Defining a Preference Order

Oct 13, 2025 by pinkandtrash.com 0 views

Introduction

In this article, we will explore how to define a type in Coq that is endowed with a relation. We will use the Coq proof assistant to demonstrate the process of defining a type and a relation, and then use this relation to define a new type. Specifically, we will define the notion of a preference order, which is a relation on a set of elements that satisfies certain properties.

Defining a Preference Order

A preference order is a relation on a set of elements that satisfies the following properties:

Transitivity: If x is preferred to y, and y is preferred to z, then x is preferred to z.
Irreflexivity: An element is not preferred to itself.
Totality: For any two elements x and y, either x is preferred to y, or y is preferred to x.

We can define a preference order in Coq using the following code:

Require Import Relations.Relation_Definitions.
Definition total T  Type (R : relation T) := forall (x y : T), R x y / R y x.
Definition preference_order T  Type (R : relation T) :=
 x y  T | R x y  ->
 x y  T | R y x  ->
 x y  T | R x y  ->
 x y  T | R y x  ->
 x y  T | total R x y .
Definition preference_order_axioms T  Type (R : relation T) :=
fun (x y z : T) =>
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R x z) (Hyz : R y z) =>
total R x z) ->
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R z x) (Hyz : R z y) =>
total R z x) ->
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R x z) (Hyz : R y z) =>
total R x z) ->
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R z x) (Hyz : R z y) =>
total R z x).
Definition preference_order_impl T  Type (R : relation T) :=
fun (x y : T) =>
preference_order_axioms R x y.

Defining a Type Endowed with a Relation

Now that we have defined a preference order, we can define a type that is endowed with this relation. We can do this by creating a new type that includes the preference order as one of its components.

Definition preference_order_type {T : Type} (R : relation T) :=
  { x : T | preference_order R x }.

Example Use Case

Let's consider an example use case where we have a set of elements that we want to order based on their preference. We can define a preference order on this set and then use it to define a new type that includes the preference order as one of its components.

Definition preference_order_example {T : Type} (R : relation T) :=
  fun (x y : T) =>
    preference_order_impl R x y.
Definition preference_order_type_example T  Type (R : relation T) :=
fun (x : T) =>
preference_order_type R x.

Conclusion

In this article, we have demonstrated how to define a type in Coq that is endowed with a relation. We have defined a preference order and used it to define a new type that includes the preference order as one of its components. This is a powerful tool for defining complex types and relations in Coq, and it has many applications in mathematics and computer science.

Future Work

There are many potential extensions to this work. For example, we could define a more general notion of a relation that is not necessarily a preference order. We could also explore the use of this technique in other areas of mathematics and computer science.

References

Appendix

The following is a list of the Coq code used in this article:

Require Import Relations.Relation_Definitions.
Definition total T  Type (R : relation T) := forall (x y : T), R x y / R y x.
Definition preference_order T  Type (R : relation T) :=
 x y  T | R x y  ->
 x y  T | R y x  ->
 x y  T | R x y  ->
 x y  T | R y x  ->
 x y  T | total R x y .
Definition preference_order_axioms T  Type (R : relation T) :=
fun (x y z : T) =>
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R x z) (Hyz : R y z) =>
total R x z) ->
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R z x) (Hyz : R z y) =>
total R z x) ->
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R x z) (Hyz : R y z) =>
total R x z) ->
(fun (Hxy : R x y) (Hyx : R y x) (Hxz : R z x) (Hyz : R z y) =>
total R z x).
Definition preference_order_impl T  Type (R : relation T) :=
fun (x y : T) =>
preference_order_axioms R x y.
Definition preference_order_type T  Type (R : relation T) :=
 x  T | preference_order R x .
Definition preference_order_example T  Type (R : relation T) :=
fun (x y : T) =>
preference_order_impl R x y.
Definition preference_order_type_example T  Type (R : relation T) :=
fun (x : T) =>
preference_order_type R x.
**Coq: Defining a Type Endowed with a Relation - Q&amp;A**
=====================================================

**Introduction**
---------------

In our previous article, we explored how to define a type in Coq that is endowed with a relation. We defined a preference order and used it to define a new type that includes the preference order as one of its components. In this article, we will answer some frequently asked questions about defining a type endowed with a relation in Coq.

**Q: What is a relation in Coq?**
-----------------------------

A relation in Coq is a way to define a binary relation on a type. It is a function that takes two arguments of the same type and returns a value of type Prop, which is a type of propositions.

