Latest 0.1.1
Homepage https://github.com/nerdsupremacist/Ogma
License MIT
Platforms ios 8.0
Authors

Ogma is a lightweight Parsing Framework written in Swift. Say goodbye to complicated state machines! Now you can easily write a Parser using only pure functions.

How do I write a Parser?

Once you have an understanding what you want to parse (it helps to have a defined Context Free Grammar already written down), you need to implement:

  • Your Model: What the result of the Parser should look like
  • Your Lexer: A component to turn an input String into Tokens
  • Your Parser: The consumer of the Tokens from the Lexer. The Parser will take the Tokens as an input and output your Model

Installation

Ogma is available using Cocoapods. To include Ogma into your project add it to your Podfile:

pod 'Ogma', '~> 0.1'

And run:

$ pod install

Example

Right now, to make it easy we will write a simple calculator using Ogma. Our calculator will only support addition and multiplication. For now we will not be doing multiplication before addition. That’s a story for another time.

Let’s go back to the Original Idea: Model, Lexer, Parser!

Model

We can model the input to our calculator as an Expression:

public indirect enum Expression {

    public struct Addition {
        let lhs: Expression
        let rhs: Expression
    }

    public struct Multiplication {
        let lhs: Expression
        let rhs: Expression
    }

    case int(Int)
    case addition(Addition)
    case multiplication(Multiplication)
}

While we’re at it we might as well just write the classic eval function:

extension Expression {

    func eval() -> Int {
        switch self {
        case .int(let int):
            return int
        case .addition(let lhs, let rhs):
            return lhs.eval() + rhs.eval()
        case .multiplication(let lhs, let rhs):
            return lhs.eval() * rhs.eval()
        }
    }

}

Lexer

First let’s define what the Tokens are going to be. So we will allow Integers, + and *, as well as opening and closing parenthesis.

So our Tokens can now be an enum. For clarity we suggest namespacing the Tokens with your model. To make it clear that they belong together:

extension Expression {

    public enum Token: TokenProtocol {
        case int(Int)
        case plus
        case times
    }

}

With the Tokens clear we can now just write a Lexer. A Lexer is a component that turns a String into Tokens. The easiest way to build one is by using what we call TokenGenerators. A TokenGenerator attempts to consume the beginning of a string as far as it can to create a single Token.

To use TokenGenerators our Lexer simply has to conform to GeneratorLexer and return the generators it will be using:

extension Expression {

    enum Lexer: GeneratorLexer {
        typealias Token = Expression.Token

        static let generators: Generators = [
                IntLiteralTokenGenerator().map(Token.int),
                RegexTokenGenerator(pattern: "\+").map(to: .plus),
                RegexTokenGenerator(pattern: "\*").map(to: .times),
            ]
    }

}

Now there’s a lot of stuff going on here. We’re mainly using two things:

  • IntLiteralTokenGenerator is a TokenGenerator already implemented by Ogma. It will take a string and attempt to parse an Int. We are then just mapping that generator to return the Tokens we modeled before.
  • RegexTokenGenerator is a TokenGenerator that will return a String matching the pattern. Then we map every instance to .plus. So if we find a + we return Token.plus

Parser

We can now make build our Parser by letting our Model implement Parsable. For an Object to be Parsable it just means that there’s a static Parser that can return the type.
For starters let’s implement it for Int. The easiest way is to write a computed property for token, when it has an Int:

extension Expression.Token {
    var int: Int? {
        guard case .int(let int) = self else { return nil }
        return int
    }
}

And give the key path to a parser:

extension Int: Parsable {
    public typealias Token = Expression.Token

    public static let parser: AnyParser<Expression.Token, Int> = .consuming(keyPath: .int)
}

And now the operations. By using the && operator we can chain multiple Parsers and expect the results sequentially:

extension Expression.Addition: Parsable {
    public typealias Token = Expression.Token

    public static let parser: AnyParser<Expression.Token, Expression.Addition> = {
        let parser = Expression.self && .plus && Expression.self
        return parser.map { Expression.Addition(lhs: .int($0), rhs: $1) }
    }()
}

extension Expression.Multiplication: Parsable {
    public typealias Token = Expression.Token

    public static let parser: AnyParser<Expression.Token, Expression.Multiplication> = {
        let parser = Expression.self && .times && Expression.self
        return parser.map { Expression.Multiplication(lhs: .int($0), rhs: $1) }
    }()
}

You can see that we are stating that the multiplication expression consists of an Expression a times operator and another Expression. Dogma will automatically match it correctly ;).

And finally we make the expression Parsable. This time using the || operator to signal that it should take the result from the first Parser that succeeds:

extension Expression: Parsable {

    public static let parser: AnyParser<Expression.Token, Expression> = Addition.map(Expression.addition) ||
        Multiplication.map(Expression.multiplication) ||
        Int.map(Expression.int)

}

Profit! 💸

Now we can use our parser:

extension String {

    func calculate() throws -> Int {
        return try Expression.parse(self, using: Expression.Lexer.self).eval()
    }

}

Awesome! We just wrote a calculator and at no point did we use a state machine. In fact all of our functions are completely pure and can easily be extended.

