monads: Fix 'mapm' so that effects happen from left to right.

* guix/monads.scm (mapm): Don't reverse LST, so that items are processed
  from left to right.  Bind the result of 'foldm' and reverse it.
* tests/monads.scm ("sequence"): Change 'frob' so it performs its side
  effect within an 'mlet' body.  Adjust call accordingly.

guix/monads.scm

@@ -209,13 +209,15 @@ (define (foldm monad mproc init lst)
 
 (define (mapm monad mproc lst)
   "Map MPROC over LST, a list of monadic values in MONAD, and return a monadic
-list."
-  (foldm monad
-         (lambda (item result)
-           (mlet monad ((item (mproc item)))
-             (return (cons item result))))
-         '()
-         (reverse lst)))
+list.  LST items are bound from left to right, so effects in MONAD are known
+to happen in that order."
+  (mlet monad ((result (foldm monad
+                              (lambda (item result)
+                                (mlet monad ((item (mproc item)))
+                                  (return (cons item result))))
+                              '()
+                              lst)))
+    (return (reverse result))))
 
 (define-inlinable (sequence monad lst)
   "Turn the list of monadic values LST into a monadic list of values, by

tests/monads.scm

@@ -166,14 +166,16 @@ (define derivation-expression
            (let* ((input (iota 100))
                   (order '()))
              (define (frob i)
-               ;; The side effect here is used to keep track of the order in
-               ;; which monadic values are bound.
-               (set! order (cons i order))
-               i)
+               (mlet monad ((foo (return 'foo)))
+                 ;; The side effect here is used to keep track of the order in
+                 ;; which monadic values are bound.  Perform the side effect
+                 ;; within a '>>=' so that it is performed when the return
+                 ;; value is actually bound.
+                 (set! order (cons i order))
+                 (return i)))
 
              (and (equal? input
-                          (run (sequence monad
-                                         (map (lift1 frob monad) input))))
+                          (run (sequence monad (map frob input))))
 
                   ;; Make sure this is from left to right.
                   (equal? order (reverse input)))))