Type Synonyms

A type synonym is a new name for an already existing type. For example, here are some type synonyms that might be useful in representing some types from linear algebra:

type point  = float * float
type vector = float list
type matrix = float list list

Anywhere that a float*float is expected, you could use point, and vice-versa. The two are completely exchangeable for one another. In the following code, getx doesn't care whether you pass it a value that is annotated as one vs. the other:

let getx : point -> float =
  fun (x,_) -> x

let pt : point = (1.,2.)
let floatpair : float*float = (1.,3.)

let one  = getx pt
let one' = getx floatpair

Type synonyms are useful because they let us give descriptive names to complex types. They are a way of making code more self-documenting.