June 2018 Update

Task/Anonymous-recursion/Haskell/anonymous-recursion-4.hs

@@ -0,0 +1,23 @@
+fib :: Integer -> Maybe Integer
+fib n
+  | n < 0 = Nothing
+  | otherwise =
+    Just $
+    (\f ->
+        let x = f x
+        in x)
+      (\f n ->
+          if n > 1
+            then f (n - 1) + f (n - 2)
+            else 1)
+      n
+
+-- TEST ----------------------------------------------------------------------
+main :: IO ()
+main =
+  print $
+  fib <$> [-4 .. 10] >>=
+  \m ->
+     case m of
+       Just x -> [x]
+       _ -> []