I am trying to understand the concept of Monads and came across this list comprehension syntax for filtering sums from 2 lists.
largeSums = [i+j | i <- [10, 20, 30], j <- [1 , 2] , (i+j)>20] I am trying to rewrite this using the do notation but do not understand what goes inside the else part:
largeSums = do i <- [10, 20, 30] j <- [1 , 2] if i+j > 20 then return (i+j) else An empty list without return will work in that case. (which means 'no result for this combination of (i, j)'. On the other hand, return (i+j) equals [i+j])
largeSums = do i <- [10, 20, 30] j <- [1 , 2] if i+j > 20 then return (i+j) else [] However, it is more idiomatic to use guard :: (Alternative f) => Bool -> f ().
import Control.Monad largeSums = do i <- [10, 20, 30] j <- [1 , 2] guard (i+j > 20) return (i+j) Related links:
pure > return