turtle_loop_vs_nondet.hs

import Control.Monad
import Turtle

main :: IO ()
main = sh $
  forM_ [(1::Int)..2] $ \i -> do
    j <- select [(1::Int)..3]
    liftIO $ print (i, j)

{-
Actual output:
(1,1)
(2,1)
(2,2)
(2,3)
(1,2)
(2,1)
(2,2)
(2,3)
(1,3)
(2,1)
(2,2)
(2,3)

What I expected:
(1,1)
(1,2)
(1,3)
(2,1)
(2,2)
(2,3)
-}

{-
What happens:
Second iteration of the loop (i.e. i=2) is executed INSIDE the branches of
the choice in the first iteration: i.e. (i,j) = (1,1), (1,2), (1,3).
Therefore, we got (2,1), (2,2), (2,3) after each of them.

Lesson learned:
Be careful when using non-determinism inside the loop.
Consider avoiding the mix of loop and non-determinism.
-}