only 1 for - bit hacky but it works - the case can be replaced with something that better scales to longer lists (put_in + Access.at ?)
for x <- 1..3, y <- 1..3, reduce: [[], [], []] do
[a, b, c] -> case x do
1 -> [a ++[{x, y}], b, c]
2 -> [a, b ++ [{x, y}], c]
3 -> [a, b, c ++ [{x, y}]]
end
end
I’m not sure whether to be proud or ashamed of myself 
iex(40)> for x <- 1..3, row = (for y <- 1..3, do: {x,y}), z <- 1..3, x == z, do: row
[[{1, 1}, {1, 2}, {1, 3}], [{2, 1}, {2, 2}, {2, 3}], [{3, 1}, {3, 2}, {3, 3}]]
previous version but without case
for x <- 3..1, y <- 3..1, reduce: [[], [], []] do
acc ->
current = Enum.at(acc, x - 1)
new = [{x, y} | current]
List.replace_at(acc, x - 1, new)
end
I had initially misread the conditions, which was “one” comprehension with three generators. I thought nested would work, I don’t really think any meaningful way makes it possible to convert that with that condition and either it’s an error, or I’d really like to know the answer (and take that course!).
The wording was indeed incorrect - no specific amount of generators is needed to solve the exercise. It will be changed with a PR. Thanks everyone for your help!
