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Updated F# code with new list and collection examples, and extra examples for data types.

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Scott Wlaschin 2013-06-29 15:54:14 +01:00
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@ -6,11 +6,11 @@ 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.
It has a powerful type system that traps many errors at compile time, but it uses type inference so that it reads more like a dynamic language.
The syntax of F# is similar to Python:
The syntax of F# is different from C-style languages:
* Curly braces are not used to delimit blocks of code. Instead, indentation is used.
* Curly braces are not used to delimit blocks of code. Instead, indentation is used (like Python).
* 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.
@ -123,7 +123,7 @@ printfn "A string %s, and something generic %A" "hello" [1;2;3;4]
// Modules are used to group functions together
// Indentation is needed for each nested module.
module Addition =
module FunctionExamples =
// define a simple adding function
let add x y = x + y
@ -132,12 +132,12 @@ module Addition =
let a = add 1 2
printfn "1+2 = %i" a
// partial application
// partial application to "bake in" parameters
let add42 = add 42
let b = add42 1
printfn "42+1 = %i" b
// composition
// composition to combine functions
let add1 = add 1
let add2 = add 2
let add3 = add1 >> add2
@ -153,30 +153,190 @@ module Addition =
printfn "1+2+3+7 = %i" d
// ================================================
// Data Types
// Lists and collection
// ================================================
// There are three types of ordered collection:
// * Lists are most basic immutable collection.
// * Arrays are mutable and more efficient when needed.
// * Sequences are lazy and infinite (e.g. an enumerator).
//
// Other collections include immutable maps and sets
// plus all the standard .NET collections
module ListExamples =
// lists use square brackets
let list1 = ["a";"b"]
let list2 = "c" :: list1 // :: is prepending
let list3 = list1 @ list2 // @ is concat
// list comprehensions (aka generators)
let squares = [for i in 1..10 do yield i*i]
// prime number generator
let rec sieve = function
| (p::xs) -> p :: sieve [ for x in xs do if x % p > 0 then yield x ]
| [] -> []
let primes = sieve [2..50]
printfn "%A" primes
// pattern matching for lists
let listMatcher aList =
match aList with
| [] -> printfn "the list is empty"
| [first] -> printfn "the list has one element %A " first
| [first; second] -> printfn "list is %A and %A" first second
| _ -> printfn "the list has more than two elements"
listMatcher [1;2;3;4]
listMatcher [1;2]
listMatcher [1]
listMatcher []
// recursion using lists
let rec sum aList =
match aList with
| [] -> 0
| x::xs -> x + sum xs
sum [1..10]
// -----------------------------------------
// Standard library functions
// -----------------------------------------
// map
let add3 x = x + 3
[1..10] |> List.map add3
// filter
let even x = x % 2 = 0
[1..10] |> List.filter even
// many more -- see documentation
module ArrayExamples =
// arrays use square brackets with bar
let array1 = [| "a";"b" |]
let first = array1.[0] // indexed access using dot
// pattern matching for arrays is same as for lists
let arrayMatcher aList =
match aList with
| [| |] -> printfn "the array is empty"
| [| first |] -> printfn "the array has one element %A " first
| [| first; second |] -> printfn "array is %A and %A" first second
| _ -> printfn "the array has more than two elements"
arrayMatcher [| 1;2;3;4 |]
// Standard library functions just as for List
[| 1..10 |]
|> Array.map (fun i -> i+3)
|> Array.filter (fun i -> i%2 = 0)
|> Array.iter (printfn "value is %i. ")
module SequenceExamples =
// sequences use curly braces
let seq1 = seq { yield "a"; yield "b" }
// sequences can use yield and
// can contain subsequences
let strange = seq {
// "yield! adds one element
yield 1; yield 2;
// "yield!" adds a whole subsequence
yield! [5..10]
yield! seq {
for i in 1..10 do
if i%2 = 0 then yield i }}
// test
strange |> Seq.toList
// Sequences can be created using "unfold"
// Here's the fibonacci series
let fib = Seq.unfold (fun (fst,snd) ->
Some(fst + snd, (snd, fst + snd))) (0,1)
// test
let fib10 = fib |> Seq.take 10 |> Seq.toList
printf "first 10 fibs are %A" fib10
// ================================================
// Data Types
// ================================================
module DataTypeExamples =
// All data is immutable by default
// Tuples are quick 'n easy anonymous types
// -- Use a comma to create a tuple
let twoTuple = 1,2
let threeTuple = "a",2,true
// Pattern match to unpack
let x,y = twoTuple //sets x=1 y=2
// Record types have named fields
// ------------------------------------
// Record types have named fields
// ------------------------------------
// Use "type" with curly braces to define a record type
type Person = {First:string; Last:string}
let person1 = {First="john"; Last="Doe"}
// Use "let" with curly braces to create a record
let person1 = {First="John"; Last="Doe"}
// Pattern match to unpack
let {First=first} = person1 //sets first="john"
// ------------------------------------
// Union types (aka variants) have a set of choices
// Only case can be valid at a time.
