Parsley is a recursive descent parser combinator library that makes it simple to write complex, type-safe parsers in Swift. Note that Parsley does not promise source compatibility for the time being. The library is in active development, and many thing might change in the near future. Specifically,

• I want to somehow modify it to support tracking indices while parsing. This might introduce a requirement that what's being parsed is indexable.
• I want to improve error handling, especially in its suggestion of expected value v. actual value.
• I want to clean up the library so that it is more lightweight with another library adding more functionality on top of it.

That said, feel free to play around with it if you'd like, it doesn't bite!

It's super easy to define lexable types in Parsley! The simplest types to lex are those whose values can be enumerated and matched against the input. For example, if you'd like to lex a finite set of operators, your operator type can conform to Matchable and it will be automatically Parsable.

Conformance to Matchable simply requires a static member all listing all the possible values of that type and an instance member matcher that will only accept input that matches, throwing a ParseError otherwise.

enum OperatorSymbol: Character, Matchable {
    case Plus  = "+"
    case Minus = "-"

    static var all: [OperatorSymbol] = [.Plus, .Minus]
    
    var matcher: Parser<Character, ()> {
        return character(rawValue).discard()
    }
}

let testPlus  = OperatorSymbol.parse("+") // Optional(Operator.Plus)
let testMinus = OperatorSymbol.parse("-") // Optional(Operator.Minus)

Sometimes we'd like to lex more complex types, literal values for example. In these cases, we'd like to manually conform our type to Parsable by defining a static member parser that will transform valid input into an instance of our type.

enum LiteralValue: Parsable {
    case StringLiteral(String)
    case IntegerLiteral(Int)
    
    static var parser = coalesce(
        between(character("\""), parseFew: any()).stringify().map(LiteralValue.StringLiteral),
        prepend(within("+-").otherwise("+"), many1(digit)).stringify().map{ Int($0)! }.map(LiteralValue.IntegerLiteral)
    )
}

let testString  = LiteralValue.parse("\"hey\"") // Optional(LiteralValue.StringLiteral("hey"))
let testInteger = LiteralValue.parse("-123")    // Optional(LiteralValue.IntegerLiteral(-123))

Once we've defined our basic token types, we might want to define some union type that can hold each of the cases. This type itself ought to be Parsable since each of its cases refer to Parsable tokens.

enum Token: Parsable {
    case Value(LiteralValue)
    case Operator(OperatorSymbol)
   
    static var parser = coalesce(
        LiteralValue.parser.map(Token.Value),
        OperatorSymbol.parser.map(Token.Operator)
    )
}

let testTokens = try! tokens(Token.self).parse("532 - 11 + \"hello\" + -3")
// [
//  Token.Value(LiteralValue.IntegerLiteral(532)), 
//  Token.Operator(OperatorSymbol.Minus),
//  Token.Value(LiteralValue.IntegerLiteral(11)),
//  Token.Operator(OperatorSymbol.Plus),
//  Token.Value(LiteralValue.StringLiteral("hello")),
//  Token.Operator(OperatorSymbol.Plus),
//  Token.Value(LiteralValue.IntegerLiteral(-3))
// ]

Note that the order we coalesce the two parsers does have an impact on the result. If we had swapped the order, we would be unable to recognize negative integer literals since we'd always parse the minus sign as an operator first.

Once we've finished lexing the input, it's time to parse! Now, the dividing line between these two stages isn't quite as clear as one might expect (as is evident by the fact we talked about a Parsable protocol in the lexing section of this document)! The lexing stage ought not care about the recursive tree-like structure of the input. Instead, the lexing stage ought to emit a linear sequence of tokens that simplifies the parsing logic. For example, the lexer ought to deal with discarding (or handling) whitespace so the parser doesn't have to complicate its logic worrying about these cases.

Impatient? Check out the documentation here or dive into the source code!