Part of the "Understanding F# types" series (link)

Overview of types in F#

A look at the big picture

Before we dive into all the specific types, let’s look at the big picture.

What are types for?

If you are coming from an object-oriented design background, one of the paradigm shifts involved in “thinking functionally” is to change how you think about types.

A well designed object-oriented program will have a strong focus on behavior rather than data, so it will use a lot of polymorphism, either using “duck-typing” or explicit interfaces, and will try to avoid having explicit knowledge of the actual concrete classes being passed around.

A well designed functional program, on the other hand, will have a strong focus on data types rather than behavior. F# puts much more emphasis on designing types correctly than an imperative language such as C#, and many of the examples in this series and later series will focus on creating and refining type definitions.

So what is a type? Types are surprisingly hard to define. One definition from a well known textbook says:

“A type system is a tractable syntactic method of proving the absence of certain program behaviors by classifying phrases according to the kinds of values they compute” (Benjamin Pierce, Types and Programming Languages)

Ok, that definition is a bit technical. So let’s turn it around – what do we use types for in practice? In the context of F#, you can think of types as being used in two main ways:

  • Firstly, as an annotation to a value that allows certain checks to be made, especially at compile time. In other words, types allow you to have “compile time unit tests”.
  • Second, as domains for functions to act upon. That is, a type is a sort of data modeling tool that allows you to represent a real world domain in your code.

These two definitions interact. The better the type definitions reflect the real-world domain, the better they will statically encode the business rules. And the better they statically encode the business rules, the better the “compile time unit tests” work. In the ideal scenario, if your program compiles, then it really is correct!

What kinds of types are there?

F# is a hybrid language, so it has a mixture of types: some from its functional background, and some from its object-oriented background.

Generally, the types in F# can be grouped into the following categories:

  • Common .NET types. These are types that conform to the .NET Common Language Infrastructure (CLI), and which are easily portable to every .NET language.
  • F# specific types. These are types that are part of the F# language and are designed for pure functional programming.

If you are familiar with C#, you will know all the CLI types. They include:

  • Built-in value types (int, bool, etc).
  • Built-in reference types (string, etc).
  • User-defined value types (enum and struct).
  • Classes and interfaces
  • Delegates
  • Arrays

The F# specific types include:

I strongly recommend that when creating new types you stick with the F# specific types rather than using classes. They have a number of advantages over the CLI types, such as:

  • They are immutable
  • They cannot be null
  • They have built-in structural equality and comparison
  • They have built-in pretty printing

Sum and Product types

The key to understanding the power of types in F# is that most new types are constructed by from other types using two basic operations: sum and product.

That is, in F# you can define new types almost as if you were doing algebra:

define typeZ = typeX "plus" typeY
define typeW = typeX "times" typeZ

I will hold off explaining what sum and product mean in practice until we get to the detailed discussion of tuples (products) and discriminated union (sum) types later in this series.

The key point is that an infinite number of new types can be made by combining existing types together using these “product” and “sum” methods in various ways. Collectively these are called “algebraic data types” or ADTs (not to be confused with abstract data types, also called ADTs). Algebraic data types can be used to model anything, including lists, trees, and other recursive types.

The sum or “union” types, in particular, are very valuable, and once you get used to them, you will find them indispensable!

How types are defined

Every type definition is similar, even though the specific details may vary. All type definitions start with a “type” keyword, followed by an identifier for the type, followed by any generic type parameters, followed by the definition. For example, here are some type definitions for a variety of types:

type A = int * int
type B = {FirstName:string; LastName:string}
type C = Circle of int | Rectangle of int * int
type D = Day | Month | Year
type E<'a> = Choice1 of 'a | Choice2 of 'a * 'a

type MyClass(initX:int) =
   let x = initX
   member this.Method() = printf "x=%i" x

As we said in a previous post, there is a special syntax for defining new types that is different from the normal expression syntax. So do be aware of this difference.

Types can only be declared in namespaces or modules. But that doesn’t mean you always have to create them at the top level – you can create types in nested modules if you need to hide them.

module sub =
    // type declared in a module
    type A = int * int

    module private helper =
        // type declared in a submodule
        type B = B of string list

        //internal access is allowed
        let b = B ["a";"b"]

//outside access not allowed
let b = sub.helper.B ["a";"b"]

Types cannot be declared inside functions.

let f x =
    type A = int * int  //unexpected keyword "type"
    x * x

Constructing and deconstructing types

After a type is defined, instances of the type are created using a “constructor” expression that often looks quite similar to the type definition itself.

let a = (1,1)
let b = { FirstName="Bob"; LastName="Smith" }
let c = Circle 99
let c' = Rectangle (2,1)
let d = Month
let e = Choice1 "a"
let myVal = MyClass 99

What is interesting is that the same “constructor” syntax is also used to “deconstruct” the type when doing pattern matching:

let a = (1,1)                                  // "construct"
let (a1,a2) = a                                // "deconstruct"

let b = { FirstName="Bob"; LastName="Smith" }  // "construct"
let { FirstName = b1 } = b                     // "deconstruct"

let c = Circle 99                              // "construct"
match c with
| Circle c1 -> printf "circle of radius %i" c1 // "deconstruct"
| Rectangle (c2,c3) -> printf "%i %i" c2 c3    // "deconstruct"

let c' = Rectangle (2,1)                       // "construct"
match c' with
| Circle c1 -> printf "circle of radius %i" c1 // "deconstruct"
| Rectangle (c2,c3) -> printf "%i %i" c2 c3    // "deconstruct"

As you read through this series, pay attention to how the constructors are used in both ways.

Field guide to the “type” keyword

The same “type” keyword is used to define all the F# types, so they can all look very similar if you are new to F#. Here is a quick list of these types and how to tell the difference between them.

Type Example Distinguishing features
Abbrev (Alias)
type ProductCode = string
type transform<'a> = 'a -> 'a
Uses equal sign only.
//not explicitly defined with type keyword
let t = 1,2
let s = (3,4)
Always available to be used and are not explicitly defined with the type keyword. Usage indicated by comma (with optional parentheses).
type Product = {code:ProductCode; price:float }
type Message<'a> = {id:int; body:'a}

let p = {code="X123"; price=9.99}
let m = {id=1; body="hello"}
Curly braces.
Uses semicolon to separate fields.
Discriminated Union
type MeasurementUnit = Cm | Inch | Mile
type Name =
    | Nickname of string
    | FirstLast of string * string
type Tree<'a> =
    | E
    | T of Tree<'a> * 'a * Tree<'a>
let u = Inch
let name = Nickname("John")
let t = T(E,"John",E)
Vertical bar character.
Uses "of" for types.
type Gender = | Male = 1 | Female = 2
let g = Gender.Male
Similar to Unions, but uses equals and an int value
type Product (code:string, price:float) =
   let isFree = price=0.0
   new (code) = Product(code,0.0)
   member this.Code = code
   member this.IsFree = isFree

let p = Product("X123",9.99)
let p2 = Product("X123")
Has function-style parameter list after name for use as constructor.
Has "member" keyword.
Has "new" keyword for secondary constructors.
type IPrintable =
   abstract member Print : unit -> unit
Same as class but all members are abstract.
Abstract members have colon and type signature rather than a concrete implementation.
type Product=
      val code:string
      val price:float
      new(code) = { code = code; price = 0.0 }

let p = Product()
let p2 = Product("X123")
Has "struct" keyword.
Uses "val" to define fields.
Can have constructor.


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