Lua iterator using an iterator - lua

Question

I have a function defined as a C binding that provides a generic for iterator:
for el in doc:each() do ... end
I want to write an iterator in Lua that iterates using this function, but return a modification of each result. How can I do this?
Edit: I'm sure my iterator has to start something like this, but I'm getting lost in the body of the function.
function myiterator()
local f, c, v = doc:each()
return (function(c2, v2)
-- ??
end), ??, ??
end

Answer 1

function myiterator()
local generator, state, prev_x = doc:each()
local function my_generator()
local x, y = generator(state, prev_x)
if x ~= nil then
prev_x = x
-- modify x, y
local modified_x = x + 100
local modified_y = "("..y..")"
-- modified_x must be non-nil
return modified_x, modified_y
end
end
return my_generator
end
Before:
local doc = {each = function() return ipairs{"aa", "bb", "cc"} end}
for x, y in doc:each() do
print(x, y)
end
Output:
1 aa
2 bb
3 cc
After:
local doc = {each = function() return ipairs{"aa", "bb", "cc"} end}
-- insert myiterator definition here
for x, y in myiterator() do
print(x, y) -- now x and y are modified
end
Output:
101 (aa)
102 (bb)
103 (cc)

Answer 2

I don't know how your C function works, but here is an iterator function that should do what you are looking for. It calls an iterator triplet repeatedly, takes the first return value, and calls a function on it to return a new value:
function map(transformer, f, c, v)
return function()
v = f(c, v)
if v ~= nil then
return transformer(v)
end
end
end
For instance, this takes the keys in the table { 'a', 'b', 'c' } and squares them. The second return value (the values corresponding to the keys) is ignored:
for v in map(function (x) return x * x end, pairs { 'a', 'b', 'c' }) do
print(v)
end
You can do map(function (elem) return do_something_to(elem) end, doc:each()).
It's easier to read the map function when it's written using a coroutine:
function map(transformer, f, c, v)
return coroutine.wrap(function ()
for val in f, c, v do
coroutine.yield(transformer(val))
end
end)
end
For completeness, either of these functions would allow you to use two return values from the original iterator triplet:
function map2(transformer, f, c, v)
return function()
local v2
v, v2 = f(c, v)
if v ~= nil then
return transformer(v, v2)
end
end
end
function map(transformer, f, c, v)
return coroutine.wrap(function ()
for v, v2 in f, c, v do
coroutine.yield(transformer(v, v2))
end
end)
end
for v in map2(function (a, b) return a .. b end, pairs { 'a', 'b', 'c' }) do
print(v)
end
-- This prints out:
-- 1a
-- 2b
-- 3c

Answer 3

You could wrap the iterator into a coroutine, see also https://www.lua.org/pil/9.3.html
-- dummy object
local doc = {
each = function()
return pairs{ 11, 22, 33 }
end
}
local myiterator = coroutine.wrap(function()
local f, c, v = doc:each()
return f, c, v
end)
for f, c, v in myiterator() do
print(f, c, v)
end
$ lua test.lua
1 11 nil
2 22 nil
3 33 nil

Answer 4

Thanks to those that helped. I learned that my iterator function does not HAVE to return 2 or 3 values. This is a valid list-table iterator:
function every(list)
local i=0
return function() -- no arguments used!
i = i+1
return list[i]
end
end
for word in every{'foo', 'bar', 'jim'} do print(word) end
--> foo
--> bar
--> jim
Because every() does not return a second value (the "invariant" value), a nil is passed as the first parameter to the anonymous function on each iteration. The second value passed to that anonymous function is the value returned from the function on a previous pass (or the third return value from every() on the very first pass)...but if that does not help us iterate, we don't need to use it.
Because none of the answers so far exactly answered what I needed, here is the solution I ended up with:
function myiterator()
local f, c, v = doc:each()
return function()
v = f(c,v)
if v then
-- NOTE: Do NOT change the `v` variable here, because
-- f() may expect it to be exactly what it returned
-- in order to iterator properly on the next invocation.
local myvalue = mutate(v)
return myvalue
end
end
end