monads: Add 'mbegin'.

* guix/monads.scm (mbegin): New macro.
* tests/monads.scm ("mbegin"): New test.
* doc/guix.texi (The Store Monad): Document it.

tests/monads.scm

@@ -32,7 +32,7 @@
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-64))
 
-;; Test the (guix store) module.
+;; Test the (guix monads) module.
 
 (define %store
   (open-connection-for-tests))
@@ -99,6 +99,21 @@
          %monads
          %monad-run))
 
+(test-assert "mbegin"
+  (every (lambda (monad run)
+           (with-monad monad
+             (let* ((been-there? #f)
+                    (number (mbegin monad
+                              (return 1)
+                              (begin
+                                (set! been-there? #t)
+                                (return 2))
+                              (return 3))))
+               (and (= (run number) 3)
+                    been-there?))))
+         %monads
+         %monad-run))
+
 (test-assert "mlet* + text-file + package-file"
   (run-with-store %store
     (mlet* %store-monad ((guile (package-file %bootstrap-guile "bin/guile"))