# Construct & The Mohr-Mascheroni Theorem

## Overview

Construct is a domain-specific language & interactive tool for expressing classical geometry. In this post we learn the basics of Construct, and in the process explore the Mohr-Mascheroni Thereom which states that any point constructible using a compass and straightedge is constructible using the compass alone.

## Classical Geometry

Geometry, the study of space & distance, was one of the first domains of mathematics to be formalized in a form recognizable to a modern audience. In the 4th century B.C. Euclid, a greek mathematician, wrote Elements which formalized the greek conception of geometry as an axiomatic system. Elements showed how geometric knowledge could be derived from 5 fundamental axioms. In modern language the axioms read:

1. Two points may be connected by a straight segment.
2. A straight segment may be extended indefinitely into a straight line.
3. A circle may be drawn about one point and through another.
4. All right angles are congruent.
5. If two lines are not parallel they intersect precisely once.

The interesting thing about these axioms is that they’re less propositional than one might think and more… constructive. Euclid’s axioms don’t just state 5 properties of different geometric objects, three of them are dedicated to stating how geometric objects can be constructed. In particular axioms 1, 2, and 3 describe how segments, lines, and circles can be constructed from, ultimately, points.1

This emphasis reveals that classical geometry is not only a system for proving propositions, but also a system for construction. That is, classical geometry is a system for constructing geometric objects (points, lines, circles, …) with particular relationships to one another. It is this facet of classical geometry that Construct expresses.

## Construct

Construct is a computer langauge which allows the user to express how geometric constructions should be done. In this section we do two constructions to give you a sense of how the language works2.

### Interactive Mode

Construct begins simply— the program starts up presenting you with a prompt and an empty canvas. You click twice on the canvas and there are two points, $A$ and $B$. Perhaps you’re looking for the perpendicular bisector… Hmm, you need two circles, their intersection, a line.

let C1 = circle(A, B)
let C2 = circle(B, A)
let D, E = intersection(C1, C2)
let l = line(D, E)


Construct’s primitives are exactly the primitives of classical geometry: the construction of fundamental geometric objects, the ability to find their intersections.

### Reusing Constructions

This is exciting, but a programming language is more than its primitives, in the same way that mathematics is more than the axioms we accept. The interesting stuff is in how primitives and axioms can be combined into new results that are worthy of their own names… We tell Construct that we want to :write this construction to a file for future use.

given point A, point B
return l
:write perpendicular_bisector my_constructions.con


“What future use?”, you wonder to yourself. You recall that the orthocenter, the center of a triangle’s circumcircle, can be found using perpendicular bisectors. You open a text file to write the construction, but stumble—“What is a triangle?”.

### Custom Geometric Objects

While the idea of a triangle is clear to you, triangles are not discussed by any of Euclid’s axioms, and are not primitives in Construct. Your problem is that the idea in your mind is not primitive, but composite: a construction of mathematicians who have come before you. You solve this problem by importing the standard library, which defines triangle.

include lib/lib.con
include my_constructions.con

construction orthocenter
given triangle t
let triangle(A, B, C) = t
let mC = perpendicular_bisector(A, B)
let mB = perpendicular_bisector(A, C)
let O = intersection(mA, mC)
return O


Construct allows you to manipulate composite geometric objects by destructuring them into their constituents.

### Conclusion

While this has been just a fleeting taste of Construct, we’ve seen that the language features geometric primitives, functional composition (construction reuse), user-defined data (custom geometric objects), and an graphical/textual interactive mode.

This set of features seems to capture classical geometry quite well—most constructions I’m aware of can be expressed using Construct’s current form. One might ask: “Are all geometric ideas expressible in Construct?” We answer this question by discussing an old and fascinating geometric theorem.

## The Mohr-Mascheroni Theorem

Thinking of geometric construction as a system of computation brings to mind the expansive work that’s been done by computer scientists on models of computation. Memories of Turing machines, linear automata, and finite state automata resurface, bringing with them theorems about what problems are solvable by which computational devices. These thoughts about computability beg a natural set of questions: “What is the relative computational power of different geometric toolboxes? What happens if we takeaway the straightedge? Or the compass?”

Enter the Mohr-Mascheroni theorem. This theorem (discovered and proved independently by Georg Mohr in 1672 and Lorenzo Macheroni in 1797) states that:

Any point constructible by a straightedge and compass is constructible by a compass alone.

The theorem’s central statement is fascinating, but first it requires us to look at constructions in a new light. Often our constructions are intended to produce certain geometric objects: the circumcircle of a triangle, the perpendicular bisector of a segment, etc. However, Mohr-Mascheroni restricts the goal of constructions to the placement of points. This isn’t as much of a restriction as one might think—all the lines, circles, and other geometric objects one might be interested in constructing are uniquely determined by a set of points (e.g. two distinct points determine a line), so constructing complex objects can be reduced to constructing the points which determine them.

