Conditionals

Conditionals in Motoko come in two forms: if-expressions and if-statements.

if-else

An if-else expression, if <c> <b1> else <b2>, has three parts:

1. A condition <c> that evaluates to a boolean value.
2. Two branches <b1> and <b2>. These branches can be simple expressions or blocks { ... }.

If the condition is true, the first branch <b1> is evaluated to produce the result of the if-else. If the conditions is false, the second branch <b2> is evaluated to produce the result of if-else. Only one of the branches is evaluated.

The type of theif-else is the type of <b1> and <b2> if they have the same type; otherwise, it is a more general type that is a supertype of their types.

For example, you might use an if-else to choose a label based on a value.

let x : Int = 1;

let identity : Text =
  if (x == 1) {
    "x is 1"
  } else {
    "x is not 1"
  }; // Produces a value

The result of the if-else is assigned to identity. Here, both branches have the same type (Text in this case) as does the entire if-else.

let n : Nat = 0;
let parity = if (n % 2 == 0) #even else #odd;

Here, the first branch has type {#even} and the second branch has type {#odd}. These types are different but they have a common supertype {#even; #odd}. The type of the if-else is then {#even; #odd}.

Motoko will infer the common supertype for you, choosing the most specific one possible. If the types are inconsistent and only have the useless common supertype Any, Motoko will issue a warning:

let n : Nat = 0;
let oops = if (n % 2 == 0) #even else 0;

if-expression

An if-expression takes the form if <c> <b1> and is like an if-else but omits the else and second branch else <b2>.

An if-expression is used purely for its side effects to conditionally evaluate a single branch when the condition is true, and do nothing otherwise. It returns the trivial value (), and its type is ().

if-expressions are best suited for situations where you need to perform conditional actions, such as logging or modifying state based on certain conditions.

Since if-expressions have type (), they can be used as declaration expressions.

let x : Int = 1;

if (x == 1) {
    Debug.print("x is 1"); // Prints and returns ()
};

Nesting if-else expressions

if-else expressions can be nested and associate to the right. This ensures the following code works as intended and the second else belongs to the second, nested if.

var age = 21;

if (age < 18) {
  "You are a minor."
} else if (age >= 18 and age < 65) {
  "You are an adult."
} else {
  "You are a senior citizen."
};