8行目をコメント解除するとコンパイル通らない

gistfile1.hs

main = print $ binsearch (\x -> x^2 - 2) (0,10) 1e-20

binsearch :: Fractional a => Ord a => (a -> a) -> (a,a) -> a -> Maybe a
binsearch f (from,to) eps =
  if (f from)*(f to) > 0 then Nothing
  else Just $ binsearch' (from,to) from
  where
--     binsearch' :: (a,a) -> a -> a
    binsearch' (from',to') bef =
      let mid = (from'+to') / fromRational 2
      in if abs (bef-mid) <= eps then mid
            else if f mid < fromRational 0 then binsearch' (mid,to') mid
                 else binsearch' (from',mid) mid