core: implement zero for Maybe and Result

core/Maybe.carp

@@ -66,4 +66,7 @@ It is the inverse of [`just?`](#just?).")
    (if (null? a)
      (Nothing)
      (Just @(Pointer.to-ref a))))
+
+  (doc zero "returns `Nothing`.")
+  (defn zero [] (Nothing))
 )

core/Result.carp

@@ -106,6 +106,9 @@ It is the inverse of [`success?`](#success?).")
         (match @b
           (Success _) false
           (Error y) (= x y))))
+
+  (doc zero "returns `(Success (zero))`.")
+  (defn zero [] (Success (zero)))
 )
 
 (defmodule Maybe