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diff --git a/fsharp.html.markdown b/fsharp.html.markdown new file mode 100644 index 00000000..69c70bc6 --- /dev/null +++ b/fsharp.html.markdown @@ -0,0 +1,431 @@ +--- +language: F# +author: Scott Wlaschin +author_url: http://fsharpforfunandprofit.com/ +--- + +F# is a general purpose functional/OO programming language. It's free and open source, and runs on Linux, Mac, Windows and more. + +It has a powerful type system that traps many errors at compile time, but it uses type inference so that it read more like a dynamic language. + +The syntax of F# is similar to Python: + +* Curly braces are not used to delimit blocks of code. Instead, indentation is used. +* Whitespace is used to separate parameters rather than commas. + +If you want to try out the code below, you can go to [tryfsharp.org](http://www.tryfsharp.org/Create) and paste it into an interactive REPL. + +```fsharp + +// single line comments use a double slash +(* multi line comments use (* . . . *) pair + +-end of multi line comment- *) + +// ================================================ +// Basic Syntax +// ================================================ + +// ------ "Variables" (but not really) ------ +// The "let" keyword defines an (immutable) value +let myInt = 5 +let myFloat = 3.14 +let myString = "hello" //note that no types needed + +// ------ Lists ------ +let twoToFive = [2;3;4;5] // Square brackets create a list with + // semicolon delimiters. +let oneToFive = 1 :: twoToFive // :: creates list with new 1st element +// The result is [1;2;3;4;5] +let zeroToFive = [0;1] @ twoToFive // @ concats two lists + +// IMPORTANT: commas are never used as delimiters, only semicolons! + +// ------ Functions ------ +// The "let" keyword also defines a named function. +let square x = x * x // Note that no parens are used. +square 3 // Now run the function. Again, no parens. + +let add x y = x + y // don't use add (x,y)! It means something + // completely different. +add 2 3 // Now run the function. + +// to define a multiline function, just use indents. No semicolons needed. +let evens list = + let isEven x = x%2 = 0 // Define "isEven" as a sub function + List.filter isEven list // List.filter is a library function + // with two parameters: a boolean function + // and a list to work on + +evens oneToFive // Now run the function + +// You can use parens to clarify precedence. In this example, +// do "map" first, with two args, then do "sum" on the result. +// Without the parens, "List.map" would be passed as an arg to List.sum +let sumOfSquaresTo100 = + List.sum ( List.map square [1..100] ) + +// You can pipe the output of one operation to the next using "|>" +// Piping data around is very common in F#, similar to UNIX pipes. + +// Here is the same sumOfSquares function written using pipes +let sumOfSquaresTo100piped = + [1..100] |> List.map square |> List.sum // "square" was defined earlier + +// you can define lambdas (anonymous functions) using the "fun" keyword +let sumOfSquaresTo100withFun = + [1..100] |> List.map (fun x -> x*x) |> List.sum + +// In F# there is no "return" keyword. A function always +// returns the value of the last expression used. + +// ------ Pattern Matching ------ +// Match..with.. is a supercharged case/switch statement. +let simplePatternMatch = + let x = "a" + match x with + | "a" -> printfn "x is a" + | "b" -> printfn "x is b" + | _ -> printfn "x is something else" // underscore matches anything + +// F# doesn't allow nulls by default -- you must use an Option type +// and then pattern match. +// Some(..) and None are roughly analogous to Nullable wrappers +let validValue = Some(99) +let invalidValue = None + +// In this example, match..with matches the "Some" and the "None", +// and also unpacks the value in the "Some" at the same time. +let optionPatternMatch input = + match input with + | Some i -> printfn "input is an int=%d" i + | None -> printfn "input is missing" + +optionPatternMatch validValue +optionPatternMatch invalidValue + +// ------ Printing ------ +// The printf/printfn functions are similar to the +// Console.Write/WriteLine functions in C#. +printfn "Printing an int %i, a float %f, a bool %b" 1 2.0 true +printfn "A string %s, and something generic %A" "hello" [1;2;3;4] + +// There are also sprintf/sprintfn functions for formatting data +// into a string, similar to String.Format in C#. + +// ================================================ +// More on functions +// ================================================ + +// F# is a true functional language -- functions are first +// class entities and can be combined easy to make powerful +// constructs + +// Modules are used to group functions together +// Indentation is needed for each nested module. +module Addition = + + // define a simple adding function + let add x y = x + y + + // basic usage of a function + let a = add 1 2 + printfn "1+2 = %i" a + + // partial application + let add42 = add 42 + let b = add42 1 + printfn "42+1 = %i" b + + // composition + let add1 = add 1 + let add2 = add 2 + let add3 = add1 >> add2 + let c = add3 7 + printfn "3+7 = %i" c + + // higher order functions + [1..10] |> List.map add3 |> printfn "new list is %A" + + // lists of functions, and more + let add6 = [add1; add2; add3] |> List.reduce (>>) + let d = add6 7 + printfn "1+2+3+7 = %i" d + +// ================================================ +// Data Types +// ================================================ + + +module DataTypeExamples = + + // All data is immutable by default + + // Tuples are quick 'n easy anonymous types + let twoTuple = 1,2 + let threeTuple = "a",2,true + + // Record types have named fields + type Person = {First:string; Last:string} + let person1 = {First="john"; Last="Doe"} + + // Union types (aka variants) have a set of choices + // Only case can be valid at a time. + type Temp = + | DegreesC of float + | DegreesF of float + let temp1 = DegreesF 98.6 + let temp2 = DegreesC 37.0 + + // Union types are great for modelling state without using flags + type EmailAddress = + | ValidEmailAddress of string + | InvalidEmailAddress of string + + let trySendEmail email = + match email with // use pattern matching + | ValidEmailAddress address -> () // send + | InvalidEmailAddress address -> () // dont send + + // Types can be combined recursively in complex ways + // without having to create subclasses + type Employee = + | Worker of Person + | Manager of Employee list + + let jdoe = {First="John";Last="Doe"} + let worker = Worker jdoe + + // The combination of union types and record types together + // provide a great foundation for domain driven design. + // You can create hundreds of little types that accurately + // reflect the domain. + + type CartItem = { ProductCode: string; Qty: int } + type Payment = Payment of float + type ActiveCartData = { UnpaidItems: CartItem list } + type PaidCartData = { PaidItems: CartItem list; Payment: Payment} + + type ShoppingCart = + | EmptyCart // no data + | ActiveCart of ActiveCartData + | PaidCart of PaidCartData + + // All complex types have pretty printing built in for free + printfn "twoTuple=%A,\nPerson=%A,\nTemp=%A,\nEmployee=%A" + twoTuple person1 temp1 worker + +// ================================================ +// Active patterns +// ================================================ + +module ActivePatternExamples = + + // F# has a special type of pattern matching called "active patterns" + // where the pattern can be parsed or detected dynamically. + + // for example, define an "active" pattern to match character types... + let (|Digit|Letter|Whitespace|Other|) ch = + if System.Char.IsDigit(ch) then Digit + else if System.Char.IsLetter(ch) then Letter + else if System.Char.IsWhiteSpace(ch) then Whitespace + else Other + + // ... and then use it to make parsing logic much clearer + let printChar ch = + match ch with + | Digit -> printfn "%c is a Digit" ch + | Letter -> printfn "%c is a Letter" ch + | Whitespace -> printfn "%c is a Whitespace" ch + | _ -> printfn "%c is something else" ch + + // print a list + ['a';'b';'1';' ';'-';'c'] |> List.iter printChar + + +// ================================================ +// Conciseness +// ================================================ + +module AlgorithmExamples = + + // F# has a high signal/noise ratio, so code reads + // almost like the actual algorithm + + // ------ Example: define sumOfSquares function ------ + let sumOfSquares n = + [1..n] // 1) take all the numbers from 1 to n + |> List.map square // 2) square each one + |> List.sum // 3) sum the results + + // test + sumOfSquares 100 |> printfn "Sum of squares = %A" + + // ------ Example: define a sort function ------ + let rec sort list = + match list with + // If the list is empty + | [] -> + [] // return an empty list + // If the list is not empty + | firstElem::otherElements -> // take the first element + let smallerElements = // extract the smaller elements + otherElements // from the remaining