Enumeration Types

Declaring a new type

OCaml’s type declaration introduces new types beyond the built-in ones. An enumeration type is a type defined by listing its possible values (called constructors):

type vehicle = Bike | Motorbike | Car | Lorry

This declaration:

• Creates a new type called vehicle.
• Creates four new constants (Bike, Motorbike, Car, Lorry) of that type.
• The constructors serve both as values and as patterns for pattern matching.

Pattern matching on constructors

Once declared, the new type behaves like a built-in type:

let wheels = function
  | Bike -> 2
  | Motorbike -> 2
  | Car -> 4
  | Lorry -> 18

The type of wheels is vehicle -> int. The compiler checks that all cases are covered and warns about missing or redundant patterns.

Why not use integers or strings?

Approach	Problem
Integers (0, 1, 2, 3)	Code is hard to read; adding a new constructor may shift all numbers
Strings (“Bike”, “Car”, …)	Comparisons are slow; typos like “MOtorbike” are silent bugs
Enumeration type	Compiler catches missing cases and typos; values are distinct types

The compiler chooses the internal integer representation automatically. Type safety prevents confusion between different enumeration types: Bike and Red (from a colour type) are distinct and cannot be accidentally interchanged.

Most programming languages support enumeration types. Common examples include days of the week, colours, and states in a state machine.