By W. Chance Coble and Ted Neward

F# is Microsoft's most recent language for the .NET platform. Developers who learn to take advantage of this new language's functional and object-oriented features will find new productivity gains and new programming design approaches not easily expressed in "just" objects alone. While functional programming can lead to some mind-bending coding, the basics are quite straightforward and should take little time to master. Functional programming is characterized by concise coding style and explicit modeling of behavior. Because F# also offers a rich set of object oriented features, its integration with other .NET languages such as C# and VB is nearly seamless.

F# is available from the Microsoft F# Developer Center (http://msdn.microsoft.com/en-us/fsharp/default.aspx) if it is not already installed in your version of Visual Studio. It runs on any version of .NET after 2.0.

F# Hello World looks like this:

#light
System.Console.WriteLine("This is one hello")
printfn "This is another hello"

Compile this file (hello.fs) with the command-line fsc.exe to produce a .NET assembly:

fsc hello.fs

This produces hello.exe, which can be executed in the usual fashion.

F# is a compiled .NET language, but can also be run as a script interactively. Fire up the F# interactive session (Under the View menu in Visual Studio or "fsi.exe" in your F# installation bin directory). Add two semi-colons "::" to terminate your interactive expressions.

The "#light" syntax is an artifact of earlier F# versions and will be removed in a future version of the language. The "System.Console.WriteLine" call is the .NET Base Class Library at work. The "printfn" is an F# function that ultimately does the same thing: prints to the console.

F# is a combination of both functional and object-oriented programming styles. As such, to the C# or Visual Basic

programmer, it can at times seem strikingly similar yet entirely foreign.

F# programs are written as composed expressions, as opposed to a series of imperative statements in C#/VB. Each expression can be named (via the "let" keyword), and referenced as such:

let files = System.IO.DirectoryInfo(@"C:\Users\Chance").GetFiles()

This defines the "files" name to a value, in this case an array of FileInfo objects. The actual type of "files" needn't be specified, since the language will infer it from the value returned from the right-hand side.

Types in F# are inferred statically by the compiler. If you are using a file editor in Visual Studio, hold your mouse over any value to see its inferred type.

We refer to the type of a value being inferred because a compile-time process attempts to figure out the types of values on its own, and in the process verify the program is type safe. Type annotations (in the form v:t, meaning v of type t) are not as necessary in a language with type inference, even though the language is still strongly typed.

Generics, which will be familiar to both .NET and Java developers, have taken on an even greater role in F# because of type inference. Notice that for any expressions without type annotations the compiler can just assume generic types. For example, the function below can be genericized to have the

following type signature, even though it does not contain any type annotations from the programmer.

let f x =
let y = g x
h y

let f (x:'a) : 'b =
let y:'c = (g:'a->'c) x
(h:'c->'b) y

The compiler does something similar to that labeling automatically. From the generic labeling, determinations can already be made about the types of these functions (assuming the expression is strongly typed). First, notice f and h must yield the same type ('b). Second, the argument f is applied to must be the same type to which g is applied ('a). Using small hints like these, the compiler is often able to completely determine the types in a program with little assistance from the programmer.

Most of type inference in F# can be boiled down to two rules. First, if a function is applied to a value then the compiler may assume that value is the type the function requires. Second, if a value is bound to the result of an expression, then that value is the type the expression yields.

There are a few times when these simple rules aren't quite enough and type annotations must be added to assist the compiler. For example, when arithmetic operators are used, F# is careful not to cast one numeric type to another without explicit instructions from the programmer. That way type inference does not become a burden to intensively numerical computing.

Declare/update mutable valuelet mutable x = 0 x <- x + 1Declare/update ref valuelet x = ref 0 x := !x + 1

Another example is in method overloading. The Write method in System.Console for example has about 18 overloads. Type inference may determine the type that is passed to it, but cannot determine a value's type in the other direction. That information is required by the overloaded method to select the proper logic to dispatch for the method.

Type inference does not just aide concise notation in the face of static typing, it is also a helpful check on functional programs. When you write a piece of code, and intellisense indicates they all have the proper type, it is one more indication that gross errors were not introduced into the program. Using this tool, F# gets much of the concise notation usually only available in dynamic languages while still maintaining a fully static type system.

Basic Syntax

The "let" expression is at the core of F#'s functional syntax, and is used in a variety of ways: defining a function, defining a sequence, and so on. F# uses significant whitespace to mark block beginnings and endings.

