F Sharp Programming/Classes
| F# : Classes and Objects |
In the real world, an object is a "real" thing. A cat, person, computer, and a roll of duct tape are all "real" things in the tangible sense. When we think about these things, we can broadly describe them in terms of a number of attributes:
- Properties: a person has a name, a cat has four legs, computers have a price tag, duct tape is sticky.
- Behaviors: a person reads the newspaper, cats sleep all day, computers crunch numbers, duct tape attaches things to other things.
- Types/group membership: an employee is a type of person, a cat is a pet, a Dell and Mac are types of computers, duct tape is part of the broader family of adhesives.
In the programming world, an "object" is, in the simplest of terms, a model of something in the real world. Object-oriented programming (OOP) exists because it allows programmers to model real-world entities and simulate their interactions in code. Just like their real-world counterparts, objects in computer programming have properties and behaviors, and can be classified according to their type.
While we can certainly create objects that represents cats, people, and adhesives, objects can also represent less concrete things, such as a bank account or a business rule.
Although the scope of OOP has expanded to include some advanced concepts such as design patterns and the large-scale architecture of applications, this page will keep things simple and limit the discussion of OOP to data modeling.
Defining an Object
editBefore an object is created, its properties and functions should be defined. You define properties and methods of an object in a class. There are actually two different syntaxes for defining a class: an implicit syntax and an explicit syntax.
Implicit Class Construction
editImplicit class syntax is defined as follows:
type TypeName optional-type-arguments arguments [ as ident ] =
[ inherit type { as base } ]
[ let-binding | let-rec bindings ] *
[ do-statement ] *
[ abstract-binding | member-binding | interface-implementation ] *
- Elements in brackets are optional, elements followed by a
*may appear zero or more times.
This syntax above is not as daunting as it looks. Here's a simple class written in implicit style:
type Account(number : int, holder : string) = class
let mutable amount = 0m
member x.Number = number
member x.Holder = holder
member x.Amount = amount
member x.Deposit(value) = amount <- amount + value
member x.Withdraw(value) = amount <- amount - value
end
The code above defines a class called Account, which has three properties and two methods. Let's take a closer look at the following:
type Account(number : int, holder : string) = class
The underlined code is called the class constructor. A constructor is a special kind of function used to initialize the fields in an object. In this case, our constructor defines two values, number and holder, which can be accessed anywhere in our class. You create an instance of Account by using the new keyword and passing the appropriate parameters into the constructor as follows:
let bob = new Account(123456, "Bob’s Saving")
Additionally, let's look at the way a member is defined:
member x.Deposit(value) = amount <- amount + value
The x above is an alias for the object currently in scope. Most OO languages provide an implicit this or self variable to access the object in scope, but F# requires programmers to create their own alias.
After we can create an instance of our Account, we can access its properties using .propertyName notation. Here's an example in FSI:
> let printAccount (x : Account) =
printfn "x.Number: %i, x.Holder: %s, x.Amount: %M" x.Number x.Holder x.Amount;;
val printAccount : Account -> unit
> let bob = new Account(123456, "Bob’s Savings");;
val bob : Account
> printAccount bob;;
x.Number: 123456, x.Holder: Bob’s Savings, x.Amount: 0
val it : unit = ()
> bob.Deposit(100M);;
val it : unit = ()
> printAccount bob;;
x.Number: 123456, x.Holder: Bob’s Savings, x.Amount: 100
val it : unit = ()
> bob.Withdraw(29.95M);;
val it : unit = ()
> printAccount bob;;
x.Number: 123456, x.Holder: Bob’s Savings, x.Amount: 70.05
Example
editLet's use this class in a real program:
open System
type Account(number : int, holder : string) = class
let mutable amount = 0m
member x.Number = number
member x.Holder = holder
member x.Amount = amount
member x.Deposit(value) = amount <- amount + value
member x.Withdraw(value) = amount <- amount - value
end
let homer = new Account(12345, "Homer")
let marge = new Account(67890, "Marge")
let transfer amount (source : Account) (target : Account) =
source.Withdraw amount
target.Deposit amount
let printAccount (x : Account) =
printfn "x.Number: %i, x.Holder: %s, x.Amount: %M" x.Number x.Holder x.Amount
