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CORRECT BEHAVIOR
THROUGH
TYPE SAFETY
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@JSPAHRSUMMERS
REACTIVECOCOA
MANTLE
CARTHAGE
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No content
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TYPES PREVENT ERRORS
(by restricting what you can do)
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NSArray
IS MORE RESTRICTED THAN
NSMutableArray
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NSArray
PREVENTS MORE ERRORS THAN
NSMutableArray
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Optional
RESTRICTS ACCESS TO ITS VALUE
PREVENTS MISTAKES WITH nil
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Array
RESTRICTS INSERTIONS
PREVENTS CRASHES AFTER RETRIEVAL
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More generally…
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TYPES = PROOFS
(the Curry–Howard correspondence)
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A → B
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A → B
func toInteger(num: Double) -> Int {
let result = round(num)
return Int(result)
}
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STRING → STRING
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STRING → STRING
func identity(s: String) -> String {
return s
}
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WE CAN USE
TYPES
TO PROVE
CORRECTNESS
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WHAT DOES NSDATA PROVE?
(Not much.)
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WHAT DOES NSSTRING PROVE?
Characters, not just bytes!
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WHAT DOES NSURL PROVE?
Valid URL, not just a string!
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EXTENDS TO
ANY KIND
OF VALIDATION
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What about tests?
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Science break!
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TESTS ARE SUPPORT
TYPES ARE PROOF
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Caveats
▸ Proofs can contain errors too
▸ Types can’t represent everything
▸ Tests may be good enough
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Case study:
ERRORS IN REACTIVECOCOA
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SIGNALS CONSIST OF…
▸ Any number of next events
▸ Optionally an error or completed event
Next* (Error | Completed)?
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RACSignal *
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No content
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PROPERTY BINDING IN RAC 2
RAC(self.imageView, image) = [RACObserve(self, model)
flattenMap:^ RACSignal * (Model *model) {
return [model fetchImage];
}];
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PROPERTY BINDING IN RAC 2
RAC(self.imageView, image) = [RACObserve(self, model)
flattenMap:^ RACSignal * (Model *model) {
return [model fetchImage];
}];
*** Received error from RACSignal in binding for key
path "image" on UIImageView:
Error Domain=NSURLErrorDomain Code=-1 "Operation
could not be completed."
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Solution:
TYPES!
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Signal
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enum NoError {}
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PROPERTY BINDING IN RAC 3
func <~ (property: MutableProperty,
signal: Signal)
func <~ (property: MutableProperty,
producer: SignalProducer)
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PROPERTY BINDING IN RAC 3
class Model {
func fetchImage() -> SignalProducer
}
self.imageProperty <~ self.modelProperty.producer
// TYPE ERROR: NSError is incompatible with NoError
|> flatMap(.Latest) { model in
return model.fetchImage()
}
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PROPERTY BINDING IN RAC 3
class Model {
func fetchImage() -> SignalProducer
}
self.imageProperty <~ self.modelProperty.producer
|> flatMap(.Latest) { model in
return model.fetchImage()
// Ignore any error event
|> catch { error in .empty }
}
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TYPED ERRORS
VS.
throws
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TYPES
CAN ALSO DESCRIBE
EFFECTS
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IO
MONAD
IN HASKELL
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WITH IO, YOU CAN…
▸ Put a value into it
▸ Perform side effects using the value
▸ “Lift” pure functions to apply to the value
▸ Never2 get the result back out
2 Except through the rarely-used unsafePerformIO.
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WITH A SIGNAL, YOU CAN…
▸ Put values into it
▸ Perform time-based operations using the values
▸ “Lift” pure functions to apply to the values
▸ Register for delivery3 to get the results back out
3 It’s possible to synchronously wait for results, but the framework highly discourages this.
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Example:
TYPES FOR CONCURRENCY
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THE UI PROBLEM™
dispatch_async(someBackgroundQueue) {
// Oops!
self.textField.text = "foobar"
}
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CAPTURING UI CODE IN THE TYPE SYSTEM
struct UIAction {
init(_ action: () -> T)
func enqueue()
func enqueue(handler: T -> ())
func map(f: T -> U) -> UIAction
func flatMap(f: T -> UIAction) -> UIAction
}
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WHAT IF…?
extension UITextField {
/// Sets the receiver's text to the given string
/// when the returned action is executed.
func jss_setText(text: String) -> UIAction<()>
}
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SLIDES AND NOTES
Available at git.io/vkjl9
(github.com/jspahrsummers/correct-behavior-through-type-safety)
THANKS TO…
Neil Pankey, Rob Rix, James Lawton, Gordon Fontenot, Eli Perkins, Javi
Soto, anyone else who I omitted in error, and you!