Parsel

Parsel is a parser combinator library that makes it easy to write parsers. Parser combinators lets you create simple parses that can be combined together to very complex ones. Take for example a parser that parses a digit from a given String: (You can use the pre-defined lexical parser for digit: L.digit)

let digit = Parser<String, Int> { input in
    guard let first = input.first, let number = Int(String(first)) else {
        return .fail(/* some pre-defined error */)
    }
    return .success(result: number, rest: String(input.dropFirst()))
}

We can now simply extend this to create a parser that parses an addition of two digits from a string:

let addition = (digit ~ L.plus ~ digit).map { a, _, b in a + b } // `L.plus` is a predefined parser that parses the `+` sign

let result = addition.parse("2+4")
try! result.unwrap() // Int: 6

Why should I use this?

Parsing is a very common task, it does not always mean to parse source code or JSON strings. Parsing means to transform an unstructured input to a structured output. In case of source code this means to parse a raw string to an AST (abstract syntax tree), in case of an addition it means to parse the result of adding two numbers out of a string. Parsing can always fail, if the input does not match the needed grammer. If the input string in the above example would have been 1+, it would have been failed because the second number is missing. The advantage of parser combinators is that you start with a very basic parser. In the above example digit parses only one digit. But it is not hard, to add a parser that parses more than one digit. A number is a repetition of mulitple digits. For repetition, we can use rep, which tries to apply the parser until it fails and collects the result as an array. Parsing an integer addition is as easy as

func intFromDigits(_ digits: [Int]) -> Int {
    return digits.reduce(0) { res, e in    
        return res * 10 + e
    }
}

let number = digit.rep.map(intFromDigits)
let addition = number ~ L.plus ~ number ^^ { a, _, b in // ^^ is convenience for map
    return a + b
}

let result = addition.parse("123+456")
try! result.unwrap() // Int: 579

(There is also a pre-defined lexical parser for numbers L.number that is able to parse a number in different formats [binary, octal, hexadecimal, decimal])

What are the disadvantages?

Since parser combinators are very high level, they abstract the whole process of parsing a lot. That means it is easier to use but it also means that the performance is not as good as an optimized, handwritten parser.

Installation

Parsel is currently available via Swift Package Manager and Cocoapods. Support for Carthage will come later.

Swift Package Manager

To use Parsel in your project, just add it as a dependency in your Package.swift file and run swift package update.

import PackageDescription

let package = Package(
  name: "MyAwesomeApp",
  dependencies: [
    .package(url: "https://github.com/BenchR267/Parsel", from: "2.2.0")
  ]
)

Cocoapods

To use Parsel with Cocoapods, just add this entry to your Podfile and run pod install:

target 'MyAwesomeApp' do
  use_frameworks!

  pod 'Parsel'
end

Requirements

• Swift 4.0
  • Parsel is written in Swift 4.0 development snapshots and will be available when Swift 4.0 is released

Example

Calculator is a small example I wrote with Parsel.

Documentation

Check out the documentation on the Github page.

Author

• @benchr, mail@benchr.de

License

Parsel is under MIT license. See the LICENSE file for more info.