elt(x,z))) \\ & \href{/posts/arithmetic-made-difficult/Or.html#thm-and-or-distribute} = & \band(\beq(a,b),\bor(\beq(x,u),\elt(x,z))) \\ & \href{/posts/arithmetic-made-difficult/Or.html#thm-or-is-if} = & \band(\beq(a,b),\bif{\beq(x,u)}{\btrue}{\elt(x,z)}) \\ & = & \band(\beq(a,b),\elt(x,\cons(u,z))) \end{eqnarray*}$ as needed. _test_elt_cons_map_cons :: (List t, Equal a, Equal (t a)) => t a -> Test (a -> a -> t a -> t (t a) -> Bool) _test_elt_cons_map_cons _ = testName "elt(cons(a,x),map(cons(b,-))(z)) == and(eq(a,b),elt(x,z))"$
\a b x z -> eq
(elt (cons a x) (map (cons b) z))
(and (eq a b) (elt x z))

## Testing

Suite:

_test_elt ::
( TypeName a, Equal a, Show a, Arbitrary a, CoArbitrary a
, TypeName (t a), List t, Arbitrary (t n), Show (t n), Equal (t n)
, Show (t a), Equal (t a), Arbitrary (t a), Equal (t (Pair a a))
, TypeName n, Equal n, Show n, Arbitrary n, Natural n
, Arbitrary (t (t a)), Show (t (t a)), Equal (t (t a))
) => Int -> Int -> t a -> n -> IO ()
_test_elt size cases t n = do
testLabel1 "elt" t

let args = testArgs size cases

runTest args (_test_elt_nil t)
runTest args (_test_elt_cons t)
runTest args (_test_elt_single t)
runTest args (_test_elt_cat t)
runTest args (_test_elt_snoc t)
runTest args (_test_elt_rev t)
runTest args (_test_elt_tails t)
runTest args (_test_elt_inits t)
runTest args (_test_elt_filter t)
runTest args (_test_elt_filter_eq t)
runTest args (_test_elt_any t)
runTest args (_test_elt_distinct t)
runTest args (_test_elt_range t n)
runTest args (_test_elt_cons_map_cons t)

Main:

main_elt :: IO ()
main_elt = do
_test_elt 20 100 (nil :: ConsList Bool)  (zero :: Unary)
_test_elt 20 100 (nil :: ConsList Unary) (zero :: Unary)