Some days ago I watched this episode from PointFree , and I wanted to come over with a couple of exercises playing with the arithmetic expression of the exercise.

## Decomposing the expresion to the minimum value

As other posts I wrote, we’re going to start decomposing an arithmetic expression to the minimum value. This is the most important step on FPP, where we work composing little functions into higher systems. Like we saw on the validators post, to build model validators we start decomposing the validator to the most simple expression and then start to grouping them into a bigger ones.

So, what is the most simple expression inside an arithmetic operation like `2 + 2`? The number. The most simple expression is the way to provide a number, and over it, we will build everything.

## The functional way

``````() -> Int
``````

Now that we have identified what is the most simple expression to start to work on, we can build an expression based on it. Let’s write an example.

``````typealias Expression = () -> Int

let two: Expression = { 2 }
let operation: Expression =  { two() + two() }
``````

As we saw, a function that returns a number can wrap the most basic expression, give us a number, and the `add` operation. But to be honest, this is far from been usable. So, like other times, let’s wrap that function inside a type. Wrapping functions inside types allow us to add functionality to the type while the base work of our functions still keeps simple.

``````struct Expression {
let run: () -> Int
}

let two = Expression { 2 }
let operation = Expression { two.run() + two.run() }
``````

Still far from been usable. We are building the `add` operation inside the expression, but we could improve the design naming inside Expression the add operation.

``````struct Expression {
let run: () -> Int
}

extension Expression {
func add(_ expression: Expression) -> Expression {
.init { self.run() + expression.run() }
}
}

let two = Expression { 2 }
``````

And like the add, we are going to add the substract function.

``````struct Expression {
let run: () -> Int
}

extension Expression {
func add(_ expression: Expression) -> Expression {
.init { self.run() + expression.run() }
}

func sub(_ expression: Expression) -> Expression {
.init { self.run() - expression.run() }
}
}

let two = Expression { 2 }
``````

And we can extend the Expression type to allow as many operations as we want. But what if we want to work with other type of values instead only Int. We can make it generic.

``````struct Expression<Value> {
let run: () -> Value
}

extension Expression where Value == Int {
func add(_ expression: Expression) -> Expression {
.init { self.run() + expression.run() }
}

func sub(_ expression: Expression) -> Expression {
.init { self.run() - expression.run() }
}
}

let two = Expression { 2 }
``````

At this point, `Expression` is built, and let us extend it with more operations without too much efforrt and we can limit those extensions bassed on the type of the `Expression`.

What if we want to print the built expression? (like 2+2)

At this point, we find that our model doesn’t allow it. Our model has coupled how we build the model and how the model works. This means we’re coupling our model with his interpretation. Being the interpretation the way of our model behaves.

So, how can we fix it?

## The enum choice.

The model proposed by point free on the (chapter)[https://www.pointfree.co/episodes/ep26-domain-specific-languages-part-1] split the model from the interpretation, building only the expression based on the operations, but not applying any logic to it.

It starts with from the same point, what is the most simple expression inside `2 + 2`?. The `Int` value, so let’s build a model that allow us to represent it.

``````enum Expression {
case value(Int)
}
``````

And then, like we have the value, we can add to our Expression model the operation values. Those operations will be cases of our enum.

``````enum Expression {
case value(Int)
case sub
}
``````

Our operations are defined, but them still need parameters to work. The paremeters of the operation could be Int, but our model already allow us to express an Int as an Expression, in our `.value` case. So let’s add the parameters needed for the operations.

``````indirect enum Expression { // indirect will allow us to have recursive enum cases
case value(Int)
case sub(Expression, Expression)
}
``````

Now we can build up our Expression with add and sub, but also our model let us concatenate more than one operation, because an Expression could be a value or a set of operations wrapped inside it.

``````let expression: Expression = .add(.value(2), .add(.value(2), .value(2)))
``````

Our model is done. Now is time to execute it. So we can have the next function:

``````func evaluate(expression: Expression) -> Int {
switch operation {
case let .value(value):
return value
return evaluate(aOP) + evaluate(bOP)
case let .sub(aOP, bOP):
return evaluate(aOP) - evaluate(bOP)
}
}
``````

