F# Tutorial - Sample Code - 讓初學者了解F#第一步
// F# Tutorial File
//
// This file contains sample code to guide you through the
// primitives of the F# language.
//
// Learn more about F# at http://fsharp.net
//
// For a larger collection of F# samples, see:
// http://go.microsoft.com/fwlink/?LinkID=124614
//
// Contents:
// - Simple computations
// - Functions on integers
// - Tuples
// - Booleans
// - Strings
// - Lists
// - Arrays
// - More Collections
// - Functions
// - Types: unions
// - Types: records
// - Types: classes
// - Types: interfaces
// - Types: classes with interface implementations
// - Printing
// open some standard namespaces
open System
// Simple computations
// ---------------------------------------------------------------
// Here are some simple computations. Note how code can be documented
// with '///' comments. Hover over any reference to a variable to
// see its documentation.
/// A very simple constant integer
let int1 = 1
/// A second very simple constant integer
let int2 = 2
/// Add two integers
let int3 = int1 + int2
// Functions on integers
// ---------------------------------------------------------------
/// A function on integers
let f x = 2*x*x - 5*x + 3
/// The result of a simple computation
let result = f (int3 + 4)
/// Another function on integers
let increment x = x + 1
/// Compute the factorial of an integer
let rec factorial n = if n=0 then 1 else n * factorial (n-1)
/// Compute the highest-common-factor of two integers
let rec hcf a b = // notice: 2 parameters separated by spaces
if a=0 then b
elif a<b then hcf a (b-a) // notice: 2 arguments separated by spaces
else hcf (a-b) b
// note: function arguments are usually space separated
// note: 'let rec' defines a recursive function
// Tuples
// ---------------------------------------------------------------
// A simple tuple of integers
let pointA = (1, 2, 3)
// A simple tuple of an integer, a string and a double-precision floating point number
let dataB = (1, "fred", 3.1415)
/// A function that swaps the order of two values in a tuple
let Swap (a, b) = (b, a)
// Booleans
// ---------------------------------------------------------------
/// A simple boolean value
let boolean1 = true
/// A second simple boolean value
let boolean2 = false
/// Compute a new boolean using ands, ors, and nots
let boolean3 = not boolean1 && (boolean2 || false)
// Strings
// ---------------------------------------------------------------
/// A simple string
let stringA = "Hello"
/// A second simple string
let stringB = "world"
/// "Hello world" computed using string concatentation
let stringC = stringA + " " + stringB
/// "Hello world" computed using a .NET library function
let stringD = String.Join(" ",[| stringA; stringB |])
// Try re-typing the above line to see intellisense in action
// Note, ctrl-J on (partial) identifiers re-activates it
// Functional Lists
// ---------------------------------------------------------------
/// The empty list
let listA = [ ]
/// A list with 3 integers
let listB = [ 1; 2; 3 ]
/// A list with 3 integers, note :: is the 'cons' operation
let listC = 1 :: [2; 3]
/// Compute the sum of a list of integers using a recursive function
let rec SumList xs =
match xs with
| [] -> 0
| y::ys -> y + SumList ys
/// Sum of a list
let listD = SumList [1; 2; 3]
/// The list of integers between 1 and 10 inclusive
let oneToTen = [1..10]
/// The squares of the first 10 integers
let squaresOfOneToTen = [ for x in 0..10 -> x*x ]
// Mutable Arrays
// ---------------------------------------------------------------
/// Create an array
let arr = Array.create 4 "hello"
arr.[1] <- "world"
arr.[3] <- "don"
/// Compute the length of the array by using an instance method on the array object
let arrLength = arr.Length
// Extract a sub-array using slicing notation
let front = arr.[0..2]
// More Collections
// ---------------------------------------------------------------
/// A dictionary with integer keys and string values
let lookupTable = dict [ (1, "One"); (2, "Two") ]
let oneString = lookupTable.[1]
// For some other common data structures, see:
// System.Collections.Generic
// Microsoft.FSharp.Collections
// Microsoft.FSharp.Collections.Seq
// Microsoft.FSharp.Collections.Set
// Microsoft.FSharp.Collections.Map
// Functions
// ---------------------------------------------------------------
/// A function that squares its input
let Square x = x*x
// Map a function across a list of values
let squares1 = List.map Square [1; 2; 3; 4]
let squares2 = List.map (fun x -> x*x) [1; 2; 3; 4]
// Pipelines
let squares3 = [1; 2; 3; 4] |> List.map (fun x -> x*x)
let SumOfSquaresUpTo n =
[1..n]
|> List.map Square
|> List.sum
// Types: unions
// ---------------------------------------------------------------
type Expr =
| Num of int
| Add of Expr * Expr
| Mul of Expr * Expr
| Var of string
let rec Evaluate (env:Map<string,int>) exp =
match exp with
| Num n -> n
| Add (x,y) -> Evaluate env x + Evaluate env y
| Mul (x,y) -> Evaluate env x * Evaluate env y
| Var id -> env.[id]
let envA = Map.of_list [ "a",1 ;
"b",2 ;
"c",3 ]
let expT1 = Add(Var "a",Mul(Num 2,Var "b"))
let resT1 = Evaluate envA expT1
// Types: records
// ---------------------------------------------------------------
type Card = { Name : string;
Phone : string;
Ok : bool }
let cardA = { Name = "Alf" ; Phone = "(206) 555-8257" ; Ok = false }
let cardB = { cardA with Phone = "(206) 555-4112"; Ok = true }
let ShowCard c =
c.Name + " Phone: " + c.Phone + (if not c.Ok then " (unchecked)" else "")
// Types: classes
// ---------------------------------------------------------------
/// A 2-dimensional vector
type Vector2D(dx:float, dy:float) =
// The pre-computed length of the vector
let length = sqrt(dx*dx + dy*dy)
/// The displacement along the X-axis
member v.DX = dx
/// The displacement along the Y-axis
member v.DY = dy
/// The length of the vector
member v.Length = length
// Re-scale the vector by a constant
member v.Scale(k) = Vector2D(k*dx, k*dy)
// Types: interfaces
// ---------------------------------------------------------------
type IPeekPoke =
abstract Peek: unit -> int
abstract Poke: int -> unit
// Types: classes with interface implementations
// ---------------------------------------------------------------
/// A widget which counts the number of times it is poked
type Widget(initialState:int) =
/// The internal state of the Widget
let mutable state = initialState
// Implement the IPeekPoke interface
interface IPeekPoke with
member x.Poke(n) = state <- state + n
member x.Peek() = state
/// Has the Widget been poked?
member x.HasBeenPoked = (state <> 0)
let widget = Widget(12) :> IPeekPoke
widget.Poke(4)
let peekResult = widget.Peek()
// Printing
// ---------------------------------------------------------------
// Print an integer
printfn "peekResult = %d" peekResult
// Print a result using %A for generic printing
printfn "listC = %A" listC
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