Implementing a Programming Language in Swift — Part 5: The Main function

This is the fifth part in a tutorial series on “Writing a Programming Language in Swift.” Be sure to check out Part 4.

In the previous tutorials, we completed implementing a Lexer and Parser for our calculator. Now all there is left i tos simply traverse our AST and “interpret” some output.

So… almost there 💪🏻!

You might have noticed that the Nodes we created when parsing (Float & InfixOperation) evaluate both quite easily to Float values. So all we have to do to be able to evaluate our AST is to add a protocol method to our Node protocol for interpreting each single Node and implement it in our Node structs:

protocol Node {
    func interpret() throws -> Float
}
// ...
extension Float: Node {
    func interpret() throws -> Float {
        return self
    }
}
struct InfixOperation: Node {
// ...
    func interpret() throws -> Float {
        let left = try lhs.interpret()
        let right = try rhs.interpret()
        switch op {
        case .divideBy:
            return left / right
        case .times:
            return left * right
        case .minus:
            return left - right
        case .plus:
            return left + right
        }
    }
}

That’s it, now we our ready create our main function, let’s call it run:

func run(code: String) throws {
    let tokens = Lexer(code: code).tokens
    let parser = Parser(tokens: tokens)
    let ast = try parser.parse()
    print(ast.interpret())
}
let code = "4 + 2 * 5 + 3 * (1 + 4)"
run(code: code) // OUTPUTS: 29

Congratulations! We now have a working calculator.

It doesn’t look like much, but as you will notice in future tutorials, adding programming language features such as variables and functions isn’t especially hard.

Next Steps

A logical next step to this is to wrap this into a command line tool. Every programming language has one.

Swift star John Sundell published a very good tutorial on this, and I’ll leave a link to it here for you to follow. Without much effort, you will be able to run our code with a command in the terminal:

> myinterpreter "(2 + 3) * 5"
> 25

Our next step however, will be to turn our simple calculator language into something that resembles a programming language. Starting by adding variables.

Stay tuned!

I hope you guys found this tutorial interesting and as always, don’t be shy to give feedback e.g. by clapping or writing a response.

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