Okey, but in a real world, we can’t have a lot of functions spreaded on all our code, so let’s wrap it inside a type. We have to pay speccial attention to `evaluate` function because is a recursive function, so we will need a way to create a type that will be created with a function that receive is own type as parameter. This is needed to be able to call itself recursively.

``````struct Runner {
private let _run: (Runner, Expression) -> Int

init(_ run: @escaping (Runner, Expression) -> Int) {
self._run = run
}

func run(_ operation: Expression) -> Int {
_run(self, operation)
}
}
``````

This way we’re able to have an evaluator runner inside our `Runner` type with the dot notation available.

``````extension Runner {
static let evaluator = Runner { runner, expression in
switch operation {
case let .value(value):
return value
return runner.run(aOP) + runner.run(bOP)
case let .sub(aOP, bOP):
return runner.run(aOP) - runner.run(bOP)
case let .mul(rExpr, lExpr):
return runner.run(rExpr) * runner.run(lExpr)
}
}
}
``````

Finally, the usage of this is really simple:

``````let expression: Expression = .add(.value(2), .add(.value(2), .value(2)))
let result = Runner.evaluator.run(expression)
``````

As we have the model interpretation outside wrapped on the `Runner` type, now we can build another runner that will print the expression instead of evaluating it. Like we want to get an `String` now instead a number, we need to make the output of our `Runner` generic.

``````struct Runner<Output> {
private let _run: (Runner, Expression) -> Output

init(_ run: @escaping (Runner, Expression) -> Output) {
self._run = run
}

func run(_ operation: Expression) -> Output {
_run(self, operation)
}
}
``````

The generic types doesn’t allow us to have `static let` inside if we don’t limit the wrapping generic types to concrete ones. So our evaluator runner will looks:

``````extension Runner where Output == Int {
static let evaluator = Runner<Int> { runner, operation in
switch operation {
case let .value(value):
return Double(value)
return runner.run(aOP) + runner.run(bOP)
case let .sub(aOP, bOP):
return runner.run(aOP) - runner.run(bOP)
}
}
}
``````

With our types already updated, let’s built the printer runner.

``````extension Runner where Output == String {
static let printer = Runner<String> { runner, operation in
switch operation {
case let .value(value):
return value
return "\(runner.run(aOP)) + \(runner.run(bOP))"
case let .sub(aOP, bOP):
return "\(runner.run(aOP)) - \(runner.run(bOP))"
}
}
}
``````

The usage will be like this.

``````let expression: Expression = .add(.value(2), .add(.value(2), .value(2)))
let result = Runner.evaluator.run(expression)
let stringResult = Runner.printer.run(expression)
``````

To improve how we run the expressions, we could extend our Expression type a little bit:

``````extension Expression {
func evaluated<Output>(by runner: Runner<Output>) -> Output {
runner.run(self)
}
}
``````

Now the usage will be.

``````let expression: Expression = .add(.value(2), .add(.value(2), .value(2)))
let result = expression.evaluated(by: .evaluator)
let stringResult = expression.evaluated(by: .printer)
``````

Another cool improvement we can make is to don’t close the value type of our `Expression` model to an `Int` value. So let’s make the `.value` case generic and update all our components.

``````indirect enum Expression<Value> {
case value(Value)
case sub(Expression, Expression)
}

struct Runner<Input, Output> {
private let _run: (Runner<Input, Output>, Expression<Input>) -> Output

init(_ run: @escaping (Runner<Input, Output>, Expression<Input>) -> Output) {
self._run = run
}

func run(_ operation: Expression<Input>) -> Output {
_run(self, operation)
}
}

extension Runner where Input == Int, Output == Double {
static let evaluatorRunner = Runner<Int, Double> { runner, operation in
switch operation {
case let .value(value):
return Double(value)
return runner.run(aOP) + runner.run(bOP)
case let .sub(aOP, bOP):
return runner.run(aOP) - runner.run(bOP)
}
}
}

extension Runner where Input == Int, Output == String {
static let printRunner = Runner<Int, String> { runner, operation in
switch operation {
case let .value(value):
return "\(value)"
return "\(runner.run(aOP)) + \(runner.run(bOP))"
case let .sub(aOP, bOP):
return "\(runner.run(aOP)) - \(runner.run(bOP))"
}
}
}
``````

Now we can build Expressions not only for Int type but also for any other type that we want to work on.