let main() =
let printAccounts() =
[homer; marge] |> Seq.iter printAccount
printfn "\nInializing account"
homer.Deposit 50M
marge.Deposit 100M
printAccounts()
printfn "\nTransferring $30 from Marge to Homer"
transfer 30M marge homer
printAccounts()
printfn "\nTransferring $75 from Homer to Marge"
transfer 75M homer marge
printAccounts()
main()
The program has the following types:
type Account =
class
new : number:int * holder:string -> Account
member Deposit : value:decimal -> unit
member Withdraw : value:decimal -> unit
member Amount : decimal
member Holder : string
member Number : int
end
val homer : Account
val marge : Account
val transfer : decimal -> Account -> Account -> unit
val printAccount : Account -> unit
The program outputs the following:
Initializing account x.Number: 12345, x.Holder: Homer, x.Amount: 50 x.Number: 67890, x.Holder: Marge, x.Amount: 100 Transferring $30 from Marge to Homer x.Number: 12345, x.Holder: Homer, x.Amount: 80 x.Number: 67890, x.Holder: Marge, x.Amount: 70 Transferring $75 from Homer to Marge x.Number: 12345, x.Holder: Homer, x.Amount: 5 x.Number: 67890, x.Holder: Marge, x.Amount: 145
Example using the do keyword
edit
The do keyword is used for post-constructor initialization. For example, to create an object which represents a stock, it is necessary to pass in the stock symbol and initialize the rest of the properties in our constructor:
open System
open System.Net
type Stock(symbol : string) = class
let url =
"http://download.finance.yahoo.com/d/quotes.csv?s=" + symbol + "&f=sl1d1t1c1ohgv&e=.csv"
let mutable _symbol = String.Empty
let mutable _current = 0.0
let mutable _open = 0.0
let mutable _high = 0.0
let mutable _low = 0.0
let mutable _volume = 0
do
(* We initialize our object in the do block *)
let webClient = new WebClient()
(* Data comes back as a comma-separated list, so we split it
on each comma *)
let data = webClient.DownloadString(url).Split([|','|])
_symbol <- data.[0]
_current <- float data.[1]
_open <- float data.[5]
_high <- float data.[6]
_low <- float data.[7]
_volume <- int data.[8]
member x.Symbol = _symbol
member x.Current = _current
member x.Open = _open
member x.High = _high
member x.Low = _low
member x.Volume = _volume
end
let main() =
let stocks =
["msft"; "noc"; "yhoo"; "gm"]
|> Seq.map (fun x -> new Stock(x))
stocks |> Seq.iter (fun x -> printfn "Symbol: %s (%F)" x.Symbol x.Current)
main()
This program has the following types:
type Stock =
class
new : symbol:string -> Stock
member Current : float
member High : float
member Low : float
member Open : float
member Symbol : string
member Volume : int
end
This program outputs the following (your outputs will vary):
Symbol: "MSFT" (19.130000) Symbol: "NOC" (43.240000) Symbol: "YHOO" (12.340000) Symbol: "GM" (3.660000)
- Note: It's possible to have any number of
dostatements in a class definition, although there's no particular reason to need more than one.
Explicit Class Definition
editClasses written in explicit style follow this format:
type TypeName =
[ inherit type ]
[ val-definitions ]
[ new ( optional-type-arguments arguments ) [ as ident ] =
{ field-initialization }
[ then constructor-statements ]
] *
[ abstract-binding | member-binding | interface-implementation ] *
Here's a class defined using the explicit syntax:
type Line = class
val X1 : float
val Y1 : float
val X2 : float
val Y2 : float
new (x1, y1, x2, y2) =
{ X1 = x1; Y1 = y1;
X2 = x2; Y2 = y2}
member x.Length =
let sqr x = x * x
sqrt(sqr(x.X1 - x.X2) + sqr(x.Y1 - x.Y2) )
end
Each val defines a field in our object. Unlike other object-oriented languages, F# does not implicitly initialize fields in a class to any value. Instead, F# requires programmers to define a constructor and explicitly initialize each field in their object with a value.
We can perform some post-constructor processing using a then block as follows:
type Line = class
val X1 : float
val Y1 : float
val X2 : float
val Y2 : float
new (x1, y1, x2, y2) as this =
{ X1 = x1; Y1 = y1;
X2 = x2; Y2 = y2;}
then
printfn "Line constructor: {(%F, %F), (%F, %F)}, Line.Length: %F"
this.X1 this.Y1 this.X2 this.Y2 this.Length
member x.Length =
let sqr x = x * x
sqrt(sqr(x.X1 - x.X2) + sqr(x.Y1 - x.Y2) )
end
Notice that we have to add an alias after our constructor, new (x1, y1, x2, y2) as this), to access the fields of our object being constructed. Each time we create a Line object, the constructor will print to the console. We can demonstrate this using fsi:
> let line = new Line(1.0, 1.0, 4.0, 2.5);;
val line : Line
Line constructor: {(1.000000, 1.000000), (4.000000, 2.500000)}, Line.Length: 3.354102
Example Using Two Constructors
editSince the constructor is defined explicitly, we have the option to provide more than one constructor.
