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import qualified Control.Monad
 
data D c a b = D (c a b) (c b a)
 
apply :: (c a b -> c a2 b2) -> (c b a -> c b2 a2) -> D c a b -> D c a2 b2
apply f1 f2 (D x y) = D (f1 x) (f2 y)
 
applyBoth = Control.Monad.join apply
 
f :: Int -> String
f = show
 
g :: String -> Int
g = read
 
h :: Int -> Int
h = (*2)
 
join :: (a -> b) -> (c -> d) -> ((a, c) -> (b, d))
join f g (x, y) = (f x, g y)
 
x1 = D f g
 
f1 = join h
 
y1 = apply f1 f1 x1 -- This compiles
y2 = apply g g x1 where g = f1 -- Also works
 
z1 = applyBoth f1 x1 -- This fails
 
main = return ()
 

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:29:19:
    Couldn't match type `[Char]' with `Int'
    Expected type: D (->) Int Int
      Actual type: D (->) Int String
    In the second argument of `applyBoth', namely `x1'
    In the expression: applyBoth f1 x1
    In an equation for `z1': z1 = applyBoth f1 x1

Standard output is empty