monads, doc: Improve mwhen and munless documentation.

* doc/guix.texi (The Store Monad) <mwhen, munless>: Document them.
* guix/monads.scm (mwhen, munless): Clarify their intended use.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

2 files changed, 22 insertions, 4 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index d0f21edeec..62f4483b85 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -4054,6 +4054,20 @@ monadic expressions are ignored.  In that sense, it is analogous to
 @code{begin}, but applied to monadic expressions.
 @end deffn
 
+@deffn {Scheme System} mwhen @var{condition} @var{mexp0} @var{mexp*} ...
+When @var{condition} is true, evaluate the sequence of monadic
+expressions @var{mexp0}..@var{mexp*} as in an @code{mbegin}.  When
+@var{condition} is false, return @code{*unspecified*} in the current
+monad.  Every expression in the sequence must be a monadic expression.
+@end deffn
+
+@deffn {Scheme System} munless @var{condition} @var{mexp0} @var{mexp*} ...
+When @var{condition} is false, evaluate the sequence of monadic
+expressions @var{mexp0}..@var{mexp*} as in an @code{mbegin}.  When
+@var{condition} is true, return @code{*unspecified*} in the current
+monad.  Every expression in the sequence must be a monadic expression.
+@end deffn
+
 @cindex state monad
 The @code{(guix monads)} module provides the @dfn{state monad}, which
 allows an additional value---the state---to be @emph{threaded} through
diff --git a/guix/monads.scm b/guix/monads.scm
index 6933f7f151..fe3d5d78f1 100644
--- a/guix/monads.scm
+++ b/guix/monads.scm
@@ -204,8 +204,10 @@ the last one."
 
 (define-syntax mwhen
   (syntax-rules ()
-    "When CONDITION is true, evaluate MEXP0..MEXP* as in an 'mbegin'.  When
-CONDITION is false, return *unspecified* in the current monad."
+    "When CONDITION is true, evaluate the sequence of monadic expressions
+MEXP0..MEXP* as in an 'mbegin'.  When CONDITION is false, return *unspecified*
+in the current monad.  Every expression in the sequence must be a monadic
+expression."
     ((_ condition mexp0 mexp* ...)
      (if condition
          (mbegin %current-monad
@@ -214,8 +216,10 @@ CONDITION is false, return *unspecified* in the current monad."
 
 (define-syntax munless
   (syntax-rules ()
-    "When CONDITION is false, evaluate MEXP0..MEXP* as in an 'mbegin'.  When
-CONDITION is true, return *unspecified* in the current monad."
+    "When CONDITION is false, evaluate the sequence of monadic expressions
+MEXP0..MEXP* as in an 'mbegin'.  When CONDITION is true, return *unspecified*
+in the current monad.  Every expression in the sequence must be a monadic
+expression."
     ((_ condition mexp0 mexp* ...)
      (if condition
          (return *unspecified*)