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Practical Functional
Programming
@kylefuller
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Imperative
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Declarative
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Describe what to do
Not how to do it.
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What makes up functional
programming?
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What makes up functional programming?
4 first class functions
4 immutable data
4 reducing
4 pipelining
4 recursing
4 currying
4 monads
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What makes up functional programming?
4 first class functions
4 immutable data
4 reducing
4 pipelining
4 recursing
4 currying
4 monads
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Side-effects
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What is not functional
function?
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Un-functional Function
var value = 0
func increment() {
value += 1
}
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Un-functional Function
func increment() {
let center = NSNotificationCenter.defaultCenter()
let value = center.valueForKey("key") as? Int ?? 0
center.setValue(value + 1, forKey: "key")
}
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Functional Function
func increment(value:Int) -> Int {
return value + 1
}
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High Order Functions
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High Order Functions
4 Functions that take other functions as arguments
4 Functions that returns a functions
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Everything is a closure
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State of callables in
Objective-C
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@implementation Object
- (void)message:(NSString *)value {
}
@end
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@implementation Object
- (void)message:(NSString *)value {
}
@end
[object message:@“Hello”];
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void message(NSString *value) {
}
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void message(NSString *value) {
}
message(@“Hello”);
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void (^message)(NSString *value) = ^void(NSString *value) {
};
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No content
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void (^message)(NSString *value) = ^void(NSString *value) {
};
message(@“Hello”);
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Callables in Swift
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(String) -> ()
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let message:(String -> ())
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let message:(String -> ()) = { value in
}
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func message(value:String) {
}
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message("dotSwift")
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Interchangeability
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func message1(value:String) {
println(“Hello \(value)”)
}
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func message1(value:String) {
println(“Hello \(value)”)
}
var message2:(String -> ()) = { value in
println(“Hi \(value)”)
}
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func message1(value:String) {
println(“Hello \(value)”)
}
var message2:(String -> Void) = { value in
println(“Hi \(value)”)
}
message2 = message1
message2(“dotSwift”)
// Hello dotSwift
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Functional programming by Example
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let groups = [
["Kyle", "Maxine"],
["André", "Katie", "Ash"]
]
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How many people are in
each group?
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How many people are in each group?
let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
var count = [Int]()
for group in groups {
let people = countElements(group)
count.append(people)
}
count
// [2, 3]
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map
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map
(source, transform) -> ([T])
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transform
(item) -> (T)
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How many people are in each group?
let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
map(groups) {
countElements($0)
}
// [2, 3]
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countElements
(sequence) -> (Int)
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transform
(item) -> (T)
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(item) -> (T)
(sequence) -> (Int)
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How many people are in each group?
let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
map(groups, countElements)
// [2, 3]
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Order the numbers of
people in each group
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[2, 3] -> [3, 2]
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sorted
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sorted
(sequence, isOrderedBefore) -> (sequence)
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isOrderedBefore
(lhs, rhs) -> (Bool)
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Order the numbers of people in each group
let count = [2, 3]
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Order the numbers of people in each group
let count = [2, 3]
sorted(count) { (lhs, rhs) in
lhs > rhs
}
// largest group is [3, 2].first
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Operators are closures
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>
(lhs, rhs) -> (Bool)
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let count = [2, 3]
sorted(count, >)
// largest group is [3, 2].first
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Building an array of all people
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Building an array of all people
let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
var people = [String]()
for group in groups {
people += group
}
// ["Kyle", "Maxine", "André", "Katie", "Ash"]
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reduce
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reduce
(sequence, initial(U), combine) -> (U)
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combine
(U, value) -> (U)
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+
(U, U) -> (U)
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reduce(groups, [], +)
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Building an array of all people
let groups = [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
reduce(groups, [], +)
// ["Kyle", "Maxine", "André", "Katie", "Ash"]
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let input = "Kyle,Maxine\nAndré,Katie,Ash"
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let input = "Kyle,Maxine\nAndré,Katie,Ash"
var groups = [[String]]()
for line in input.componentsSeparatedByString("\n") {
let group = line.componentsSeparatedByString(",")
groups.append(group)
}
groups
// [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
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let input = "Kyle,Maxine\nAndré,Katie,Ash"
let groups = map(input.componentsSeparatedByString("\n")) { line in
line.componentsSeparatedByString(",")
}
groups
// [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
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let input = "Kyle,Maxine\nAndré,Katie,Ash"
func commaSeparator(input:String) -> [String] {
return input.componentsSeparatedByString(",")
}
map(input.componentsSeparatedByString("\n"), commaSeparator)
// [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
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High Order Functions
4 Functions that returns a functions
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func separateBy(separator:String) -> ((String) -> [String]) {
func inner(source:String) -> [String] {
return source.componentsSeparatedByString(separator)
}
return inner
}
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func separateBy(separator:String) -> ((String) -> [String]) {
func inner(source:String) -> [String] {
return source.componentsSeparatedByString(separator)
}
return inner
}
let lineSeparator = separateBy("\n")
let commaSeparator = separateBy(",")
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func separateBy(separator:String)(source:String) -> [String] {
return source.componentsSeparatedByString(separator)
}
let lineSeparator = separateBy("\n")
let commaSeparator = separateBy(",")
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let input = "Kyle,Maxine\nAndré,Katie,Ash"
map(lineSeparator(input), commaSeparator)
// [["Kyle", "Maxine"], ["André", "Katie", "Ash"]]
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Why are map and reduce
better?
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Declarative
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Immutability
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Don’t iterate over arrays
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Don’t iterate over arrays
4 Use map
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Don’t iterate over arrays
4 Use map
4 Use reduce
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Write declaratively
Not imperatively