transform.hs

{-
Based on
- https://www.schoolofhaskell.com/school/to-infinity-and-beyond/pick-of-the-week/guide-to-ghc-extensions/list-and-comprehension-extensions
-}

{-# LANGUAGE ParallelListComp, TransformListComp #-}

f :: (Num a, Ord a) => [a] -> [a] -> [(a, a)]
f xs ys = [(x, y) | x <- xs,
                    then (filter (\z -> z > 10))
                  | y <- ys,
                    then reverse]

main = print $ f [1..5] [11..15]

{-
runhaskell transform.hs produces the following errors:

transform.hs:10:43:
    Could not deduce (Ord a1) arising from a use of ‘>’
    from the context (Num a, Ord a)
      bound by the type signature for
                 f :: (Num a, Ord a) => [a] -> [a] -> [(a, a)]
      at transform.hs:8:6-45
    Possible fix:
      add (Ord a1) to the context of
        a type expected by the context: [a1] -> [a1]
    In the expression: z > 10
    In the first argument of ‘filter’, namely ‘(\ z -> z > 10)’
    In the expression: (filter (\ z -> z > 10))

transform.hs:10:45:
    Could not deduce (Num a1) arising from the literal ‘10’
    from the context (Num a, Ord a)
      bound by the type signature for
                 f :: (Num a, Ord a) => [a] -> [a] -> [(a, a)]
      at transform.hs:8:6-45
    Possible fix:
      add (Num a1) to the context of
        a type expected by the context: [a1] -> [a1]
    In the second argument of ‘(>)’, namely ‘10’
    In the expression: z > 10
    In the first argument of ‘filter’, namely ‘(\ z -> z > 10)’

-}