Define any valuelet x = 2Define a function valuelet f a = a + xDefine a recursive function

open System.IO
let rec printSubDirFiles dir =
   let files = Directory.GetFiles dir
   let dirs = Directory.GetDirectories dir
   printf "%s\n%A\n\n" dir files
   Array.iter printSubDirFiles dirs

Anonymous functionfun x -> Console.WriteLine (x.ToString())

The language also provides traditional imperative looping and iteration constructs, such as "if", "for" and "while". Note that "if/then" and "if/then/else" are slightly different from traditional O-O languages, in that they, like most F# expressions, must yield a value, and so both sides of the "if/then/else" must result in the same type of value.

if/thenif x=10 then printf "Was 10\n"if/then/elseif x=10 then "Was 10\n" else "Was not 10\n"For loopfor x in xs do printf "%s" x.ToString()While loop

let ls =
  System.Collections.Generic.List<int>()
while (ls.Count<10) do
  ls.Add(ls.Count)

Like most .NET languages, F# also provides some mechanism for organizing code; in fact, F# provides two, modules and namespaces, the latter acting the same way as C#/VB namespaces. Modules provide not only lexical scoping, but also space for module-level values, such as constants, fields and functions.

Basic Code Organization: namespaces, types and modules

namespace MyFSharpProg
open System.Net
type Foo () =
  member x.GetRequest = WebRequest.Create
 module Main = begin
// values and functions here
end

While most identifiers in F# are immutable, in accordance with traditional functional programming principles, F# permits the definition and modification of values using the "mutable" keyword, or by taking the reference of the value in question by preceding the value with "ref". Assignment to mutable values is done using the left-hand arrow operator ("<-"). Assignment to "ref" values is done with the ":=" operator. Obtaining the value of a "ref" value uses the "!" operator.

Declare/update mutable valuelet mutable x = 0 x <- x + 1Declare/update ref valuelet x = ref 0 x := !x + 1

Function Composition

Because programs are built as composed expressions, the sequence of program logic is typically defined through function composition. Arguments to a function are evaluated prior to the function body, making programming by composition intuitive for programmers coming from a C#/VB/Java background.

Operators such as >> (for function chaining) and |> (for value piping) allow programmers to chain composed functions together in the same order in which they will be evaluated.

Composing functions to sum 5 largest array values

let sumLargestFive:int array->int =
    Array.sort
    < Array.rev
    <fun a -> Array.sub a 0 5
    <Array.sum

Composing functions starting with a value (piping)

let tabAndWrite (s:string) =
    s
    |> (+) "\t"
    |> Console.WriteLine

tupleslet (name,value) = ("Two",2)record typestype Person = {name:string;age:int}let p = { name="Bob";age=20 }discriminated unions

type WebTree =
  | Page of string * WebTree
  | Address of string

let rec listFromWebTree wt =
 match wt with
   | Page(url,subTree) ->
      url : listFromWebTree subTree
   | Address(url) -> [ url ]

let rec containsZero ls = function
  | first::rest -> if first=0 then true
             else containsZero rest
  | [] -> false

let startsWithZero ls = function
  | 0::rest -> true
  | ls -> false

let startsWithZero ls = function
  | first::rest when first=0 -> true
  | _ -> false

[ 1..10] =
  List.map
       (fun x -> x / 2)
        [for x in 1..10 -> x*2]

[|2..2..20|] =
   Array.map ((*) 2)
     [|for x in 1..10 -> x|]

[|1..20|].[..9]
[|1..20|].[10..]

seq
{
   let dirs =
     System.IO.Directory.GetFiles @"C:\"
   for x in dirs do
      yield x }
|> Seq.iter (printf "%s\t")

[1..10]
|> List.map (fun x -> x * 10)
|> List.reduce (fun s x -> s + x)
|> printfn "%d"

let rec factorial n =
  match n with
  | 0 -> 1
  | v -> v * factorial(v-1)

let rec listFromWebTree wt =
  match wt with
    | Page(url,subTree) ->
           url :: listFromWebTree subTree
    | Address(url) -> [ url ]

let rec containsZero ls = function
  | first::rest -> if first=0 then true
             else containsZero rest
  | [] -> false

let startsWithZero ls = function
  | 0::rest -> true
  | ls -> false

let startsWithZero ls = function
  | first::rest when first=0 -> true
  | _ -> false

let startsWithZero ls = function
  | 0::_ -> true
  | _ -> false

open System.IO
try
 File.ReadAllText(@"C:\dir\myfile.txt")
with
| :? DirectoryNotFoundException as ex
     -> "Dir does not exist"
| ex -> ex.Message

