2013-06-28 23:26:34 +00:00
---
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.
2013-06-29 03:53:43 +00:00
```csharp
2013-06-28 23:26:34 +00:00
// 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/ ).
