README.md

swift-parser-generator

This code contains an attempt to make something like the Scala parser combinators in Swift.

It has been partially successful in that it can be used to make simple parsers, but I have not as yet implemented the Packrat style parsers.

How it works

Using operator overloading a nested set of functions is created that forms the parser. The functions take a parser instance and a reader object that provides the function with characters to parse.

The following parsing operations are currently supported

// "a" followed by "b"
let rule = "a" ~ "b"

// "a" or "b"
let rule = "a" | "b"

// "a" followed by something other than "b"
let rule = "a" ~ !"b"

// "a" followed by one or more "b"
let rule = "a" ~ "b"+

// "a" followed by zero or more "b"
let rule = "a" ~ "b"*

// "a" followed by a numeric digit
let rule = "a" ~ ("0"-"9")

// "a" followed by the rule named "blah"
let rule = "a" ~ ^"blah"

// "a" followed by the end of input
let rule = "a"*!*

To have the parser call your code when a rule matches use the => operator. For example:

import SwiftParser
class Adder : Parser {
    var stack: Int[] = []
    
    func push() {
        stack.append(self.text.toInt()!)
    }
    
    func add() {
        let left = stack.removeLast()
        let right = stack.removeLast()
        
        stack.append(left + right)
    }
    
    override func rules() {
        let number = ("0"-"9")+ => push
        let expr = (number ~ "+" ~ number) => add
        
        start_rule = expr
    }
}

This example displays several details about how to work with the parser. The parser is defined in a method called rules and it must set the start_rule to tell the parser where to begin. The following code snippet is taken from one of the unit tests. It show how to implement a parser containing mutually recursive rules:

  override func rules() {
      start_rule = (^"primary")*!*
        
      let number = ("0"-"9")+ => push
      add_named_rule("primary",   rule: ^"secondary" ~ (("+" ~ ^"secondary" => add) | ("-" ~ ^"secondary" => sub))*)
      add_named_rule("secondary", rule: ^"tertiary" ~ (("*" ~ ^"tertiary" => mul) | ("/" ~ ^"tertiary" => div))*)
      add_named_rule("tertiary",  rule: ("(" ~ ^"primary" ~ ")") | number)
  }