open System
open System.Net
type Car = class
val used : bool
val owner : string
val mutable mileage : int
(* first constructor *)
new (owner) =
{ used = false;
owner = owner;
mileage = 0 }
(* another constructor *)
new (owner, mileage) =
{ used = true;
owner = owner;
mileage = mileage }
end
let main() =
let printCar (c : Car) =
printfn "c.used: %b, c.owner: %s, c.mileage: %i" c.used c.owner c.mileage
let stevesNewCar = new Car("Steve")
let bobsUsedCar = new Car("Bob", 83000)
let printCars() =
[stevesNewCar; bobsUsedCar] |> Seq.iter printCar
printfn "\nCars created"
printCars()
printfn "\nSteve drives all over the state"
stevesNewCar.mileage <- stevesNewCar.mileage + 780
printCars()
printfn "\nBob commits odometer fraud"
bobsUsedCar.mileage <- 0
printCars()
main()
This program has the following types:
type Car =
class
val used: bool
val owner: string
val mutable mileage: int
new : owner:string * mileage:int -> Car
new : owner:string -> Car
end
Notice that our val fields are included in the public interface of our class definition.
This program outputs the following:
Cars created c.used: false, c.owner: Steve, c.mileage: 0 c.used: true, c.owner: Bob, c.mileage: 83000 Steve drives all over the state c.used: false, c.owner: Steve, c.mileage: 780 c.used: true, c.owner: Bob, c.mileage: 83000 Bob commits odometer fraud c.used: false, c.owner: Steve, c.mileage: 780 c.used: true, c.owner: Bob, c.mileage: 0
Differences Between Implicit and Explicit Syntaxes
editAs you've probably guessed, the major difference between the two syntaxes is related to the constructor: the explicit syntax forces a programmer to provide explicit constructor(s), whereas the implicit syntax fuses the primary constructor with the class body. However, there are a few other subtle differences:
- The explicit syntax does not allow programmers to declare
letanddobindings. - Even though
valfields can be used in the implicit syntax, they must have the attribute[<DefaultValue>]and be mutable. It is more convenient to useletbindings in this case. Publicmemberaccessors can be added when they need to be public. - In the implicit syntax, the primary constructor parameters are visible throughout the whole class body. By using this feature, it is not necessary to write code that copies constructor parameters to instance members.
- While both syntaxes support multiple constructors, when you declare additional constructors with the implicit syntax, they must call the primary constructor. In the explicit syntax all constructors are declared with new() and there is no primary constructor that needs to be referenced from others.
| Class with primary (implicit) constructor | Class with only explicit constructors |
|---|---|
// The class body acts as a constructor
type Car1(make : string, model : string) = class
// x.Make and x.Model are property getters
// (explained later in this chapter)
// Notice how they can access the
// constructor parameters directly
member x.Make = make
member x.Model = model
// This is an extra constructor.
// It calls the primary constructor
new () = Car1("default make", "default model")
end
|
type Car2 = class
// In this case, we need to declare
// all fields and their types explicitly
val private make : string
val private model : string
// Notice how field access differs
// from parameter access
member x.Make = x.make
member x.Model = x.model
// Two constructors
new (make : string, model : string) = {
make = make
model = model
}
new () = {
make = "default make"
model = "default model"
}
end
|
In general, its up to the programmer to use the implicit or explicit syntax to define classes. However, the implicit syntax is used more often in practice as it tends to result in shorter, more readable class definitions.
Class Inference
editF#'s #light syntax allows programmers to omit the class and end keywords in class definitions, a feature commonly referred to as class inference or type kind inference. For example, there is no difference between the following class definitions:
| Class Inference | Class Explicit |
|---|---|
type Product(make : string, model : string) =
member x.Make = make
member x.Model = model
|
type Car(make : string, model : string) = class
member x.Make = make
member x.Model = model
end
|
Both classes compile down to the same bytecode, but the code using class inference allows us to omit a few unnecessary keywords.
Class inference and class explicit styles are considered acceptable. At the very least, when writing F# libraries, don't define half of your classes using class inference and the other half using class explicit style—pick one style and use it consistently for all of your classes throughout your project.
Class Members
editInstance and Static Members
editThere are two types of members you can add to an object:
- Instance members, which can only be called from an object instance created using the
newkeyword. - Static members, which are not associated with any object instance.
