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main (print)

# T ? E is a success value or an error value. Return either directly.
half(n int) int ? str
    if n % 2 != 0
        "odd"
    n / 2

forward(error str) str from error
    error

# ?: unwraps or falls back. ? unwraps or returns the handler's value
# from the current function.
quarter(n int) int ? str
    h = half(n) ? forward
    half(h)

main()
    a = half(8) ?: 0
    b = half(7) ?: 0
    when quarter(12) is
        value print.line("{a} {b} {value}")
        error print.line("{error}")

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