monads: Fix 'anym'.

* guix/monads.scm (anym): Fix successful case.
* tests/monads.scm ("anym"): New test.

guix/monads.scm

@@ -228,9 +228,10 @@ lifted in MONAD, for which PROC returns true."
         (()
          (return #f))
         ((head tail ...)
-         (mlet monad ((value head))
+         (mlet* monad ((value  head)
-           (or (and=> (proc value) return)
+                       (result -> (proc value)))
-               head
+           (if result
+               (return result)
                (loop tail))))))))
 
 (define-syntax listm

tests/monads.scm

@@ -163,6 +163,18 @@
          %monads
          %monad-run))
 
+(test-assert "anym"
+  (every (lambda (monad run)
+           (eq? (run (with-monad monad
+                       (let ((lst (list (return 1) (return 2) (return 3))))
+                         (anym monad
+                               (lambda (x)
+                                 (and (odd? x) 'odd!))
+                               lst))))
+                'odd!))
+         %monads
+         %monad-run))
+
 (test-end "monads")