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Type Driven Development
Попов Валерий
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TESTS
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TESTS TESTS
TESTS
ESTS TEST
TESTS TESTS
TESTS
ESTS TEST
TESTS TESTS
TESTS
ESTS TEST
TESTS TESTS
TESTS
ESTS TEST
TESTS TESTS
TESTS
ESTS TEST
TESTS TESTS
TESTS
ESTS TEST
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TYPES
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Practical
› Type systems
› Using types
Agenda
Theoretical
› Dependent types in Swift
› Type DD approach
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Static | Dynamic
Categories
Strong | Weak Implicit | Explicit
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C/C++
Objective-C
Swift
Java
C# Haskell
Clojure
Python
Ruby
JavaScript
STATIC DYNAMIC
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Objective-C id
Haskell.Dynamic
STATIC DYNAMIC
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STRONG WEAK
Swift
Java
C# Haskell Clojure
Python
Ruby
JavaScript
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Clojure
Scala
PHP Python C/C++
Java
Haskell
Objective-C
IMPLICIT EXPLICIT
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Scala
Haskell
Swift
IMPLICIT EXPLICIT
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Types define code
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Python: weak, dynamic
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def binary_search(l, value):
low = 0
high = len(l)-1
while low <= high:
mid = (low+high)
if l[mid] > value: high = mid-1
elif l[mid] < value: low = mid+1
else: return mid
return -1
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Python: weak, dynamic
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def binary_search(list, value):
…
else: return mid
return -1
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Python: weak, dynamic
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def binary_search(list, value):
…
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Swift: static, strong
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func binarySearch(xs: [T], x: T) -> Int? {
var recurse: ((Int, Int) -> Int?)!
recurse = {(low, high) in switch (low + high) / 2 {
case _ where high < low: return nil
case let mid where xs[mid] > x:
return recurse(low, mid - 1)
case let mid where xs[mid] < x:
return recurse(mid + 1, high)
case let mid:
return mid
}}
return recurse(0, xs.count - 1)
}
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Swift: static, strong
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func binarySearch
(xs: [T], x: T) -> Int? {
…
}
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Swift: static, strong
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func binarySearch
(xs: [T], x: T) -> Int? {
…
}
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USING TYPES
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Money transfer
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struct FailedTransaction {
let isRejected: Bool
let isCancelled: Bool
let notEnoughMoney: Amount?
}
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struct FailedTransaction {
let isRejected: Bool
let isCancelled: Bool
let notEnoughMoney: Amount?
}
let transaction = FailedTransaction
(
isRejected: true,
isCancelled: true,
notEnoughMoney: nil
)
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Product type
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struct FailedTransaction {
let isRejected: Bool
let isCancelled: Bool
let notEnoughMoney: Amount?
}
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Sum type
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enum FailedTransaction {
case rejected
case cancelled
case outOfMoney(Amount)
}
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let failedTransaction: FailedTransaction
failedTransaction = .rejected
enum FailedTransaction {
case rejected
case cancelled
case outOfMoney(Amount)
}
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Keep types simple
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Message sending
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struct Message {
let text: String
var isEncrypted: Bool
}
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func send(message: Message) throws {
if message.isEncrypted {
print(message.text)
} else {
throw MessageError.unsecured
}
}
struct Message {
let text: String
var isEncrypted: Bool
}
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func send(message: Message) throws {
if message.isEncrypted {
print(message.text)
} else {
throw MessageError.unsecured
}
}
let message = Message(text: "Hi!",
isEncrypted: false)
send(message: message)
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Runtime exception :(
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let message = Message(text: "Hi!",
isEncrypted: false)
send(message: message)
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Compile time?
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struct Message {
let text: String
}
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Compile time?
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struct Message {
let text: String
}
enum Encrypted {}
enum PlainText {}
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func send(message: Message) {
print(message.text)
}
struct Message {
let text: String
}
enum Encrypted {}
enum PlainText {}
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func send(message: Message) {
print(message.text)
}
let message = Message(text: “Hi!”)
send(message: message)
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Compile time :)
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let message = Message(text: “Hi!")
send(message: message)
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Phantom Types
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let m1 = Message(text: “”)
let m2 = Message(text: “”)
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Make invalid states
unacceptable
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ERROR HANDLING
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Error codes
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int foo() {
if (failed) {
return -1;
}
return 0;
}
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Completion
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typedef void (^Compl)(NSData,NSError*)
- (void)performTask:(Compl)completion{
…
completion(nil, error)
}
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typedef void (^Compl)(NSData,NSError*)
- (void)performTask:(Compl)completion{
…
completion(nil, nil) // ???
}
typedef void (^Compl)(NSData,NSError*)
- (void)performTask:(Compl)completion{
…
completion(data, error) // ???
}
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Exceptions
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void performTask(String[] args)
throws FileNotFoundException {
...
throw new FileNotFoundException()
}
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Unchecked exceptions
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void task() throws RuntimeException {
...
}
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Result
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enum Result {
case success(T)
case failure(Error)
}
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func contentsOfFile(
path: String) -> Result {
return .success("")
}
func performTask() -> Result {
return .failure(Err.some)
}
enum Result {
case success(T)
case failure(Error)
}
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Errors have types
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EVEN MORE
POWERFUL TYPES
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Array sort
1 6 2 8 3 4
1 2 3 4 6 8
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Idris
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insSort : Vect n elem -> Vect n elem
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Dependent types
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insSort : Vect n elem -> Vect n elem
FUNC NAME
ARGUMENT TYPE
DEPENDENT TYPE
ARGUMENT NAME
RETURN TYPE
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DEPENDENT TYPES
IN SWIFT?
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1 ≠ 2
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protocol Nat { init() }
struct Zero: Nat {}
protocol NonZero: Nat {
associatedtype Predecessor: Nat
}
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protocol NonZero: Nat {
associatedtype Predecessor: Nat
}
struct Successor: NonZero {
typealias Predecessor = N
}
typealias One = Successor
typealias Two = Successor
typealias Three = Successor
…
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protocol BinaryOperation {
associatedtype A: Nat
associatedtype B: Nat
}
struct Successor: NonZero {
typealias Predecessor = N
}
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struct EQ {}
extension BinaryOperation where A == B {
var r: EQ { return EQ() }
}
struct NEQ {}
extension BinaryOperation {
var r: NEQ { return NEQ() }
}
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struct Compare : BinaryOperation {
typealias A = L
typealias B = R
}
Compare().r //EQ
Compare().r //NEQ
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//struct NEQ {}
//extension BinaryOperation {
// var r: NEQ { return NEQ() }
//}
Compare().r //Compile time error
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Not type dependent
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BEN-G/VALIDATED
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typealias LoggedInUser =
Validated
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TYPE DRIVEN
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TDD
RED -> GREEN -> REFACTOR
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TypeDD
TYPE -> DEFINE-> REFINE
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Type Holes in Swift
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func typeHole
(_ description: String = "") -> T {
fatalError("unfilled hole \(description)")
}
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› Types and type systems
› Painless types
› Swift type system is almost good
› Type DD
Summary
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› Finite-state machine
› Hindley-Milner algorithm
› Curry-Howard isomorphism
› Dependent types in Idris
Even more