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Functional Swift
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Chris Eidhof | @chriseidhof
August 07, 2014
Technology
1
140
Functional Swift
Łódź wiOSłuje - August, 2014
Chris Eidhof | @chriseidhof
August 07, 2014
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Transcript
Functional Swift @chriseidhof Łódź wiOSłuje - August, 2014
What's Functional Programming? • Pure • Referentially transparent • Typed
Our data set let cities : [String:Int] = [ "Warszawa":
1706624 , "Kraków": 766583 , "Łódź": 753192 , "Wrocław": 632930 , "Poznań": 567932 ] let names = Array(cities.keys) let populations = Array(cities.values)
Names > [Poznań, Warszawa, Wrocław, Kraków, Łódź]
Populations > [567932, 1706624, 632930, 766583, 753192]
Map func addCity(s: String) -> String { return s +
" is a city" } names.map(addCity) > [Poznań is a city, Warszawa is a city, Wrocław is a city, Kraków is a city, Łódź is a city]
Filter func isLodz(s: String) -> Bool { return s ==
"Łódź" } names.filter(isLodz) > [Łódź]
Filter, simplified names.filter({ (s: String) -> Bool in return s
== "Łódź" }) > [Łódź]
Filter, more simplified names.filter({ s in return s == "Łódź"
}) > [Łódź]
Filter, even more simplified names.filter({ return $0 == "Łódź" })
> [Łódź]
Filter, simplest names.filter { $0 == "Łódź" } > [Łódź]
populations.filter { $0 > 1000000 } > [1706624]
Sum of an array func sum(arr: [Int]) -> Int {
var result = 0 for i in arr { result += i } return result } sum(Array(1..<10)) > 45
Product of an array func product(arr: [Int]) -> Int {
var result = 1 for i in arr { result *= i } return result } product(Array(1..<10)) > 362880
Reduce func reduce(initialValue: Int, combine: (Int,Int) -> Int, arr: [Int])
-> Int { var result = initialValue for i in arr { result = combine(result,i) } return result }
Reduce reduce(0, +, Array(1..<10)) > 45 reduce(1, *, Array(1..<10)) >
362880
Sum and Product let sum = { reduce(0,+,$0) } let
product = { reduce(1,*,$0) }
Concatenate func concat(strings: [String]) -> String { var result =
"" for x in strings { result += x } return result } concat(names) > PoznańWarszawaWrocławKrakówŁódź
Generics func reduce<A>(initialValue: A, combine: (A,A) -> A, arr: [A])
-> A { var result = initialValue for i in arr { result = combine(result,i) } return result } reduce("", +, names) > PoznańWarszawaWrocławKrakówŁódź
Adding line-breaks reduce("", { $0 + "\n" + $1 },
names) > Poznań > Warszawa > Wrocław > Kraków > Łódź
Making reduce more generic func reduce<A,R>(initialValue: R, combine: (R,A) ->
R, arr: [A]) -> R { var result = initialValue for i in arr { result = combine(result,i) } return result }
Example: Core Image
The Objective-C way CIFilter *hueAdjust = [CIFilter filterWithName:@"CIHueAdjust"]; [hueAdjust setDefaults];
[hueAdjust setValue: myCIImage forKey: kCIInputImageKey]; [hueAdjust setValue: @2.094f forKey: kCIInputAngleKey];
A Swift Filter typealias Filter = CIImage -> CIImage
Blur func blur(radius: Double) -> Filter { return { image
in let parameters : Parameters = [kCIInputRadiusKey: radius, kCIInputImageKey: image] let filter = CIFilter(name:"CIGaussianBlur", parameters:parameters) return filter.outputImage } }
Example let url = NSURL(string: "http://bit.ly/1pabRsM"); let image = CIImage(contentsOfURL:
url) let blurBy5 = blur(5) let blurred = blurBy5(image)
None
Color Generator func colorGenerator(color: NSColor) -> Filter { return {
_ in let filter = CIFilter(name:"CIConstantColorGenerator", parameters: [kCIInputColorKey: color]) return filter.outputImage } }
Composite Source Over func compositeSourceOver(overlay: CIImage) -> Filter { return
