Numeric for ranges

The first rc.3 for form walks an inclusive integer range:

for index = 1, 5 do
    print(index)
end

This prints 1 through 5. The last bound is included when the progression reaches it.

Choosing a step

Omit the third expression for a step of 1, or provide one explicitly:

for even = 2, 10, 2 do
    print(even)
end

for countdown = 5, 1, -1 do
    print(countdown)
end

A positive step continues while the binding is <= the last bound. A negative step continues while it is >= the last bound. A range can be empty when its direction cannot reach the bound.

Static rules

The first bound, last bound, and optional step:

• are evaluated exactly once, from left to right, before the loop;
• must have one identical fixed integer type;
• cannot use a floating-point type;
• use checked progression arithmetic.

The loop binding is immutable and exists only inside the body:

for index: Int8 = 1, 3 do -- type annotation is not part of the range grammar
    index = 2              -- also invalid: the binding cannot be assigned
end

Write the bounds with the desired context instead:

local first: Int8 = 1
local last: Int8 = 3

for index = first, last do
    print(String(index))
end

A statically known zero step is rejected. A runtime zero step traps with InvalidRangeStep before the first iteration.

Arrays today

Arrays are one-based, so an index range covers their positions cleanly:

for index = 1, Array.length(names) do
    print(Array.get(names, index))
end

rc.3 does not implement for name in names. Generalized iteration remains deferred until the nominal Iterable<T> and Iterator<T> protocols are complete; there is no dynamic iterator fallback.