Example
Stateful iterators carry some additional information about the current state of the iterator.
Using Tables
The addition state can be packed into the generic for loop's invariant state.
local function chars_iter(t, i)
local i = i + 1
if i <= t.len then
return i, t.s:sub(i, i)
end
end
local function chars(s)
-- the iterators state
local t = {
s = s, -- the subject
len = #s -- cached length
}
return chars_iter, t, 0
end
for i, c in chars 'abcde' do
print(i, c) --> 1 a, 2 b, 3 c, 4 d, 5 e
end
Using Closures
Additional state can be wrapped within a function closure. Since the state is fully contained in the scope of the closure the invariant state and control variable are not needed.
local function chars(s)
local i, len = 0, #s
return function() -- iterator function
i = i + 1
if i <= len then
return i, s:sub(i, i)
end
end
end
for i, c in chars 'abcde' do
print(i, c) --> 1 a, 2 b, 3 c, 4 d, 5 e
end
Using Coroutines
Additional state can be contained within a coroutine, again the invariant state and control variable are not needed.
local function chars(s)
return coroutine.wrap(function()
for i = 1, #s do
coroutine.yield(s:sub(i, i))
end
end)
end
for c in chars 'abcde' do
print(c) --> a, b, c, d, e
end
