Generators

A generator uses a coroutine to produce a sequence of values one at a time.

In this page:

Generators

Generators

A generator is a function that yields successive values, and wrapping it in coroutine.wrap gives an iterator usable in a for loop. Values are produced lazily, only when the loop asks for the next one. This works even for infinite sequences, as long as the loop stops on its own.

Note: A for loop stops when the iterator returns nil, so a finished generator ends the loop.

Example: Generators

lua

local function range(n)
  return coroutine.wrap(function()
    for i = 1, n do coroutine.yield(i) end
  end)
end
for v in range(4) do io.write(v, " ") end
print()

local function fib()
  return coroutine.wrap(function()
    local a, b = 0, 1
    while true do
      coroutine.yield(a)
      a, b = b, a + b
    end
  end)
end
for v in fib() do
  if v > 50 then break end
  io.write(v, " ")
end
print()

-- Output:
-- 1 2 3 4
-- 0 1 1 2 3 5 8 13 21 34

Common Mistakes

Building the whole list first instead of yielding
Writing an infinite generator without a break
Calling the generator function instead of using its result

Chapter Summary

Yield values one by one
wrap turns it into an iterator
Values are produced lazily
The loop ends when the function returns nil

🔒

Chapter Quiz — Complete all 7 topics to unlock

0/7 topics done

Complete these topics first:

← Previous Lesson coroutine.wrap Next Lesson → Producers and consumers

Generators

Generators

Chapter Quiz — Complete all 7 topics to unlock

Login to run this code