Functional programming with F#

Functional programming with F#

• 1.
  Functional programming with F# Remigiusz Koczapski http://blog.koczapski.com
• 2.
  Functional programming • Declarativeinstead imperative (express what do you want instead how to get it) • Evaluation of mathematical functions instead statements that can change state • Output value of a function depends on an input only. Function will always produce the same result • Easier to understand and more predictable behavior of the program
• 3.
  Rules • Higher-order functions- takes one or more functions as an argument or returns a function as a result • Pure functions (Idempotence) - have no side effect on memory or I/O • Recursion - functions invoke themselves • Strict (eager) or non-strict evaluation (lazy) - arguments are/are not evaluated if their values are not necessary to execute a function • Type systems - strong compile-time type checking • Referential transparency – assigned values are never changed
• 4.
  Functional Languages • Strict •Hope • Rex • … • Non-strict • Lisp (Clojure, Logo) • ML (OCaml) • F# • R • Mathematica • Wolfram Language • …
• 5.
  F# • Invented inMicrosoft Research • Developed by F# Software Foundation (with Microsoft’s support) • Open source • OCaml implementation for .NET platform • Functional-first language • Object Oriented Programming support • Interoperable with all .NET languages • Available for Windows, Linux and OSX
• 6.
  F# in VisualStudio
• 7.
  F# syntax let SayHello()= printf "Hello" [<EntryPoint>] let main argv = SayHello() System.Console.ReadKey() |> ignore 0
• 8.
  „let” identifier andfunctions • Binds identifier with value or a function • Identifier with immutable value • Simple one-line function • Function with overridden types • Function invocation let hi = "Hello" let Square x = x * x let Area (length : float) (height : float) : float = length * height SayHello() let rectArea = Area 2.1, 3.2
• 9.
  Type interface • Staticallytyped language • Type error checking at compilation step • Strong type deduction mechanism let ShowValue myVal = printfn "This value is: %f" myVal
• 10.
  Branching and looping •Branching – if instruction let Greeting name = if name = "" then "whoever you name" else name let PrintNumbers min max = for x in min..max do printf "%i " x • For-in loop • For-to/downto loop let ForToLoop() = for i = 1 to 10 do printf "%d " i • While-do loop let LookForValue value maxValue = let mutable continueLooping = true let generator = new Random() while continueLooping do let rand = generator.Next(maxValue) if rand = value then printfn "nFound a %d!" value continueLooping <- false
• 11.
  Arrays, lists andsequences • Arrays – fast, fixed size, with mutable elements • Lists – works like an array, with immutable elements let arr = [|1; 2; 3|] let squares = Array.init 10 (fun index -> index * index) let myList = [2; 3; 5; 4] arr.[1] <- 99 let even = List.filter (fun x -> x % 2 = 0) myList • Sequences – works like IEnumerable; variable size let smallNumbers = {0..99} let smallNumbers2 = Seq.init 100 (fun x -> x)
• 12.
  Forward-pipe operator |> •Gets previous’ instruction result as an argument of next instruction let PrintLongWords (words : string[]) = let longWords : string [] = Array.filter(fun w -> w.Length > 8) words let sortedLongWords = Array.sort longWords Array.iter(fun w -> printfn "%s" w) sortedLongWords let PrintLongWords2 (words: string[]) = words |> Array.filter (fun w -> w.Length > 8) |> Array.sort |> Array.iter (fun w -> printfn "%s" w)
• 13.
  Records and optiontypes • Records – lightweight containers for small group of values • option – creates nullable type type Company = { Name : string SalesTaxNumber : int option } let company = { Name = "HCL"; SalesTaxNumber = None } printfn "Company %s %i" company.Name company.SalesTaxNumber.Value let company2 = { Name = "UBS"; SalesTaxNumber = Some 12314 }
• 14.
  Discriminated Unions • DiscriminatedUnion – generalized types without object hierarchy type Shape = | Square of float | Rectangle of float * float | Circle of float let s = Square 3.4 let r = Rectangle (2.2, 1.9) let c = Circle (1.0) let drawning = [|s; r; c|]
• 15.
  Pattern matching • Allowsto compare value with conditions • Warning if not every case is covered • Knows how to extract fields from type let Area (shape : Shape) = match shape with | Square x -> x * x | Rectangle(h, w) -> h * w | Circle r -> System.Math.PI * r * r let total = drawning |> Array.sumBy Area let one = [|50|] let two = [|60; 61|] let many = [|0..99|] let Describe arr = match arr with | [|x|] -> sprintf "One elem: %i " x | [|x; y|] -> sprintf "Two elems: %i, %i" x y | _ -> sprintf "A longer array"
• 16.
  Immutability and shadowing •Values are immutable by default • „mutable” keyword creates mutable variable • „<-” operator allows to assign new value • Assigning new value using „let” and „=” will shadow previous value let ImmutabilityDemo2() = let x = 42 printfn "X: %i" x if 1 = 1 then let x = x + 1 printfn "x: %i" x printfn "X: %i" x > ImmutabilityDemo2();; X: 42 x: 43 X: 42 let MutabilityDemo() = let mutable x = 43 printfn "x: %i" x x <- x + 1 printfn "x: %i" x > MutabilityDemo();; x: 43 x: 44
• 17.
  Object Oriented Programming •Objects are immutable by default but can be mutable • Objects can have members, methods and constructors • Interfaces and inheritance is supported • Autoproperties (getters and setters) type Person(forename : string, surname : string) = member __.Forename = forename member __.Surname = surname let p1 = Person("John", "Deere") let forename = p1.Forename type MutablePerson2(forename : string, surname : string) = member val Forename = forename with get, set member val Surname = surname with get, set
• 18.
  Unit testing inF# [<TestCase("23:59:59", 23, 59, 59)>] let ``is time properly parsed``(time, parsedHour, parsedMinute, parsedSecond) = //act let parsedTime = Time.Parse(time) //assert Assert.That(parsedTime.Hour, Is.EqualTo(parsedHour)) Assert.That(parsedTime.Minute, Is.EqualTo(parsedMinute)) Assert.That(parsedTime.Second, Is.EqualTo(parsedSecond))
• 19.
  More? • http://fsharp.org/ -F# foundation site with tutorials • https://fsharpforfunandprofit.com/ - F# in practice • https://app.pluralsight.com/library/courses/fsharp-jumpstart/table-of- contents - basics video tutorial • https://app.pluralsight.com/library/courses/functional-architecture- fsharp/table-of-contents - more practical tutorial • Examples taken from „F# Jumpstart” by Kit Eason – watch here