ones + |> List.filter (fun e -> e < firstElem) + |> sort // and sort them + let largerElements = // extract the larger ones + otherElements // from the remaining ones + |> List.filter (fun e -> e >= firstElem) + |> sort // and sort them + // Combine the 3 parts into a new list and return it + List.concat [smallerElements; [firstElem]; largerElements] + + // test + sort [1;5;23;18;9;1;3] |> printfn "Sorted = %A" + +// ================================================ +// Asynchronous Code +// ================================================ + +module AsyncExample = + + // F# has some built-in features to help with async code + // without encountering the "pyramid of doom" + // + // The following example downloads a set of web pages in parallel. + + open System.Net + open System + open System.IO + open Microsoft.FSharp.Control.CommonExtensions + + // Fetch the contents of a URL asynchronously + let fetchUrlAsync url = + async { + let req = WebRequest.Create(Uri(url)) + use! resp = req.AsyncGetResponse() + use stream = resp.GetResponseStream() + use reader = new IO.StreamReader(stream) + let html = reader.ReadToEnd() + printfn "finished downloading %s" url + } + + // a list of sites to fetch + let sites = ["http://www.bing.com"; + "http://www.google.com"; + "http://www.microsoft.com"; + "http://www.amazon.com"; + "http://www.yahoo.com"] + + // do it + sites + |> List.map fetchUrlAsync // make a list of async tasks + |> Async.Parallel // set up the tasks to run in parallel + |> Async.RunSynchronously // start them off + +// ================================================ +// .NET compatability +// ================================================ + +module NetCompatibilityExamples = + + // F# can do almost everything C# can do, and it integrates + // seamlessly with .NET or Mono libraries. + + // ------- work with existing library functions ------- + + let (i1success,i1) = System.Int32.TryParse("123"); + if i1success then printfn "parsed as %i" i1 else printfn "parse failed" + + // ------- Implement interfaces on the fly! ------- + + // create a new object that implements IDisposable + let makeResource name = + { new System.IDisposable + with member this.Dispose() = printfn "%s disposed" name } + + let useAndDisposeResources = + use r1 = makeResource "first resource" + printfn "using first resource" + for i in [1..3] do + let resourceName = sprintf "\tinner resource %d" i + use temp = makeResource resourceName + printfn "\tdo something with %s" resourceName + use r2 = makeResource "second resource" + printfn "using second resource" + printfn "done." + + // ------- Object oriented code ------- + + // F# is also a fully fledged OO language. + // It supports classes, inheritance, virtual methods, etc. + + // interface + type IEnumerator<'a> = + abstract member Current : 'a + abstract MoveNext : unit -> bool + + // abstract base class with virtual methods + [<AbstractClass>] + type Shape() = + //readonly properties + abstract member Width : int with get + abstract member Height : int with get + //non-virtual method + member this.BoundingArea = this.Height * this.Width + //virtual method with base implementation + abstract member Print : unit -> unit + default this.Print () = printfn "I'm a shape" + + // concrete class that inherits from base class and overrides + type Rectangle(x:int, y:int) = + inherit Shape() + override this.Width = x + override this.Height = y + override this.Print () = printfn "I'm a Rectangle" + + //test + let r = Rectangle(2,3) + printfn "The width is %i" r.Width + printfn "The area is %i" r.BoundingArea + r.Print() + + // ------- extension methods ------- + + //Just as in C#, F# can extend existing classes with extension methods. + type System.String with + member this.StartsWithA = this.StartsWith "A" + + //test + let s = "Alice" + printfn "'%s' starts with an 'A' = %A" s s.StartsWithA + + // ------- events ------- + + type MyButton() = + let clickEvent = new Event<_>() + + [<CLIEvent>] + member this.OnClick = clickEvent.Publish + + member this.TestEvent(arg) = + clickEvent.Trigger(this, arg) + + // test + let myButton = new MyButton() + myButton.OnClick.Add(fun (sender, arg) -> + printfn "Click event with arg=%O" arg) + + myButton.TestEvent("Hello World!") + +``` + +## More Information + +For more demonstrations of F#, go to the [Try F#](http://www.tryfsharp.org/Learn) site, or my [why use F#](http://fsharpforfunandprofit.com/why-use-fsharp/) series. + +Read more about F# at [fsharp.org](http://fsharp.org/). + + + + |