type-abbreviations.md

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post	Type abbreviations	Also known as aliases	fsharp-types	Understanding F# types	3

Let's start with the simplest type definition, a type abbreviation or alias.

It has the form:

type [typename] = [existingType]

where "existing type" can be any type: one of the basic types we have already seen, or one of the extended types we will be seeing soon.

Some examples:

type RealNumber = float
type ComplexNumber = float * float
type ProductCode = string
type CustomerId = int
type AdditionFunction = int->int->int
type ComplexAdditionFunction = 
       ComplexNumber-> ComplexNumber -> ComplexNumber

And so on -- pretty straightforward.

Type abbreviations are useful to provide documentation and avoid writing a signature repeatedly. In the above examples, ComplexNumber and AdditionFunction demonstrate this.

Another use is to provide some degree of decoupling between the usage of a type and the actual implementation of a type. In the above examples, ProductCode and CustomerId demonstrate this. I could easily change CustomerId to be a string without changing (most of) my code.

However, one thing is to note is that this really is just an alias or abbreviation; you are not actually creating a new type. So if I define a function that I explicitly say is an AdditionFunction:

type AdditionFunction = int->int->int
let f:AdditionFunction = fun a b -> a + b

the compiler will erase the alias and return a plain int->int->int as the function signature.

In particular, there is no true encapsulation. I could use an explicit int anywhere I used a CustomerId and the compiler would not complain. And if I had attempted to create safe versions of entity ids such as this:

type CustomerId = int
type OrderId = int

then I would be disappointed. There would be nothing preventing me from using an OrderId in place of a CustomerId and vice versa. To get true encapsulated types like this, we will need to use single case union types, as described in a later post.