Options and Results

In this chapter we discuss the Option type and the Result variant type.

They are different concepts but they are generally used for the same purpose, namely specifying the return type of functions. Options are used more widely beyond function return types, where as Results mostly appear as function return type.


An option type is a type that can either have some value or have no value at all.

An option type is a type preceded by a question mark ?.
An option value is a value preceded by a question mark ?.

Some examples:

let a : ?Nat = ?202;

let b : ?Text = ?"DeFi";

let c : ?Bool = ?true;

Variable a is annotated with option type ?Nat and assigned the option value ?202. The option value must be of the same type as the option type, meaning the value 202 is of type Nat.

Every option value can have the value null, which is a special 'value' indicating the absence of a value. The null value has type Null. For example:

let x : ?Nat = null;

let y : ?Text = null : Null;

We assign the value null to variable x which has option type ?Nat. In the second line we type annotated the null value with its Null type.

Lets use an option type as the return type of two functions:

func returnOption() : ?Nat {

func returnNull() : ?Nat {

The two functions both have ?Nat as their return type. The first returns an option value and the second returns null.

In the next chapter about control flow we will demonstrate how we can use option types in a useful way.


To fully understand Results we have to understand generics first, which are an advanced topic that we will cover later in this book. For now we will only cover a limited form of the Result variant type to give a basic idea of the concept.

A Result type is a variant type. A simple definition could be:

type Result = {

A value of this type may be one of two possible variants, namely #ok or #err. These variants are used to indicate either the successful result of a function or a possible error during the evaluation of a function.

Lets use the Result type in the same way we used Options above:

func returnOk() : Result {

func returnErr() : Result {

Both functions have Result as their return type. The one returns the #ok variant and the other return the #err variant.

In the next chapter about control flow we will demonstrate how we can use the Result variant type in a useful way.