The following class has a static method and an instance method:
type SomeClass(prop : int) = class
member x.Prop = prop
static member SomeStaticMethod = "This is a static method"
end
We invoke a static method using the form className.methodName. We invoke instance methods by creating an instance of our class and calling the methods using classInstance.methodName. Here is a demonstration in fsi:
> SomeClass.SomeStaticMethod;; (* invoking static method *)
val it : string = "This is a static method"
> SomeClass.Prop;; (* doesn't make sense, we haven't created an object instance yet *)
SomeClass.Prop;; (* doesn't make sense, we haven't created an object instance yet *)
^^^^^^^^^^^^^^^
stdin(78,1): error FS0191: property 'Prop' is not static.
> let instance = new SomeClass(5);;
val instance : SomeClass
> instance.Prop;; (* now we have an instance, we can call our instance method *)
val it : int = 5
> instance.SomeStaticMethod;; (* can't invoke static method from instance *)
instance.SomeStaticMethod;; (* can't invoke static method from instance *)
^^^^^^^^^^^^^^^^^^^^^^^^^^
stdin(81,1): error FS0191: property 'SomeStaticMethod' is static.
We can, of course, invoke instance methods from objects passed into static methods, for example, let's say we add a Copy method to our object defined above:
type SomeClass(prop : int) = class
member x.Prop = prop
static member SomeStaticMethod = "This is a static method"
static member Copy (source : SomeClass) = new SomeClass(source.Prop)
end
We can experiment with this method in fsi:
> let instance = new SomeClass(10);;
val instance : SomeClass
> let shallowCopy = instance;; (* copies pointer to another symbol *)
val shallowCopy : SomeClass
> let deepCopy = SomeClass.Copy instance;; (* copies values into a new object *)
val deepCopy : SomeClass
> open System;;
> Object.ReferenceEquals(instance, shallowCopy);;
val it : bool = true
> Object.ReferenceEquals(instance, deepCopy);;
val it : bool = false
Object.ReferenceEquals is a static method on the System.Object class which determines whether two objects instances are the same object. As shown above, our Copy method takes an instance of SomeClass and accesses its Prop property.
When should you use static methods rather than instance methods?
When the designers of the .NET framework were designing the System.String class, they had to decide where the Length method should go. They had the option of making the property an instance method (s.Length) or making it static (String.GetLength(s)). The .NET designers chose to make Length an instance method because it is an intrinsic property of strings.
On the other hand, the String class also has several static methods, including String.Concat which takes a list of string and concatenates them all together. Concatenating strings is instance-agnostic, its does not depend on the instance members of any particular strings.
The following general principles apply to all OO languages:
- Instance members should be used to access properties intrinsic to an object, such as
stringInstance.Length. - Instance methods should be used when they depend on state of a particular object instance, such as
stringInstance.Contains. - Instance methods should be used when its expected that programmers will want to override the method in a derived class.
- Static methods should not depend on a particular instance of an object, such as
Int32.TryParse. - Static methods should return the same value as long as the inputs remain the same.
- Constants, which are values that don't change for any class instance, should be declared as a static members, such as
System.Boolean.TrueString.
Getters and Setters
editGetters and setters are special functions which allow programmers to read and write to members using a convenient syntax. Getters and setters are written using this format:
member alias.PropertyName
with get() = some-value
and set(value) = some-assignment
Here's a simple example using fsi:
> type IntWrapper() = class
let mutable num = 0
member x.Num
with get() = num
and set(value) = num <- value
end;;
type IntWrapper =
class
new : unit -> IntWrapper
member Num : int
member Num : int with set
end
> let wrapper = new IntWrapper();;
val wrapper : IntWrapper
> wrapper.Num;;
val it : int = 0
> wrapper.Num <- 20;;
val it : unit = ()
> wrapper.Num;;
val it : int = 20
Getters and setters are used to expose private members to outside world. For example, our Num property allows users to read/write to the internal num variable.
Since getters and setters are glorified functions, we can use them to sanitize input before writing the values to our internal variables. For example, we can modify our IntWrapper class to constrain our to values between 0 and 10 by modifying our class as follows:
type IntWrapper() = class
let mutable num = 0
member x.Num
with get() = num
and set(value) =
if value > 10 || value < 0 then
raise (new Exception("Values must be between 0 and 10"))
else
num <- value
end
We can use this class in fsi:
> let wrapper = new IntWrapper();;
val wrapper : IntWrapper
> wrapper.Num <- 5;;
val it : unit = ()
> wrapper.Num;;
val it : int = 5
> wrapper.Num <- 20;;
System.Exception: Values must be between 0 and 10
at FSI_0072.IntWrapper.set_Num(Int32 value)
at <StartupCode$FSI_0076>.$FSI_0076._main()
stopped due to error
Adding Members to Records and Unions
editIts just as easy to add members to records and union types as well.
