could not deduce b ~ b1

main.hs

module Main where

class A a where
class B a where

foo :: (A a, B b) => IO a -> (a -> IO b) -> IO ()
foo _ func = return ()
    where
        -- this breaks because func is not passed in
        barBroken :: (A a, B b) => a -> IO b
        barBroken = func

        barWorks :: (A a, B b) => (a -> IO b) -> a -> IO b
        barWorks f a = f a
        
main = return ()