vvv

import Data.Foldable (foldl')
import Debug.Trace (trace)

prnEqual :: (Eq a) => a -> a -> IO ()
prnEqual a b = if a == b then print "True" else print "False"

-- Note the absence of parentheses around `Eq a` and having exactly one `print`.
prnEqual' :: Eq a => a -> a -> IO ()
prnEqual' a b = print $ if a == b then "True" else "False"

vvv

{-- XXX Your definition of `checkChar` is okay, but the abstraction itself
 -- is very odd.  It is fine as an exercise of using "do" notation and
 -- `return`. In real code though explicit statements are both short and
 -- expressive (see lines 21-22).
checkChar :: IO Bool
checkChar = do
    c <- getChar
    return (c == 'y')
--}

vvv

listsum :: Num a => [a] -> a
listsum [] = 0
listsum (x:xs) = x + listsum xs

main :: IO ()
main = print $ listsum [1..5]