## Mapping

Expression has a generic type inside it, this allow us to have the same expressions with different types inside, maybe we even need to transform them. Tranforming an `Expression<Int>` to `Expression<String>` could be really simple if we implement high order functions like map operation inside our `Expression` type.

``````extension Expression {
func map<NewValue>(_ transform: (Value) -> NewValue) -> Expression<NewValue> {
switch self {
case .value(let value):
return .value(transform(value))
case let .sub(lExpression, rExpression):
return .sub(lExpression.map(transform), rExpression.map(transform))
}
}
}
``````

As you may see, the key point of the map, as on the other functions we’ve built to use oeprations, is the recurisivity on it due to the indirect cases of the enum. We are interating over all the expressions inside the initial expression and only mutate the value case.

Mutating an expression will be like:

``````let expression: Expression<Int> = .add(.value(2), .add(.value(2), .value(2)))
let stringExpression = expression.map { "\(\$0)" }
``````

Like the people from Pointfree does, let’s add the var case. This case allow us to create vars inside our expression that will be suplied by values when evaluating them.

``````indirect enum Expression { // indirect will allow us to have recursive enum cases
case value(Int)
case sub(Expression, Expression)
case `var`(String)
}
``````

The Runner object also needs to be update, it needs to receive a Dictionary of String and values, but unlike pointfree, instead of Ints, we will use Expression.

Using expressions as the value of the variables, if you saw the video, will avoid us to implement the `bind` case.

The bind case was a special case to bind an Expression to a var, but it add complexity and is really unneded if we model the dictionary to have Expressions. Also, thanks to the Runner generic type, we can limit the dictionary Expression to only allow Expressions with the same type.

Finally our code look like:

``````indirect enum Expression<Value> {
case value(Value)
case sub(Expression, Expression)
case `var`(String)
}

extension Expression {
func map<NewValue>(_ transform: (Value) -> NewValue) -> Expression<NewValue> {
switch self {
case .value(let value):
return .value(transform(value))
case let .sub(lExpression, rExpression):
return .sub(lExpression.map(transform), rExpression.map(transform))
case let .var(key):
return .var(key)
}
}

func evaluate<Output>(_ runner: Runner<Value, Output>, environment: [String: Expression] = [:]) -> Output {
runner.run(self, environment: environment)
}
}

struct Runner<Input, Output> {
private let _run: (Runner<Input, Output>, Expression<Input>, [String: Expression<Input>]) -> Output

init(_ run: @escaping (Runner<Input, Output>, Expression<Input>, [String: Expression<Input>]) -> Output) {
self._run = run
}

func run(_ operation: Expression<Input>, environment: [String: Expression<Input>] = [:]) -> Output {
_run(self, operation, environment)
}
}

extension Runner where Input == Int, Output == Double {
static let evaluatorRunner = Runner<Int, Double> { runner, operation, environment in
switch operation {
case let .value(value):
return Double(value)
return runner.run(aOP, environment: environment) + runner.run(bOP, environment: environment)
case let .sub(aOP, bOP):
return runner.run(aOP, environment: environment) - runner.run(bOP, environment: environment)
case let .var(key):
guard let value = environment[key] else {
}
return runner.run(value, environment: environment)
}
}
}

extension Runner where Input == String, Output == String {
static let printRunner = Runner<String, String> { runner, operation, environment in
switch operation {
case let .value(value):
return value
return "\(runner.run(aOP, environment: environment)) + \(runner.run(bOP, environment: environment))"
case let .sub(aOP, bOP):
return "\(runner.run(aOP, environment: environment)) - \(runner.run(bOP, environment: environment))"
case let .var(key):
guard let value = environment?[key] else {
}
return runner.run(value, environment: environment)
}
}
}
``````

Then use the vars will be:

``````let expression: Expression<Int> = .add(.var("x"), .value(6))
let result = expression.evaluate(.evaluatorRunner, environment: ["x": .value(2)])
``````

## Friendly sintax

The model is working. The interpretation is abstracted from the model creation. That’s okey, but, why if we’re building an expression based on arithmetic operations, don’t we use operators?

