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if / elif / else

Use if to run code only when a condition is true.

Use elif for more cases and else for the fallback case.

def describe(n):
    if n < 0:
        return "negative"
    elif n > 0:
        return "positive"
    else:
        return "zero"

actor main(env):
    print(describe(-7))
    print(describe(0))
    print(describe(5))
    env.exit(0)

The conditions in if, elif, and while are expressions that evaluate to True or False.

n = 7

if n % 2 == 0:
    print("even")
else:
    print("odd")

Conditionals compose with Acton's expression model, so guards often mix comparisons, membership tests, and short-circuiting in one place. Once the branching logic starts encoding type or protocol decisions, helper functions or explicit narrowing usually read better than a long ladder.