Nested loop equivalent

Question

I want to do a list of concatenations in Haskell. I have [1,2,3] and [4,5,6] and i want to produce [14,15,16,24,25,26,34,35,36]. I know I can use zipWith or sth, but how to do equivalent of: foreach in first_array foreach in second_array

I guess I have to use map and half curried functions, but can't really make it alone :S

Answer 1

You could use list comprehension to do it:

[x * 10 + y | x <- [1..3], y <- [4..6]]

In fact this is a direct translation of a nested loop, since the first one is the outer / slower index, and the second one is the faster / inner index.

Answer 2

You can exploit the fact that lists are monads and use the do notation:

do
  a <- [1, 2, 3]
  b <- [4, 5, 6]
  return $ a * 10 + b

You can also exploit the fact that lists are applicative functors (assuming you have Control.Applicative imported):

(+) <$> (*10) <$> [1,2,3] <*> [4,5,6]

Both result in the following:

[14,15,16,24,25,26,34,35,36]