With this understanding of the goal of a construction Mohr-Mascheroni states that the straightedge is superfluous: the compass alone can construct any point the compass and straightedge could construct together. One immediately wonders – how would you prove such a statement?

The answer is easier than you would think. Since the goal of constructions is the placement of points, we begin by asking what points straightedge allows us to place. The answer? That the straightedge allows us to find the intersection between lines and other objects. Whenever we find the intersection of a line and a circle, or between two lines, we are using a straightedge, and ultimately, these are the only times we use a straightedge.

Proving Mohr-Masheroni boils down to showing that

• the intersection(s) of a line and circle
• the intersection of a line and line

where lines are represented by two distinct points on their extent, can be found using only a compass. While we will not fully present each construction here, we’ll look briefly at each.

### The Intersection of a Line and Circle

Finding the intersection of a line (determined by two points on it) and circle (determined by its center and a point on its extent) is at first glance rather straightforward. The construction is guided by a core strategy: reflect the circle about the line, and the circle will intersect its reflection precisely where the circle intersects the line. Done in construct:

construction line_circle_intersect
given line l, circle K
let line(C, D) = l
let circle(B, A) = K
let K2 = circle(C, B)
let K3 = circle(D, B)
let B2 = intersection(K2, K3) - {B}
let A2 = c_move(B2, segment(B, A))
let K4 = circle(B2, A2)
let F, G = intersection(K, K4)
return F, G


where we’ve used c_move a construction that places a point a certain distance away from another3.

what this construction doesn’t immediately show is that it doesn’t always work. In particular, if the center of the circle is already on the line, then the circle reflects onto itself, its self-intersection is the entire circle, and we fail to find the two points where the circle intersects the line.

This situation is straightforward to detect, and there is an alternate construction that handles it appropriately. We need the ability to take stock of the situation and chose the appropriate construction. This ability is branching, as is typically provided to a programming language by if/else, match, or switch statements. However, Construct has none of these control flow structures. In fact, Construct has no idea of branching whatsoever—all constructions must have linear control flow.

For this reason Construct can not express the line-circle construction for the Mohr-Macheroni theorem.

### The Intersection of Two Lines

Finding the intersection of two lines is a much more complex process. For that reason we won’t go through it here, but will instead highlight one step in the construction: the extension step.

The extension step takes two segments, $A$ and $B$, and repeated doubles $A$ until it is longer than $B$.

The act of doubling $A$ until it’s longer than $B$ is an example of an iterative process—a process that should be repeated until some condition is met. In this situation, the Archimedean Principle, which states there are no infinitely small or large lengths, guarantees that eventually $A$ will be larger than $B$. This proves that the process terminates.

However, our problem isn’t whether the process terminates. Rather, the issue is that the Construct language has no support for iteration. Programming languages typically allow for iteration through for/while loops or recursion, both of which are absent from Construct.

### Limitations of Construct

Exploring the Mohr-Mascheroni theorem has shown some of the limits of construct: that it supports neither branching nor iteration. However, this exploration could be even more important than that. Viewed in a different light, it might reveal limitations of the Mohr-Mascheroni theorem itself.

The Mohr-Macheroni theorem is a grand statement about the computational power of two different systems. It says that the system with a compass can do just as much as the system with a compass and straightedge. However, it makes this statement without addressing the computational character of these systems: without any discussion of kinds of computational capabilities each of these systems need in order to correctly use the tools they’re given.

This omission could be inconsequential—perhaps at the end of the day our two sets of tools can compute the same points given the same set of computational primitives. But our work here suggests otherwise. We saw that the Mohr-Mascheroni constructions rely on branching and iteration, computation primitives missing from Construct. Should these primitives be included in Construct?

I suspect the answer is no. I didn’t omit them on accident, or because I was lazy. I omitted them because all the constructions I’ve seen (save Mohr-Mascheroni) don’t use them. It seems that iteration and branching have no place in a Mohr-Mascheroni-free world, suggesting that the classical geometric tools (a compass and straightedge) can do with only linear control flow what the compass requires branching & iteration to do. Essentially, the world might look like this:

System Geometric Primitives Computational Primitives Constructive Power
A compass, straightedge sequencing equal to B
B compass sequencing, branching, iteration equal to A
C compass sequencing less than A and B

Which is rather interesting.

## Closing Thoughts

While I’m no longer making large changes to Construct, I’m still very curious about the computational quandry that it has brought to light. This post glosses over some of the details, but if you’re interested in this, let me know.

If you think I’m totally off-base, let me know about that too :).

1. It turns out that having substantial constructive character in mathematical axioms is not unusual. As an example, Zermelo-Fraenkel set theory is an axiomatic system used to discuss the most foundational and logically dangerous parts of mathematics. It too is constructive in nature. ↩︎

2. Don’t take this to be a working introduction to the language. I’m giving you a taste of some exotic dish—not the recipe for it. ↩︎

3. Doing this construction without a straightedge is left to the reader, but is relatively straight-forward. ↩︎