{ image in let parameters : Parameters = [kCIInputBackgroundImageKey: image, kCIInputImageKey: overlay] let filter = CIFilter(name:"CISourceOverCompositing", parameters: parameters) return filter.outputImage.imageByCroppingToRect(image.extent()) } }
Color Overlay func colorOverlay(color: NSColor) -> Filter { return {
image in let overlay = colorGenerator(color)(image) return compositeSourceOver(overlay)(image) } }
Combining everything let blurRadius = 5.0 let overlayColor = NSColor.redColor().colorWithAlphaComponent(0.2)
let blurredImage = blur(blurRadius)(image) let overlaidImage = colorOverlay(overlayColor)(blurredImage)
None
Combining everything, take 2 let result = colorOverlay(overlayColor)(blur(blurRadius)(image))
Filter composition func composeFilters(filter1: Filter, filter2: Filter) -> Filter {
return {img in filter1(filter2(img)) } }
Using filter composition let myFilter1 = composeFilters(blur(blurRadius), colorOverlay(overlayColor)) let result1
= myFilter1(image)
Filter composition with an operator infix operator |> { associativity
left } func |> (filter1: Filter, filter2: Filter) -> Filter { return {img in filter1(filter2(img))} }
Using filter composition let myFilter2 = blur(blurRadius) |> colorOverlay(overlayColor) let
result2 = myFilter2(image)
Function composition func |> (f1: B -> C, f2: A
-> B) -> A -> C { return {x in f1(f2(x))} }
Example: Spreadsheet
None
None
None
Expressions enum Expression { case Number(Int) // e.g. 10 case
Reference(String,Int) // A0 case BinaryExpression(String,Expression,Expression) // 1 + A9 case FunctionCall(String,Expression) // SUM(...) }
Parsing references let reference = { Token.Reference($0,$1) } </> capital
<*> naturalNumber
Parsing expressions prim = numberOrReference <|> functionCall <|> parens(expression)
Parsing expressions let operators : [[String]] = [ [":"] ,
["*", "/"] , ["+", "-"] ] let expression = pack(operators, prim)
Parsing results We can now convert this: parse(expression, "SUM(A1:A9)") into
this: Expression.FunctionCall("SUM", Expression.BinaryExpression( ":", Expression.Reference("A",1), Expression.Reference("A",9) ) )
Evaluating expressions
The result enum enum Result { case IntResult(Int) case StringResult(String)
case ListResult([Result]) case EvaluationError(String) }
The evaluation function func evaluate(expressions: [Expression?]) -> [Result] { return
expressions.map(evaluateExpression(expressions)) }
Evaluating an expression evaluateExpression([42,10*10,A0+A1])('A1') > 100 evaluateExpression([42,10*10,A0+A1])('A2') > 142
Evaluating an expression func evaluateExpression(context: [Expression?]) -> Expression? -> Result
{ return {e in e.map { expression in let compute = evaluateExpression(context) switch (expression) { case .Number(let x): return Result.IntResult(x) case .Reference("A", let idx): return compute(context[idx]) case .BinaryExpression(let s, let l, let r): return evaluateBinary(s, compute, l, r) case .FunctionCall(let f, let p): return evaluateFunction(f, compute(p)) default: return .EvaluationError("Couldn't evaluate expression") } } ?? .EvaluationError("Couldn't parse expression") } }
Mixing FP and OO class SpreadsheetDatasource : NSObject, NSTableViewDataSource, EditedRow
Mixing FP and OO var arr: [String] func tableView(aTableView: NSTableView,
objectValueForTableColumn: NSTableColumn, row: Int) -> AnyObject { return editedRow == row ? arr[row] : results[row] }
Mixing FP and OO func calculateExpressions() { let expressions: [Expression?]
= arr.map { if let tokens = parse(tokenize(), $0) { return parse(expression(), tokens) } return nil } results = evaluate(expressions) }
Conclusion FP is a massively powerful tool in your toolbox.
Use it together with OO, and build awesome stuff.
By Chris Eidhof, Florian Kugler and Wouter Swierstra
@chriseidhof