ReasonML Quick Start Guide

unit

In Reason, unit is a type that means "nothing." A function whose return type is unit cannot return anything other than unit. There is exactly one value of type unit: () (that is, a pair of empty parentheses, which is also called unit).

In contrast, there are exactly two values of type bool: true and false. There are an infinite number of values of type int.

As discussed in Chapter 1, Introduction to ReasonML, the idiomatic way to represent a nullable value in Reason is with the option type. The major difference between the option type and the unit type is that a value of type option could be nothing, or it could be some value where as a value of type unit is always ().

A function that accepts and/or returns unit likely causes side effects. ...