Try
  File.ReadAllText(@"C:\dir\myfile.txt")
finally
  printf "Always run this\n"

type Person(fn:string,ln:string,a:int) =
    member p.FirstName = fn
    member p.LastName = ln
    member p.Age = a
    override p.ToString() =
        String.Format("{0} {1})",
            p.FirstName, p.LastName)

type Student(fn:string,ln:string,a:int)=
inherit Person(fn, ln, a)
override s.ToString() = ...

type AccessCounter() =
let mutable i = 0
member me.Access () = i <- i + 1
member me.Count
with get () = i
set v = i <- v

type Person
   (fn:string,ln:string,age:int) =
new (age:int) =
Person("John","Doe",age)
member p.FirstName = fn
member p.LastName = ln
member p.Age = age
override p.ToString() =
 String.Format("{0} {1})",
p.FirstName, p.LastName)

type Shape =
   abstract Area: unit -> float
type Rectangle(l,w) =
  interface Shape with
    member me.Area () = l * w

[<AbstractClass>]
type Shape(nm:string) =
   member me.Name = nm
   abstract Area : unit -> float
type Rectangle(l,w) =
   inherit Shape("Rectangle")
   override me.Area() = l * w

type PersonRec =
  {fn:string;ln:string;age:int}
  with
    member
me.AgePlusOne = me.age + 1

type PersonKinds =
  | Male of Person
  | Female of Person
  with
    member me.AgePlusOne =
      match me with
      | Male p -> p.Age + 1
      | Female p -> p.Age + 1

(Male (Person("John","Doe",24)))
  .AgePlusOne

type Person
  with
    member x.AgePlusOne = x.Age + 1

let dirty =
   { new System.IDisposable with
     member me.Dispose() = () } // clean up

<#time;;
<[1..10000];;
Real: 00:00:00.028, CPU: 00:00:00.015, GC gen0: 1, gen1: 0, gen2:0

let velocity
      (d:float<meters>)
      (t:float<seconds>) =
           d / t

type [<Measure>] inches
velocity 5.0<inches>
         10.0

Async.RunWithContinuations
   (con,exc,can,aExpr)

let aMap f xs =
  Seq.map
    (fun x -> async {return f x})
     xs
  |> Async.Parallel

let (|Xml|NoXml|)
      (doc:XmlDocument) =
  if doc.InnerXml = ""
  then NoXml
  else Xml(doc.InnerXml)

open System.IO
let (|NoFiles|_|) (fs:FileInfo []) =
  if fs.Length=0 then Some ()
  else None
let (|TooManyFiles|_|) n
           (fs:FileInfo []) =
  if fs.Length > n then Some ()
  else None
 let (|FilesContents|_|)
   maxReadSize (fs:FileInfo [])
     :byte array array option=
   let buff =
      Array.create maxReadSize 0uy
   fs
   |> Array.map
       (fun file ->  
            let len =
                file
                .OpenRead()
                .Read
(buff,0,maxReadSize)
                (len,buff))
 |> Array.map
    (fun (len,contents) ->
          Array.init
             len
       (fun i -> contents.[i]))
 |> Some

let processFiles =
  match files with
  | TooManyFiles 50 () -> ">>oh no!"
  | NoFiles () -> "0 Oh no!"
  | FilesContents 1000 fs ->
      "Got "
       + fs.Length.ToString()
       + " files"

F#'s type system does 
not automatically cast 
numbers

This expression 
has type
         float
but is here used 
with type
 int

let add x y = x + y
let isum = 1 + 1
let fsum = 1.0 + 1.0

Values should be 
eventually typed
through inference, 
or annotation (i.e. 
generic types can't be 
instantiated)

Value 
restriction. The 
value 'x' has 
been inferred 
to have generic 
type.

let rec f x = f x
//Alternatively
let id x = x
let arr =
Array.create
10
id

Different type 
expectations: 
Expression is typed as 
'b, but 'a was put in 
the code

This expression
has type 
   'a 
but here is used
with type
   'b

let printStr = 
   printf "%s" 
printStr 10

Incomplete Pattern
Match Warning 
(unhandled patterns 
possible)

Incomplete 
pattern 
matches 
on this 
expression

match x with
| 1 -> "Was one"
| 2 -> "Was two"
// The wildcard _ can
// fix this

Overloaded functions 
often require type 
annotations to select 
the correct method call

The method 
'Write' is 
overloaded

open System
let print s =
   Console.Write s