Further Leveraging the Type System

My SwiftSummit 2015 lightning talk about further leveraging Swift's type system. Replaces the planned talk "Making the Illegal Impossible".

Transcript

1. Making the Illegal
   Impossible
   

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2. Question about Type
   Does every value that can
   be created make sense?
   

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3. Making the Illegal
   Impossible
   

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4. Further Leveraging the
   Type System
   

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5. ⛳️
   When using strict types, we often have
   to (temporarily) fill holes
   

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6. undefined can fill holes
   func undefined(_ message:String="") -> T {
   fatalError("undefined \(message)")
   }
   Can be used for simple things like
   let a : String = undefined("special string")
   let b : Int = undefined("some int")
   let t = NSTimer(timeInterval: undefined(), target: undefined(),
   selector: undefined(), userInfo: undefined(),
   repeats: undefined())
   

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7. Function Holes
   func parseIntFromString(input : String) -> Int? {
   let parsedInt = (undefined() as String -> Int?)(input)
   return parsedInt
   }
   func mayFail(x : Int) -> Result
   func addUpSuccesses(xs : [Int]) -> Int {
   let combine : (Int, Result) -> Int = undefined("fancy +")
   return map(xs, mayFail).reduce(0, combine:combine)
   }
   -> fatal error: undefined fancy +: file main.swift, line 214
   

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8. Handling the impossible
   func lexicographicallyLowestString(x:String, y:String, z:String) -> String {
   let xyz = [x, y, z]
   return xyz.sorted({ $0 < $1 }).first ??
   undefined("the impossible happened!")
   }
   

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9. !
   Phantoms
   

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10. Phantom types
    protocol Currency {}
    enum GBP : Currency {}
    enum EUR : Currency {}
    enum USD : Currency {}
    struct Money {
    let amount : NSDecimalNumber
    }
    

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11. Phantom type example
    func convertGBPtoEUR(gbp:Money) -> Money {
    let forex : NSDecimalNumber = NSDecimalNumber(mantissa: 133263,
    exponent: -5,
    isNegative: false)
    return Money(amount:gbp.amount.decimalNumberByMultiplyingBy(forex))
    }
    

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12. Thank you!
    @johannesweiss | johannes@jweiss.io
    

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