Wrong value/infinite loop in for-loop when using iterators with tail calls

[yuin]

Wrong value/infinite loop in for-loop when using iterators with tail calls

[cschomburg]

I tried to find a workaround for my iterators, but I noticed it does not only affect coroutines. I think it is a problem with tail calls in the iterator function, maybe the stack gets confused.

This example results in an endless loop:

local get_val = function(i)
        if i == 1 then return i, "a" end
        if i == 2 then return i, "b" end
        return nil
end

local my_next = function(invariant, i)
        i = (i or 0) + 1
        print("next:", i)
        return get_val(i)
end

for k, v in my_next do
        print("for:", k, v)
end

Output:

next:   1
for:    1       1
for:    1       1
for:    1       1
... and so on

It looks like the for-loop tries to call the inner function get_val and not the iterator my_next. It does not happen if we inline the get_val or eliminate the tail call by saving its results:

local get_val = function(i)
        if i == 1 then return i, "a" end
        if i == 2 then return i, "b" end
        return nil
end

local my_next = function(invariant, i)
        i = (i or 0) + 1
        print("next:", i)
        local a, b = get_val(i)
        return a, b
end

for k, v in my_next do
        print("for:", k, v)
end

Works correctly:

next:   1
for:    1       a
next:   2
for:    2       b
next:   3

[yuin]

Thank you again, @xconstruct .
I've fixed the bug in dd0fd53 thanks to your detailed report 👍