gistfile1.txt

"the cow ate the grass"
"the cow ate the grass"

app/Main.hs:34:8-15: error:
    • Occurs check: cannot construct the 
infinite type: a ~ a -> a
      Expected type: [a]
        Actual type: [a -> a]
    • In the expression: map f xs
      In the expression:
        let
          a' = f a b
          xs = b : cs
        in map f xs
      In an equation for ‘map2’:
          map2 f (a : b : cs)
            = let
                a' = f a b
                xs = b : cs
              in map f xs
    • Relevant bindings include
        a' :: a (bound at app/Main.hs:32:9)
        xs :: [a] (bound at app/Main.hs:33:9)
        cs :: [a] (bound at app/Main.hs:31:13)
        b :: a (bound at app/Main.hs:31:11)
        a :: a (bound at app/Main.hs:31:9)
        f :: a -> a -> a (bound at app/Main.hs:31:6)
        (Some bindings suppressed; use 
-fmax-relevant-binds=N or -fno-max-relevant-binds)
   |
34 |     in map f xs
   |        ^^^^^^^^

map2

map2 :: (a -> a -> a) -> [a] -> [a]
map2 f [] = []
map2 f [a] = [a]
map2 f (a:[b]) = [f a b, b]
map2 f (a:b:cs) = 
    let a' = f a b
        xs = b:cs
    in map f xs

main :: IO ()
main = do
    let s = "the cow ate the grass"
    print s
    print $ map2 (\c1 c2 -> if c1 `elem` "aeiou" then '!' else c1) s