mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-11-22 21:52:31 +03:00
652 lines
21 KiB
Markdown
652 lines
21 KiB
Markdown
---
|
|
language: F#
|
|
contributors:
|
|
- ["Scott Wlaschin", "http://fsharpforfunandprofit.com/"]
|
|
filename: learnfsharp.fs
|
|
---
|
|
|
|
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 reads more like a dynamic language.
|
|
|
|
The syntax of F# is different from C-style languages:
|
|
|
|
* 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 [https://try.fsharp.org](https://try.fsharp.org) 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
|
|
|
|
// Mutable variables
|
|
let mutable a=3
|
|
a <- 4 // a is now 4.
|
|
|
|
// Somewhat mutable variables
|
|
// Reference cells are storage locations that enable you to create mutable values with reference semantics.
|
|
// See https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/reference-cells
|
|
let xRef = ref 10
|
|
printfn "%d" xRef.Value // 10
|
|
xRef.Value <- 11
|
|
printfn "%d" xRef.Value // 11
|
|
|
|
let a=[ref 0; ref 1] // somewhat mutable list
|
|
a[0].Value <- 2
|
|
|
|
// ------ 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. Note
|
|
// that equality operator is single char "=".
|
|
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 easily to make powerful
|
|
// constructs
|
|
|
|
// Modules are used to group functions together
|
|
// Indentation is needed for each nested module.
|
|
module FunctionExamples =
|
|
|
|
// 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 to "bake in" parameters
|
|
let add42 = add 42
|
|
let b = add42 1
|
|
printfn "42 + 1 = %i" b
|
|
|
|
// composition to combine functions
|
|
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
|
|
|
|
// ================================================
|
|
// 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]
|
|
|
|
// A prime number generator
|
|
// - this is using a short notation for the pattern matching syntax
|
|
// - (p::xs) is 'first :: tail' of the list, could also be written as p :: xs
|
|
// this means this matches 'p' (the first item in the list), and xs is the rest of the list
|
|
// this is called the 'cons pattern'
|
|
// - uses 'rec' keyword, which is necessary when using recursion
|
|
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
|
|
| first :: _ -> printfn "the list has more than two elements, first element %A" first
|
|
|
|
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
|
|
// ------------------------------------
|
|
|
|
// Use "type" with curly braces to define a record type
|
|
type Person = {First:string; Last:string}
|
|
|
|
// 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 one 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
|
|
|
|
// ------------------------------------
|
|
// Modeling with types
|
|
// ------------------------------------
|
|
|
|
// Union types are great for modeling 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 -> () // don't send
|
|
|
|
// 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
|
|
|
|
// ------------------------------------
|
|
// 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
|
|
// ================================================
|
|
|
|
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
|
|
|
|
// You can use "elif" instead of "else if" in conditional expressions.
|
|
// They are equivalent in F#
|
|
|
|
// for example, define an "active" pattern to match character types...
|
|
let (|Digit|Letter|Whitespace|Other|) ch =
|
|
if System.Char.IsDigit(ch) then Digit
|
|
elif System.Char.IsLetter(ch) then Letter
|
|
elif 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
|
|
|
|
// -----------------------------------
|
|
// FizzBuzz using active patterns
|
|
// -----------------------------------
|
|
|
|
// You can create partial matching patterns as well
|
|
// Just use underscore in the definition, 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
|
|
// ================================================
|
|
|
|
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 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 { // "async" keyword and curly braces
|
|
// creates an "async" object
|
|
let req = WebRequest.Create(Uri(url))
|
|
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
|
|
}
|
|
|
|
// 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 compatibility
|
|
// ================================================
|
|
|
|
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 with generic type
|
|
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 my [why use F#](http://fsharpforfunandprofit.com/why-use-fsharp/) series.
|
|
|
|
Read more about F# at [fsharp.org](http://fsharp.org/) and [dotnet's F# page](https://dotnet.microsoft.com/languages/fsharp).
|