Other cool features

Ignoring Tokens

Some languages choose to ignore certain characters and expressions entirely.
These are Tokens that should never make it to the Parser in the first place. For example: Comments and Whitespaces.

If you already have a TokenGenerator that consumes this, you simply call ignore in your Lexer.
For example if we want to ignore Whitespaces in our calculator:

enum Lexer: GeneratorLexer {
    ...

    static let generators: Generators = [
            WhiteSpaceTokenGenerator().ignore(),
            ...
        ]
}

Binary Operators and Operator Precedence

Another common scenario when writing a Parser is handling operator precedence. Even as in mentioned in the example above, with the precendence between + and -. Ogma already comes with a model for writing Binary Operations and handling the precendence between operators.

Note: for now this only works when you know all possible operators before hand. If you’re planning on writing a language that supports custom operators, this API isn’t for you.

To use the Binary Operator API you have to implement two protocols:

  • BinaryOperator: basically an Identifier for the operator that will run. This has to be comparable so that we know the precedence. The smaller the value, the higher the precedence.
  • and MemberOfBinaryOperation: This is basically the left and right hand side of your operator. The Member also knows which operators it supports, and can be instantiated from an Operation.

Example

Back in our calculator case we can now ditch the Addition and Multiplication structs in favor of BinaryOperation. We begin by implementing the Operator:

extension Expression {

    public enum Operator: Int, BinaryOperator {
        case multiplication
        case addition

        var token: Expression.Token {
            switch self {
            case .multiplication:
                return .times
            case .addition:
                return .plus
            }
        }
    } 

}

Please note that multiplication was written before addition. This means that now, since Operator is RawRepresentable by Int, the de facto precedence is multiplication before addition.
We make a few adjustments to our expression

public indirect enum Expression {
    case int(Int)
    case operation(BinaryOperation<Expression>) 
}

and implement MemberOfBinaryOperation:

extension Expression: MemberOfBinaryOperation {

    public init(from operation: BinaryOperation<Expression>) {
        self = .operation(operation)
    }

}

We’re almost done, now we just have to update the parser for the new model:

extension Expression: Parsable {

    public static let parser: AnyParser<Expression.Token, Expression> = BinaryOperation<Expression>.map(Expression.operation) ||
        Int.map(Expression.int)

}

String Annotation

Sometimes you’re not expecting all of the user’s input to be valid. Maybe you want them to be able to inline your languange inside of some other text. For that there’s AnnotatedString<T>. An annotated string is actually just an array of the enum:

public enum AnnotationElement<Annotation> {
    case text(String)
    case annotated(String, Annotation)
}

Where it can be some text that wasn’t matched of an annotated section with a parsed value.

Now you can support cool features like parsing inlined JSON:

Hello, here's some JSON: { "Hello": "World" }

Which would output:

[
    .text("Hello, here's some JSON:"),
    .annotated("{ "Hello": "World" }",
               .dictionary(["Hello": .string("World"])),
]

In fact you don’t even need a full Parser architecture to enjoy this feature. If you just want to annotate this expressible by a Regular Expression, a Lexer will be enough.

For example if you’re parsing mentions and hashtags in a tweet:

First: Build a lexer.

enum Twitter {

    enum Token: TokenProtocol {
        case mention(String)
        case hashtag(String)
    }

    enum Lexer: GeneratorLexer {
        typealias Token = Twitter.Token

        static let generators: Generators = [
            RegexTokenGenerator(pattern: "@(\w+)", group: 1).map { .mention($0) },
            RegexTokenGenerator(pattern: "#(\w+)", group: 1).map { .hashtag($0) },
        ]
    }

}

Second: Call annotate:

let tweet = try Twitter.Lexer.annotate(input) as AnnotatedString<Twitter.Token>

Future Topics and Known Issues

  • A type can only implement Parsable once. And for one type of Tokens. So if you plan on writing more than one parser, you should not let types from the Standard Library implement Parsable.

Authors

Acknowledgements

This project was made possible with the help, supervision and valuable feedback from:

Latest podspec

{
    "name": "Ogma",
    "version": "0.1.1",
    "summary": "Framework for Parsing in Swift using pure functions",
    "description": "Ogma is a lightweight Parsing Framework written in Swift. Say goodbye to complicated state machines! Now you can easily write a Parser using only pure functions.",
    "homepage": "https://github.com/nerdsupremacist/Ogma",
    "license": {
        "type": "MIT",
        "file": "LICENSE"
    },
    "authors": {
        "mathiasquintero": "[email protected]"
    },
    "source": {
        "git": "https://github.com/nerdsupremacist/Ogma.git",
        "tag": "0.1.1"
    },
    "social_media_url": "https://twitter.com/nerdsupremacist",
    "platforms": {
        "ios": "8.0"
    },
    "source_files": "Ogma/Classes/**/*",
    "swift_versions": "4.2"
}

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