Loops

Nucleus has one condition-controlled loop and one counted loop. Between them they cover indefinite work, fixed ranges and array traversal without hiding a counter or iterator object.

while tests a condition

while value <= 0
    value = value + 1
end

The condition is Boolean and is tested before every iteration. while true is the indefinite form. When such a loop has no exit that targets it, the compiler knows it cannot fall through. A value routine may therefore return from inside it without adding an unreachable return after end.

for uses a declared local

The counter in a for header must already be a scalar local:

var index as i8

for index = -3 to 3 step + (1 + 1)
    // body
end

The compiler evaluates the start and bound once. to includes the bound; until excludes it. The step defaults to positive one and may instead be a nonzero compile-time constant magnitude with an optional sign. The companion writes step + (1 + 1), making both the direction and folded magnitude visible.

The counter is read-only inside the active loop. This prevents the body and the loop machinery from disagreeing about its next value. A value that would continue but overflow the counter type causes the loop-range safety trap rather than wrapping.

Leaving and continuing

continue moves to the next condition test in while, or to the increment and next test in for. exit leaves the nearest enclosing loop. Both are unlabelled; a nested loop receives them before an outer loop does.

The companion skips -1, leaves before processing 3, and calls a routine whose constant-true loop returns the first positive value. It finishes with observed equal to 3.

var observed as u16 = 0

sub firstPositive(value as i8) as i8
    while true
        if value > 0
            return value
        end
        value = value + 1
    end
end

sub main()
    var index as i8

    for index = -3 to 3 step + (1 + 1)
        if index = -1
            continue
        elseif index = 3
            exit
        end
        observed = observed + 1
    end
    observed = observed + u16(firstPositive(-2))
end

Summary

• while tests a Boolean condition before each iteration.
• A folded constant-true while with no targeting exit does not fall through.
• A for counter is a previously declared integer local.
• to is inclusive and until is exclusive.
• exit and continue target the nearest loop.

See loop control and routine fallthrough. The checked companion is 05-loops.nu.