✨ unitary list is instance monad-fail

Problem:
- The unitary list context is a monad but not monad fail.

Solution:
- Extend the unitary list context to monad fail.

src/unitary-list.lisp

@@ -13,7 +13,10 @@
   "Return the monad operators for a unitary list.
 
 A unitary list is a list with 1 or zero elements"
-  (make-instance 'monad-operators
+  (make-instance 'monad-fail-operators
     :mreturn #'list
     :flatmap (lambda (f mx)
-               (and mx (funcall f (car mx))))))
+               (and mx (funcall f (car mx))))
+    :fail (lambda (str)
+            (declare (ignore str))
+            nil)))

test/unitary-list-test.lisp

@@ -30,7 +30,8 @@
     (is (equal '("X") (ctx-run context (fmap #'symbol-name (pure 'x)))))
     (is (equal '("X") (ctx-run context (fapply (pure #'symbol-name) (pure 'x)))))
     (is (equal '((x y)) (ctx-run context (product (pure 'x) (pure 'y)))))
-    (is (equal '("X") (ctx-run context (flatmap (lambda (x) (mreturn (symbol-name x))) (pure 'x)))))))
+    (is (equal '("X") (ctx-run context (flatmap (lambda (x) (mreturn (symbol-name x))) (pure 'x)))))
+    (is (equal '() (ctx-run context (fail "Yikes!"))))))
 
 (test binding
   (let ((context (make-unitary-list-context)))