Record example:
> type Line =
{ X1 : float; Y1 : float;
X2 : float; Y2 : float }
with
member x.Length =
let sqr x = x * x
sqrt(sqr(x.X1 - x.X2) + sqr(x.Y1 - x.Y2))
member x.ShiftH amount =
{ x with X1 = x.X1 + amount; X2 = x.X2 + amount }
member x.ShiftV amount =
{ x with Y1 = x.Y1 + amount; Y2 = x.Y2 + amount };;
type Line =
{X1: float;
Y1: float;
X2: float;
Y2: float;}
with
member ShiftH : amount:float -> Line
member ShiftV : amount:float -> Line
member Length : float
end
> let line = { X1 = 1.0; Y1 = 2.0; X2 = 5.0; Y2 = 4.5 };;
val line : Line
> line.Length;;
val it : float = 4.716990566
> line.ShiftH 10.0;;
val it : Line = {X1 = 11.0;
Y1 = 2.0;
X2 = 15.0;
Y2 = 4.5;}
> line.ShiftV -5.0;;
val it : Line = {X1 = 1.0;
Y1 = -3.0;
X2 = 5.0;
Y2 = -0.5;}
Union example
> type shape =
| Circle of float
| Rectangle of float * float
| Triangle of float * float
with
member x.Area =
match x with
| Circle(r) -> Math.PI * r * r
| Rectangle(b, h) -> b * h
| Triangle(b, h) -> b * h / 2.0
member x.Scale value =
match x with
| Circle(r) -> Circle(r + value)
| Rectangle(b, h) -> Rectangle(b + value, h + value)
| Triangle(b, h) -> Triangle(b + value, h + value);;
type shape =
| Circle of float
| Rectangle of float * float
| Triangle of float * float
with
member Scale : value:float -> shape
member Area : float
end
> let mycircle = Circle(5.0);;
val mycircle : shape
> mycircle.Area;;
val it : float = 78.53981634
> mycircle.Scale(7.0);;
val it : shape = Circle 12.0
Generic classes
editClasses which take generic types can be created:
type 'a GenericWrapper(initialVal : 'a) = class
let mutable internalVal = initialVal
member x.Value
with get() = internalVal
and set(value) = internalVal <- value
end
We can use this class in FSI as follows:
> let intWrapper = new GenericWrapper<_>(5);;
val intWrapper : int GenericWrapper
> intWrapper.Value;;
val it : int = 5
> intWrapper.Value <- 20;;
val it : unit = ()
> intWrapper.Value;;
val it : int = 20
> intWrapper.Value <- 2.0;; (* throws an exception *)
intWrapper.Value <- 2.0;; (* throws an exception *)
--------------------^^^^
stdin(156,21): error FS0001: This expression has type
float
but is here used with type
int.
> let boolWrapper = new GenericWrapper<_>(true);;
val boolWrapper : bool GenericWrapper
> boolWrapper.Value;;
val it : bool = true
Generic classes help programmers generalize classes to operate on multiple different types. They are used in fundamentally the same way as all other generic types already seen in F#, such as Lists, Sets, Maps, and union types.
Pattern Matching Objects
editWhile it's not possible to match objects based on their structure in quite the same way that we do for lists and union types, F# allows programmers to match on types using the syntax:
match arg with
| :? type1 -> expr
| :? type2 -> expr
Here's an example which uses type-testing:
type Cat() = class
member x.Meow() = printfn "Meow"
end
type Person(name : string) = class
member x.Name = name
member x.SayHello() = printfn "Hi, I'm %s" x.Name
end
type Monkey() = class
member x.SwingFromTrees() = printfn "swinging from trees"
end
let handlesAnything (o : obj) =
match o with
| null -> printfn "<null>"
| :? Cat as cat -> cat.Meow()
| :? Person as person -> person.SayHello()
| _ -> printfn "I don't recognize type '%s'" (o.GetType().Name)
let main() =
let cat = new Cat()
let bob = new Person("Bob")
let bill = new Person("Bill")
let phrase = "Hello world!"
let monkey = new Monkey()
handlesAnything cat
handlesAnything bob
handlesAnything bill
handlesAnything phrase
handlesAnything monkey
handlesAnything null
main()
This program outputs:
Meow Hi, I'm Bob Hi, I'm Bill I don't recognize type 'String' I don't recognize type 'Monkey' <null>