In order to do this, we’re going to extend the `Expression` model to be `ExpressibleByIntegerLiteral` limited to the Int expression.

``````extension Expression: ExpressibleByIntegerLiteral where Value == Int {
init(integerLiteral value: IntegerLiteralType) {
self = .value(value)
}
}
``````

To build the `.var` case, we’re going to implement the `ExpressibleByStringLitleral`, this has a trade-off that we must consider. We don’t be able to build an `Expression<String>` from litlerals, because we will use the string literal to build the var case on any type of Expression.

``````extension Expression: ExpressibleByStringLiteral {
init(stringLiteral value: StringLiteralType) {
self = .var(value)
}
}
``````

And then, we only need to create the operators.

``````extension Expression {
static func +(lhs: Expression, rhs: Expression) -> Expression {
}

static func -(lhs: Expression, rhs: Expression) -> Expression {
.sub(lhs, rhs)
}
}
``````

Now, we can build Expressions like if we are writting code.

``````let expression: Expression<Int> = 6 + 2 + "x"
let result = expression.evaluate(.evaluatorRunner, environment: ["x": .value(2)])
``````

## Conclusion

At the end, all of these ways of coding looks pretty similar. First you have to find what is the minimum expression to your goal, usually that minimum expression will be a function. Then wrap it inside a type, and then extend the type to compose or whatever.

For the functional choice, we’ve started from `() -> Int`, wrapped it inside a Type. Added a couple of functions inside the type to allow `add` and `sub`, and made it generic. I didn’t implements the oeprators and `Expressible` protocols inside this case, but it could be implemented too like in the `enum` choice.

On the other hand, for the enum choice, we’ve started from the most simple case `.value(Int)` and then add the other cases. We’ve created the runner to wrap inside an object how the Expression is interpreted, based on the function `(Expression) -> Int`. Then we’ve updated it to be generic. Added the `.var` case with the environment dictionary, and some sugar syntax to write the expressions with operators.

### All the code

``````import Foundation

indirect enum Expression<Value> {
case value(Value)
case sub(Expression, Expression)
case mul(Expression, Expression)
case `var`(String)
}

extension Expression {
func map<NewValue>(_ transform: (Value) -> NewValue) -> Expression<NewValue> {
switch self {
case .value(let value):
return .value(transform(value))
case let .sub(lExpression, rExpression):
return .sub(lExpression.map(transform), rExpression.map(transform))
case let .mul(lExpression, rExpression):
return .mul(lExpression.map(transform), rExpression.map(transform))
case let .var(key):
return .var(key)
}
}

func evaluate<Output>(_ runner: Runner<Value, Output>, environment: [String: Expression]? = nil) -> Output {
runner.run(self, environment: environment)
}
}

extension Expression: ExpressibleByIntegerLiteral where Value == Int {
init(integerLiteral value: IntegerLiteralType) {
self = .value(value)
}
}

extension Expression: ExpressibleByStringLiteral {
init(stringLiteral value: StringLiteralType) {
self = .var(value)
}
}

extension Expression {
static func +(lhs: Expression, rhs: Expression) -> Expression {
}
static func -(lhs: Expression, rhs: Expression) -> Expression {
.sub(lhs, rhs)
}
static func *(lhs: Expression, rhs: Expression) -> Expression {
.mul(lhs, rhs)
}
}

struct Runner<Input, Output> {
private let _run: (Runner<Input, Output>, Expression<Input>, [String: Expression<Input>]?) -> Output

init(_ run: @escaping (Runner<Input, Output>, Expression<Input>, [String: Expression<Input>]?) -> Output) {
self._run = run
}

func run(_ operation: Expression<Input>, environment: [String: Expression<Input>]? = nil) -> Output {
_run(self, operation, environment)
}
}

extension Runner where Input == Int, Output == Double {
static let evaluatorRunner = Runner<Int, Double> { runner, operation, environment in
switch operation {
case let .value(value):
return Double(value)
return runner.run(aOP, environment: environment) + runner.run(bOP, environment: environment)
case let .sub(aOP, bOP):
return runner.run(aOP, environment: environment) - runner.run(bOP, environment: environment)
case let .mul(rExpr, lExpr):
return runner.run(rExpr, environment: environment) * runner.run(lExpr, environment: environment)
case let .var(key):
guard let value = environment?[key] else {
}
return runner.run(value, environment: environment)
}
}
}

extension Runner where Input == String, Output == String {
static let printRunner = Runner<String, String> { runner, operation, environment in
switch operation {
case let .value(value):
return value
return "\(runner.run(aOP, environment: environment)) + \(runner.run(bOP, environment: environment))"
case let .sub(aOP, bOP):
return "\(runner.run(aOP, environment: environment)) - \(runner.run(bOP, environment: environment))"
case let .mul(rExpr, lExpr):
return "\(runner.run(rExpr, environment: environment)) * \(runner.run(lExpr, environment: environment))"
case let .var(key):
guard let value = environment?[key] else {