September Morn Update

Task/Nth-root/Haskell/nth-root-2.hs

@@ -0,0 +1,28 @@
+import Control.Applicative (liftA2)
+
+nthRoot :: Double -> Double -> Double
+nthRoot n x =
+  fst $
+  until
+    (uncurry (==))
+    (((,) <*> ((/ n) . liftA2 (+) ((n - 1) *) ((x /) . (** (n - 1))))) . snd)
+    (x, x / n)
+
+-- TESTS --------------------------------------------------
+main :: IO ()
+main =
+  putStrLn $
+  fTable
+    "Nth roots:"
+    (\(a, b) -> show a ++ " `nthRoot` " ++ show b)
+    show
+    (uncurry nthRoot)
+    [(2, 2), (5, 34), (10, 734 ^ 10), (0.5, 7)]
+
+-- FORMAT OF RESULTS --------------------------------------
+fTable :: String -> (a -> String) -> (b -> String) -> (a -> b) -> [a] -> String
+fTable s xShow fxShow f xs =
+  let w = maximum (length . xShow <$> xs)
+      rjust n c = liftA2 drop length (replicate n c ++)
+  in unlines $
+     s : fmap (((++) . rjust w ' ' . xShow) <*> ((" -> " ++) . fxShow . f)) xs