Conditionals#

Use if to run a block when a Boolean condition is true. Add else to run a different block when it is false. Only the selected branch runs.

fn status(active: Bool) -> String:
    if active:
        return "Active"
    else:
        return "Inactive"

A short branch can use one simple statement on the same line:

fn status(active: Bool) -> String:
    if active: return "Active"
    else: return "Inactive"

See the suite rules for mixing inline and indented bodies.

Conditions require Bool. The condition in this example is already a Boolean; numeric and text values do not acquire truth values implicitly.

if is a statement and does not produce a value. Evaluate its condition once; execute exactly one selected branch, or neither branch when the condition is false and no else is present. else and else if start on separate lines, aligned with the corresponding if. An additional conditional uses else if, not elif.

Each branch has its own lexical scope. Names declared in a branch are not visible after the conditional; assignments to an outer var affect that existing binding. An else if chain checks conditions in order and stops after selecting a branch.

fn classify(value: Int) -> String:
    if value < 0:
        return "negative"
    else if value == 0:
        return "zero"
    else:
        return "positive"

if 1: is rejected because its condition is Int. let result = if active: ... is rejected because if is not an expression. Use branch-local assignment to a declared var or explicit returns to select a result. The function return rules determine whether all reachable paths return.

Validating inputs#

Validation uses the same ordinary control flow as any other decision:

fn nonnegative(value: Int) -> Bool:
    if value < 0:
        return false

    return true

The early return exits the function on the negative path. The final return runs on the remaining path. Expected failures uses this structure to reject invalid input with a domain error.

Bindings after a branch#

Every binding is initialized at declaration. To select a value for use after the conditional, initialize an outer var and assign to it in the branches. Branch-local names stay inside their branch. See bindings .

Use pattern matching when the decision depends on which case of an alternative value is present.