Parser Combinators

Transcript

1. Parser Combinators
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2. Self Intro
   » Name: Joshua Kaplan
   » Interests:
   !
   λ
   "
   » Company: GMO Pepabo
   » Service: minne
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3. What are Parser Combinators?
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4. » Higher-order function that takes multiple parsers
   and /combines/ them into a new parser
   » CS theory
   » Parsec, Haskell
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5. In Swift
   struct Parser {
   let run: (inout Substring) throws -> A
   }
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6. Characteristics
   » Monadic
   » Composable
   » Generic
   » Immutable
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7. Use
   let int = Parser { input in
   let num = input.prefix(while: { $0.isNumber })
   guard let number = Int(num) else { throw ParserError.IntError.notANumber(num) }
   input.removeFirst(num.count)
   return number
   }
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8. func removingLiteral(_ string: String) -> Parser {
   return Parser { input in
   guard input.hasPrefix(string) else {
   throw ParserError.StringError.literalNotFound(string[...])
   }
   input.removeFirst(string.count)
   }
   }
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9. Higher order functions
   » map
   » flatMap (bind, >>=)
   » zip
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10. struct Coordinate { let x, y: Int }
    let str = "1,2"
    let coordinateParser = zip(
    int,
    removingLiteral(","),
    int
    ).map { x, _, y in Coordinate(x: x, y: y) }
    let (coordinate, _) = try coordinateParser.run(str[...])
    ▿ Coordinate
    - x: 1
    - y: 2
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11. func substring(while predicate: @escaping (Character) -> Bool) -> Parser {
    return Parser { input in
    let p = input.prefix(while: predicate)
    input.removeFirst(p.count)
    return p
    }
    }
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12. Let's make another parser!
    struct Person { let name: String; let age: Int }
    let str = "name: John, age: 90"
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13. Name and age parsers
    let nameParser = zip(
    removingLiteral("name: "),
    substring(while: { $0.isLetter })
    ).map { _, name in return String(name) }
    let ageParser = zip(
    removingLiteral("age: "),
    int
    ).map { _, age in return age }
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14. Person parser
    let personParser = zip(
    nameParser,
    removingLiteral(", "),
    ageParser
    ).map { name, _, age in return Person(name: name, age: age) }
    let (person, _) = try personParser.run(str[...])
    ▿ Person
    - name: "John"
    - age: 90
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15. Comparison
    » By hand
    » Scanner
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16. Why and when?
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17. References
    » https://github.com/pointfreeco/episode-code-
    samples/tree/master/0064-parser-combinators-pt3
    » https://talk.objc.io/episodes/S01E13-parsing-
    techniques
    » https://github.com/johnpatrickmorgan/Sparse
    » https://github.com/davedufresne/SwiftParsec
    » https://github.com/thoughtbot/Argo
    » https://github.com/tryswift/TryParsec
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