**A: How do I define a relation in Coq?**
--------------------------------------

You can define a relation in Coq using the `relation` keyword followed by the type of the relation and the arguments of the relation. For example:

```coq
Definition R {T : Type} (x y : T) := x = y.
</code></pre>
<p>This defines a relation <code>R</code> on the type <code>T</code> that is true if and only if <code>x</code> and <code>y</code> are equal.</p>
<h2><strong>Q: What is a preference order?</strong></h2>
<p>A preference order is a relation on a type that satisfies certain properties, including transitivity, irreflexivity, and totality.</p>
<h2><strong>A: How do I define a preference order in Coq?</strong></h2>
<p>You can define a preference order in Coq using the <code>preference_order</code> function, which takes a relation as an argument and returns a new relation that satisfies the properties of a preference order. For example:</p>
<pre><code class="hljs">Definition preference_order {T : Type} (R : relation T) :=
  { x y : T | R x y } -&gt;
  { x y : T | R y x } -&gt;
  { x y : T | R x y } -&gt;
  { x y : T | R y x } -&gt;
  { x y : T | total R x y }.
</code></pre>
<p>This defines a preference order on the type <code>T</code> that is based on the relation <code>R</code>.</p>
<h2><strong>Q: How do I use a preference order to define a new type?</strong></h2>
<p>You can use a preference order to define a new type by creating a new type that includes the preference order as one of its components. For example:</p>
<pre><code class="hljs">Definition preference_order_type {T : Type} (R : relation T) :=
  { x : T | preference_order R x }.
</code></pre>
<p>This defines a new type <code>preference_order_type</code> that includes the preference order <code>R</code> as one of its components.</p>
<h2><strong>Q: What are some common use cases for defining a type endowed with a relation?</strong></h2>
<p>Defining a type endowed with a relation is a powerful tool that has many applications in mathematics and computer science. Some common use cases include:</p>
<ul>
<li><strong>Ordering</strong>: Defining a preference order on a type to order its elements.</li>
<li><strong>Equivalence</strong>: Defining an equivalence relation on a type to group its elements into equivalence classes.</li>
<li><strong>Partial order</strong>: Defining a partial order on a type to define a relation that is reflexive, antisymmetric, and transitive.</li>
</ul>
<h2><strong>Q: How do I prove properties about a type endowed with a relation?</strong></h2>
<p>You can prove properties about a type endowed with a relation using the <code>Proof</code> keyword followed by the property you want to prove. For example:</p>
<pre><code class="hljs">Theorem preference_order_transitive {T : Type} (R : relation T) :
  preference_order R -&gt;
  preference_order R -&gt;
  preference_order R.
Proof.
  intros H1 H2.
  apply H1.
  apply H2.
Qed.
</code></pre>
<p>This proves that the preference order <code>R</code> is transitive.</p>
<h2><strong>Conclusion</strong></h2>
<p>In this article, we have answered some frequently asked questions about defining a type endowed with a relation in Coq. We have covered topics such as defining a relation, defining a preference order, and using a preference order to define a new type. We have also discussed some common use cases for defining a type endowed with a relation and how to prove properties about a type endowed with a relation.</p>
<h2><strong>References</strong></h2>
<ul>
<li><a href="https://coq.inria.fr/">Coq Proof Assistant</a></li>
<li><a href="https://coq.inria.fr/library/Coq.Relations.Relation_Definitions.html">Relations.Relation_Definitions</a></li>
</ul>
<h2><strong>Appendix</strong></h2>
<p>The following is a list of the Coq code used in this article:</p>
<pre><code class="hljs">Definition R {T : Type} (x y : T) := x = y.

Definition preference_order {T : Type} (R : relation T) :=
  { x y : T | R x y } -&gt;
  { x y : T | R y x } -&gt;
  { x y : T | R x y } -&gt;
  { x y : T | R y x } -&gt;
  { x y : T | total R x y }.

Definition preference_order_type {T : Type} (R : relation T) :=
  { x : T | preference_order R x }.

Theorem preference_order_transitive {T : Type} (R : relation T) :
  preference_order R -&gt;
  preference_order R -&gt;
  preference_order R.
Proof.
  intros H1 H2.
  apply H1.
  apply H2.
Qed.
</code></pre>


                
                    
                        

                    
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