services: configuration: Add a define-maybe/no-serialization syntax.

Before this change, using define-maybe along define-configuration with the
no-serialization syntactic keyword would result in the following warning:

  warning: possibly unbound variable `VARIABLE-NAME'

This change introduces the define-maybe/no-serialization variant that does
away with defining a serialization helper procedure, which makes it possible
to avoid the above warning.

* gnu/services/configuration.scm (define-maybe/no-serialization): New syntax.
(define-maybe-helper): New procedure.
(define-maybe): Define syntax using the above procedure.
* tests/services/configuration.scm (tests): Fix module name.
(custom-number-serializer): Do not print to standard output.
(maybe-number?, serialize-maybe-number): New procedures defined via the
define-maybe macro.
(config-with-maybe-number): New configuration.
(serialize-number): New procedure.
("maybe value serialization"): New test.
(maybe-string?): New procedure defined via the define-maybe/no-serialization
macro.
(config-with-maybe-string/no-serialization): New configuration.
("maybe value without serialization no procedure bound"): New test.

1 files changed, 26 insertions, 11 deletions

diff --git a/gnu/services/configuration.scm b/gnu/services/configuration.scm
index 21cb829382..72b1d1cec6 100644
--- a/gnu/services/configuration.scm
+++ b/gnu/services/configuration.scm
@@ -48,6 +48,7 @@
 
             serialize-configuration
             define-maybe
+            define-maybe/no-serialization
             validate-configuration
             generate-documentation
             configuration->documentation
@@ -107,20 +108,34 @@ does not have a default value" field kind)))
   "Assemble PARTS into a raw (unhygienic) identifier."
   (datum->syntax ctx (symbol-append (syntax->datum parts) ...)))
 
+(define (define-maybe-helper serialize? syn)
+  (syntax-case syn ()
+    ((_ stem)
+     (with-syntax
+         ((stem?            (id #'stem #'stem #'?))
+          (maybe-stem?      (id #'stem #'maybe- #'stem #'?))
+          (serialize-stem   (id #'stem #'serialize- #'stem))
+          (serialize-maybe-stem (id #'stem #'serialize-maybe- #'stem)))
+       #`(begin
+           (define (maybe-stem? val)
+             (or (eq? val 'disabled) (stem? val)))
+           #,@(if serialize?
+                  (list #'(define (serialize-maybe-stem field-name val)
+                            (if (stem? val)
+                                (serialize-stem field-name val)
+                                "")))
+                  '()))))))
+
 (define-syntax define-maybe
   (lambda (x)
-    (syntax-case x ()
+    (syntax-case x (no-serialization)
+      ((_ stem (no-serialization))
+       (define-maybe-helper #f #'(_ stem)))
       ((_ stem)
-       (with-syntax
-           ((stem?                (id #'stem #'stem #'?))
-            (maybe-stem?          (id #'stem #'maybe- #'stem #'?))
-            (serialize-stem       (id #'stem #'serialize- #'stem))
-            (serialize-maybe-stem (id #'stem #'serialize-maybe- #'stem)))
-         #'(begin
-             (define (maybe-stem? val)
-               (or (eq? val 'disabled) (stem? val)))
-             (define (serialize-maybe-stem field-name val)
-               (if (stem? val) (serialize-stem field-name val) ""))))))))
+       (define-maybe-helper #t #'(_ stem))))))
+
+(define-syntax-rule (define-maybe/no-serialization stem)
+  (define-maybe stem (no-serialization)))
 
 (define (define-configuration-helper serialize? syn)
   (syntax-case syn ()