# Theories and models §

The concept of theories and models is a powerful tool for understanding important programming ideas like interfaces and implementations, DSL design, reification and automation, and even programming languages themselves. Really, theories and models help to show how all of these are different views of the same thing.

More than this, the tension between theory and model is the critical tension of creative abstraction, generalization, and synthesis. Getting a good handle on this idea can make some pieces of the nebulous, creative process of programming more tangible.

In this article I'll define theories and models and give a few examples. I'll also sketch out processes for moving from collections of models to a theory and from a designed theory onward to its models.

## Theories and models §

A theory is an external description of a class of things that captures the essential nature shared by that whole class. A model of a theory is one of those things, or, equivalently, a concrete construction which is accurately described by the theory.

As an example, let's consider the theory of monoids. To talk about a theory we provide a "presentation" of the theory which looks like the following

The theory of monoids

A type $M$ closed under the operations

• $\text{unit}$ and
• $a \diamond b$ for all $a, b \in M$

following the laws

• (left identity) for all $a \in M$, $\text{unit} \diamond a = a$
• (right identity) for all $a \in M$, $a \diamond \text{unit} = a$
• (associativity) for all $a, b, c \in M$, $(a \diamond b) \diamond c = a \diamond (b \diamond c)$

In reading this note that it consists of 3 parts

• a declaration of the involved types (or sets)
• a set of operations (means of introduction)
• a set of laws (equalities)

and that the operations (here: $\text{unit}$ and $\_ \diamond \_$) are just abstract names. They are given meaning by the theory iteself.

This abstract presentation of a theory can be modeled in some programming languages. For instance, in Scala we can write a trait.

trait MONOID {
type M

def unit: M
def combine(a: M, b: M): M
}


though the laws would need to just be specified in a comment.

The "typeclass" pattern as implemented explicitly in Haskell or modeled by Scala's implicits system is doing approximately the same thing as what this article is discussing. Feel free to port all of the ideas over to that formulation if it's more familiar.

A model of this theory is a concrete instantiation of it. In particular, we need to

• give a concrete definition for $M$,
• define all of the operations, and
• prove that the definition of $M$ and its operations satisfy all of the laws

There are many models of the theory of monoids. Here are a few

Let $M$ by the set of all integers and define

• $\text{unit}$ as $0$ and
• $a \diamond b$ as $a + b$.

The monoid of multiplication

Let $M$ by the set of all integers and define

• $\text{unit}$ as $1$ and
• $a \diamond b$ as $a \times b$.

You can probably trivially prove that the law equations hold for these models. More interestingly, we can have families of models

The family of monoids of lists, one for each type A

Let $M$ be the type of lists of values of type $A$ and define

• $\text{unit}$ to be the empty list and
• $a \diamond b$ to be the concatenation of lists $a$ and $b$

We can give these models as Scala vals and defs

val AdditionModel: MONOID { type M = Int } =
new MONOID {
type M = Int
val unit = 0
def combine(a: M, b: M): M = a + b
}

val MultiplicationModel: MONOID { type M = Int } =
new MONOID {
type M = Int
val unit = 1
def combine(a: M, b: M): M = a * b
}

def ListModel[A]: MONOID { type M = List[A] } =
new MONOID {
type M = List[A]
val unit = List.empty[A]
def combine(a: M, b: M): M = a ++ b
}


For these, I could have used object and class definitions, but it is cleaner to use type ascription like Model: THEORY rather than extends. Also note that we use a refinement type to expose the meaning of M.

## The essence of abstraction §

Typically, we start with things that are concrete—models—and discover theories which explain their key behavior only later. It's not usually the case that we just develop a theory out of the blue.

Instead, theories arise through careful stripping away of inessential details from a collection of models. For instance, the theory of monoids above arises from noting the repeating pattern of certain operations available on several familiar, concrete types. Each of $(+)$, $(\times)$, and list concatenation are associative and the theory of monoids allows us to focus intently on that.

Once you've created a theory—and carefully tuned it to really focus on just the part you want—you are able to speak to the properties of every model at once. Generally, this arises from taking properties you're familiar with from a given model of the theory and seeing if you can "port" them to work on theory itself.

This is the essence of abstraction, and when it's done like this the properties of theories translate perfectly to their models. Programmers are often familiar with the notion of the "leaky abstraction". In fact, often people believe that all abstractions are "leaky" in that they may contain sharp edges and fail unexpectedly.

Focusing on the notions of the theories and models helps you to design leak-free abstractions. By recognizing the operations and laws of your theory and proving that the models satisfy them, you establish equivalence at the strength of your laws.

## Ways theories can fail §

Theories can fail by becoming inconsistent. This arises when their laws are so restrictive that no models exist. Typically, this isn't an issue because we've designed our theory with several models in mind and tuned it so that everything remains in alignment. That said, there are still times when it's useful to try to develop something "theory first" without concrete models in mind.

Theories can also fail by being incomplete if they don't actually cover some interesting model like you wish they did. This arises when their operations are too weak, but in practice it's rarer to see. Usually, we're pretty good at making our theories powerful and big enough.

In practice, the more frequent issue is when working "theory first" you find it's just difficult to find a model. This is a common feeling in programming again where you can design the API you love, but then struggle to implement it.