// ------------------------------------
// Use "type" with bar/pipe to define a union type
type Temp =
| DegreesC of float
| DegreesF of float
// Use one of the cases to create one
let temp1 = DegreesF 98.6
let temp2 = DegreesC 37.0
// Pattern match on all cases to unpack
let printTemp = function
| DegreesC t -> printfn "%f degC" t
| DegreesF t -> printfn "%f degF" t
printTemp temp1
printTemp temp2
// ------------------------------------
// Recursive types
// ------------------------------------
// 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
// ------------------------------------
// Modelling with types
// ------------------------------------
// Union types are great for modelling state without using flags
type EmailAddress =
| ValidEmailAddress of string
@ -186,15 +346,6 @@ module DataTypeExamples =
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.
@ -211,10 +362,35 @@ module DataTypeExamples =
| ActiveCart of ActiveCartData
| PaidCart of PaidCartData
// All complex types have pretty printing built in for free
// ------------------------------------
// Built in behavior for types
// ------------------------------------
// Core types have useful "out-of-the-box" behavior, no coding needed.
// * Immutability
// * Pretty printing when debugging
// * Equality and comparison
// * Serialization
// Pretty printing using %A
printfn "twoTuple=%A,\nPerson=%A,\nTemp=%A,\nEmployee=%A"
twoTuple person1 temp1 worker
// Equality and comparison built in.
// Here's an example with cards.
type Suit = Club | Diamond | Spade | Heart
type Rank = Two | Three | Four | Five | Six | Seven | Eight
| Nine | Ten | Jack | Queen | King | Ace
let hand = [ Club,Ace; Heart,Three; Heart,Ace;
Spade,Jack; Diamond,Two; Diamond,Ace ]
// sorting
List.sort hand |> printfn "sorted hand is (low to high) %A"
List.max hand |> printfn "high card is %A"
List.min hand |> printfn "low card is %A"
// ================================================
// Active patterns
// ================================================
@ -224,6 +400,8 @@ module ActivePatternExamples =
// F# has a special type of pattern matching called "active patterns"
// where the pattern can be parsed or detected dynamically.
// "banana clips" are the syntax for active patterns
// for example, define an "active" pattern to match character types...
let (|Digit|Letter|Whitespace|Other|) ch =
if System.Char.IsDigit(ch) then Digit
@ -242,7 +420,26 @@ module ActivePatternExamples =
// print a list
['a';'b';'1';' ';'-';'c'] |> List.iter printChar
// -----------------------------------
// FizzBuzz using active patterns
// -----------------------------------
// You can create partial matching patterns as well
// Just use undercore in the defintion, and return Some if matched.
let (|MultOf3|_|) i = if i % 3 = 0 then Some MultOf3 else None
let (|MultOf5|_|) i = if i % 5 = 0 then Some MultOf5 else None
// the main function
let fizzBuzz i =
match i with
| MultOf3 & MultOf5 -> printf "FizzBuzz, "
| MultOf3 -> printf "Fizz, "
| MultOf5 -> printf "Buzz, "
| _ -> printf "%i, " i
// test
[1..20] |> List.iter fizzBuzz
// ================================================
// Conciseness
// ================================================
@ -289,7 +486,7 @@ module AlgorithmExamples =
module AsyncExample =
// F# has some built-in features to help with async code
// F# has 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.
@ -301,10 +498,14 @@ module AsyncExample =
// Fetch the contents of a URL asynchronously
let fetchUrlAsync url =
async {
async { // "async" keyword and curly braces
// creates an "async" object
let req = WebRequest.Create(Uri(url))
use! resp = req.AsyncGetResponse()
use! resp = req.AsyncGetResponse()
// use! is async assignment
use stream = resp.GetResponseStream()
// "use" triggers automatic close()
// on resource at end of scope
use reader = new IO.StreamReader(stream)
let html = reader.ReadToEnd()
printfn "finished downloading %s" url
@ -360,7 +561,7 @@ module NetCompatibilityExamples =
// F# is also a fully fledged OO language.
// It supports classes, inheritance, virtual methods, etc.
// interface
// interface with generic type
type IEnumerator<'a> =
abstract member Current : 'a
abstract MoveNext : unit -> bool