monads: Fix 'liftN' fallback case.

Reported by Andy Wingo <wingo@igalia.com>.

* guix/monads.scm (define-lift) <fallback case>: Add missing #'.  Remove
  extra formal parameter.
* tests/monads.scm ("lift"): Add test with 'lift1' as a procedure.

guix/monads.scm

@@ -239,10 +239,10 @@ (define-syntax liftn
             (identifier? #'id)
             ;; Slow path: Return a closure-returning procedure (we don't
             ;; guarantee (eq? LIFTN LIFTN), but that's fine.)
-            (lambda (liftn proc monad)
-              (lambda (args ...)
-                (with-monad monad
-                  (return (proc args ...))))))))))))
+            #'(lambda (proc monad)
+                (lambda (args ...)
+                  (with-monad monad
+                    (return (proc args ...))))))))))))
 
 (define-lift lift0 ())
 (define-lift lift1 (a))

tests/monads.scm

@@ -95,10 +95,12 @@ (define (g x)
 
 (test-assert "lift"
   (every (lambda (monad run)
-           (let ((f (lift1 1+ monad)))
+           (let ((f (lift1 1+ monad))
+                 (g (apply lift1 1+ (list monad))))
              (with-monad monad
                (let ((number (random 777)))
                  (= (run (>>= (return number) f))
+                    (run (>>= (return number) g))
                     (1+ number))))))
          %monads
          %monad-run))