## Theory-first case study: DSL design §

A common situation where this arises is in DSL design where the shape of your language takes form first by considering the domain of the problem and the human-factors design of the language. You decide what you want to say before you figure out how it'll be said.

This is "theory first" because we'll decide the operations of our language and likely have a sense for rules for how they interact well before we know how things are going to actually work.

### Account ledgers §

For instance, imagine the theory of a (very trivial) account ledger. The ledger may contain atomic transactions like $\text{add}(a)$ and $\text{subtract}(b)$ and have them be glued together with an associative operation $e_1 ; e_2$. We're concerned about the account remaining positive, so any given $\text{subtract}$ operation could fail, but $\text{add}$s cannot, and we have laws like

$\text{add}(a) ; \text{add}(b) = \text{add}(a + b)$

This actually "hoists" a whole set of laws up from the properties of addition. For instance, we know that $\text{add}(a); \text{add}(b) = \text{add}(b) ; \text{add}(b)$.

It's probably clear that ledgers like this have as a model a list of numbers, positive representing $\text{add}$s and negative representing $\text{subtract}$s, but we may very well want to specify behavior like above before getting caught up in the specifics of a model. For instance, in this list-of-numbers model, how do we handle $0$? Is it an $\text{add}$?

### Graphics §

Another popular theory is a grammer of graphics. For this, we create a set of atomic parameterized "shapes" like $\text{rectangle}(width, height)$ and $\text{circle}(radius)$ and glue them together with operations like $\text{above}(a, b)$ and $\text{beside}(a, b)$ which places the diagrams $a$ and $b$ in the field either atop or next to one another.

trait GRAPHICS {
type Img
def rectangle(width: Double, height: Double): Imt
def above(a: Img, b: Img): Img
def beside(a: Img, b: Img): Img
}


Here the laws are a bit harder to define until we've added more operations, but already we have a couple with rectangles

above(rectangle(a, b), rectangle(a, c)) = rectangle(a, b + c)
beside(rectangle(a, b), rectangle(c, b)) = rectangle(a + c, b)


This law demonstrates that we only care about what the Imgs look like—we're happy to forget two rectangles into one if it appears identical.

But what is a model for GRAPHICS?

## Syntactic models §

Finding models can be tough, but there is a mechanical process for getting started. You can always begin with the "syntactic model"—the AST generated by your operations. It is trivially guaranteed to support all of the needed operations, but may fail to satisfy the laws.

val SyntacticGraphics = new GRAPHICS {
sealed abstract class Img
final case class Circle(radius: Double) extends Img
final case class Rectangle(width: Double, height: Double) extends Img
final case class Above(a: Img, b: Img) extends Img
final case class Beside(a: Img, b: Img) extends Img

def rectangle(width: Double, height: Double): Img = Rectangle(width, height)
def above(a: Img, b: Img): Img = Above(a, b)
def beside(a: Img, b: Img): Img = Beside(a, b)
}


For simple laws, we can conform the syntactic model to our theory by being careful with the constructors. For instance, here above, and beside, could investigate their arguments to see if they're both Rectangles.

def above(a: Img, b: Img): Img = (a, b) match {
case (Recatangle(wa, ha), Rectangle(wb, hb)) if wa == wb =>
Rectangle(wa, ha + hb)
case _ => Above(a, b)
}


But for more complex laws this might not scale. We'll often find that the syntactic model is no model at all: it is "too large".

## Laws are often up to interpretation §

A common way of resolving this is to treat the syntactic model as a language AST and "interpret" it into one or more meanings. These meanings need not preserve every property of the AST as long as they capture some of them. For instance, we might create an "interpreter" which tells us the width and height of the image.

def width(a: Img): Double
def height(a: Img): Double


and then instead of expecting the laws to hold precisely, we ask them to hold "up to observation" through each interpreter

width(above(rectangle(a, b), rectangle(a, c))) = width(rectangle(a, b + c))
width(height(rectangle(a, b), rectangle(a, c))) = height(rectangle(a, b + c))


Interestingly, interpreters help us identify valid models. We now have two models for GRAPHICS, one defined by width and one by height. For instance, here's the "width model" and it clearly satisfies the GRAPHICS laws we've described.

val WidthModel: GRAPHICS { type Img = Double } =
new GRAPHICS {
type Img = Double

def rectangle(width: Double, height: Double): Img = width
def above(a: Img, b: Img): Img = max(a, b)
def beside(a: Img, b: Img): Img = a + b
}


In practice, one might prefer to use the "syntactic model" for a while even though it doesn't follow the laws since there may be multiple law-abiding interpretations. A single construction of a value of Img can then be interpreted in many valid ways.

In the world of embedded domain specific languages, the "syntactic model" is often called a "deep embedding" and its interpretations "shallow embeddings" and both have their uses.

## Wrap up §

The concept of theories and models helps to make explicit the process of abstraction and to give handles for doing it more effectively. Once you're familiar with the idea, you'll see them everywhere and want to ask questions about how a theory's laws are being satisfied.

Generally, the types, operations, and laws of a theory have to all fit together and each one contributes an important component of the functionality of the theory. Furthermore, models which are known to support the operations and follow the laws can be considered "leak free"—at least up to the properties truly locked in by the laws.

Interplay between the two is sometimes a process of abstraction from a group of similar models to their master theory and sometimes a process of "theory first" development where we choose the sort of words we'd like to use and then go seeking interpretations of them.