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Expression declarations

An expression declaration is a declaration that consists of a single expression. The expression is evaluated solely for its value and side effects. Unlike other declarations, it does not declare any new names.

When used as an intermediate declaration within a block, a declaration expression must have type (). If it produces a value of another type, it must be preceded by the ignore keyword to discard the value and ensure the expression evaluates to ().

However, when used as the final declaration in a block, the declaration expression determines the type and value of the entire block. In this case, it may be of any valid type, not just (). For example:

var n : Nat  = 0;

func regUser(name : Text) : Text {
  ignore bumpUsers(); // Returns Nat, but result is not needed
  let greeting = "Welcome, " # name # "!";
  greeting
};

func bumpUsers() : Nat {
   n += 1;
   n
};

In Motoko, expressions of type () play the role of statements in other languages.

Basic usage​

Expression declarations are commonly used for functions or operations that produce side effects, such as printing or modifying state.

Debug.print("Hello, Motoko!");

In this example, the string "Hello, Motoko!" is printed, but the expression is not assigned to any variable.

Expression declarations in a sequence​

If an expression appears inside a sequence of declarations but is not the last declaration, it must return () (the unit type).

let x = 10;
Debug.print("Processing..."); // Expression declaration with side effects
let y = x * 2;                // Valid, since Debug.print() returns ()

In this example, Debug.print() is used for its side effect. Because it returns (), it can safely appear before let y = x * 2; in the sequence.

In comparison, below is an invalid expression declaration:

let x = 10;
x * 2;  // This expression produces a non-`()` value but is not named or ignored.
let y = 5;

The expression x * 2; returns a value of type Nat, but since it is not assigned to a variable and is not the last declaration, this is invalid. Declaration expressions in intermediate positions must return () or be marked ignore.

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