replaceMin problem & value recursion

gistfile1.hs

{-# LANGUAGE DoRec #-}

data Tree a = L a | B (Tree a) (Tree a)
    deriving (Show)

rpMinWithIO :: (Ord a, Show a) => (Tree a, a) -> IO (Tree a, a)
rpMinWithIO (L a, m)   = print a >> return (L m, a)
rpMinWithIO (B l r, m) =
    rpMinWithIO (l, m) >>=
        \(l', lmin) ->
            rpMinWithIO (r, m) >>=
                \(r', rmin) ->
                    return (B l' r', min lmin rmin)

replaceMinWithIO :: (Ord a, Show a) => Tree a -> IO (Tree a)
replaceMinWithIO tree = do { rec (tree', m) <- rpMinWithIO (tree, m)
                           ; return tree'
                           }