Is Haskell's mapM not lazy?

Question

UPDATE: Okay this question becomes potentially very straightforward.

q <- mapM return [1..]

Why does this never return

Answer 1

Okay this question becomes potentially very straightforward.

q <- mapM return [1..]

Why does this never return?

It's not necessarily true. It depends on the monad you're in.

For example, with the identity monad, you can use the result lazily and it terminates fine:

newtype Identity a = Identity a

instance Monad Identity where
  Identity x >>= k = k x
  return = Identity

-- "foo" is the infinite list of all the positive integers
foo :: [Integer]
Identity foo = do
  q <- mapM return [1..]
  return q

main :: IO ()
main = print $ take 20 foo -- [1 .. 20]