Routines and Calls

Routines give a calculation a name and define the values it needs. Nucleus has one routine family, introduced by sub; a result type distinguishes a value routine from a result-free one.

sub choose(left as u8, right as u8) as u8
    if left > right
        return left
    end
    return right
end

Parameters are declared individually. Scalar arguments are copied into a new activation for the call. Scalar locals belong to that activation and must be declared before its statements. A recursive call receives its own parameter and local values.

Calls evaluate arguments left to right

The order is observable when arguments call routines or change storage:

observed = u16(choose(mark(2), mark(7))) + u16(sequence)

mark(2) completes before mark(7) begins. The compiler retains the first result while evaluating the second, then starts choose. The companion uses that order to build sequence = 27.

Returning successfully

A result-free routine may use bare return or reach its closing end. A value routine uses return expression; every reachable path must return a compatible value. The compiler checks this from the structured statement forms rather than inventing a default result.

return may occur inside a conditional or loop. It ends the current call immediately. Nucleus has no destructors or deferred cleanup to run on the way out.

Routine scope contains its parameters and scalar locals. A parameter or local may shadow a program name, but it cannot duplicate another parameter or earlier local in the same routine. Outside the routine, the program binding is visible again.

The companion proves left-to-right argument evaluation and early return. It leaves observed equal to 34.

var sequence as u8 = 0
var observed as u16 = 0

sub mark(value as u8) as u8
    sequence = sequence * 10 + value
    return value
end

sub choose(left as u8, right as u8) as u8
    if left > right
        return left
    end
    return right
end

sub main()
    observed = u16(choose(mark(2), mark(7))) + u16(sequence)
end

Summary

• sub declares both result-free and result-bearing routines.
• Scalar parameters and locals belong to one call activation.
• Arguments are evaluated from left to right before the call begins.
• Value routines must return a value on every reachable path.
• return ends the current activation immediately.

See names and scopes and routines and calls. The checked companion is 10-routines.nu.