Conditionals

An if expression specifies that some code should only be evaluated if a certain condition is true. For example:

script {
  fun example() {
    if (x > 5) x = x - 5
  }
}

The condition must be an expression of type bool.

An if expression can optionally include an else clause to specify another expression to evaluate when the condition is false.

script {
  fun example() {
    if (y <= 10) y = y + 1 else y = 10
  }
}

Either the “true” branch or the “false” branch will be evaluated, but not both. Either branch can be a single expression or an expression block.

The conditional expressions may produce values so that the if expression has a result.

script {
  fun example() {
    let z = if (x < 100) x else 100;
  }
}

The expressions in the true and false branches must have compatible types. For example:

script {
  fun example() {
    // x and y must be u64 integers
    let maximum: u64 = if (x > y) x else y;
 
    // ERROR! branches different types
    let z = if (maximum < 10) 10u8 else 100u64;
 
    // ERROR! branches different types, as default false-branch is () not u64
    if (maximum >= 10) maximum;
  }
}

If the else clause is not specified, the false branch defaults to the unit value. The following are equivalent:

script {
  fun example() {
    if (condition) true_branch // implied default: else ()
    if (condition) true_branch else ()
  }
}

Commonly, if expressions are used in conjunction with expression blocks.

script {
  fun example() {
    let maximum = if (x > y) x else y;
    if (maximum < 10) {
        x = x + 10;
        y = y + 10;
    } else if (x >= 10 && y >= 10) {
        x = x - 10;
        y = y - 10;
    }
  }
}
 

Grammar for Conditionals

if-expression → if ( expression ) expression else-clause_opt

else-clause → else expression