services: configuration: Support (field1 maybe-number "") format.

As opposed to explicitly using 'disabled as value, or using the
(field1 (maybe-number) "") format.

It's mostly the work of Maxime Devos shared under #54674, with some
modifications by Attila Lendvai.

* gnu/services/configuration.scm (normalize-field-type+def): New function.
(define-configuration-helper) (define-configuration): Support new field
format.
* tests/services/configuration.scm (config-with-maybe-number->string): New
function.
("maybe value serialization of the instance"): New test.
("maybe value serialization of the instance, unspecified"): New test.

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

gnu/services/configuration.scm

@@ -5,6 +5,7 @@
 ;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
 ;;; Copyright © 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2021 Andrew Tropin <andrew@trop.in>
+;;; Copyright © 2022 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -162,10 +163,26 @@ (define-maybe-helper #t #f #'(_ stem))))))
 (define-syntax-rule (define-maybe/no-serialization stem)
   (define-maybe stem (no-serialization)))
 
+(define (normalize-field-type+def s)
+  (syntax-case s ()
+    ((field-type def)
+     (identifier? #'field-type)
+     (values #'(field-type def)))
+    ((field-type)
+     (identifier? #'field-type)
+     (values #'(field-type 'disabled)))
+    (field-type
+     (identifier? #'field-type)
+     (values #'(field-type 'disabled)))))
+
 (define (define-configuration-helper serialize? serializer-prefix syn)
   (syntax-case syn ()
-    ((_ stem (field (field-type def ...) doc custom-serializer ...) ...)
+    ((_ stem (field field-type+def doc custom-serializer ...) ...)
-     (with-syntax (((field-getter ...)
+     (with-syntax
+         ((((field-type def) ...)
+           (map normalize-field-type+def #'(field-type+def ...))))
+       (with-syntax
+           (((field-getter ...)
              (map (lambda (field)
                     (id #'stem #'stem #'- field))
                   #'(field ...)))
@@ -176,12 +193,8 @@ (define (define-configuration-helper serialize? serializer-prefix syn)
             ((field-default ...)
              (map (match-lambda
                     ((field-type default-value)
-                            default-value)
+                     default-value))
-    			   ((field-type)
+                  #'((field-type def) ...)))
-                            ;; Quote `undefined' to prevent a possibly
-                            ;; unbound warning.
-                            (syntax 'undefined)))
-    			 #'((field-type def ...) ...)))
             ((field-serializer ...)
              (map (lambda (type custom-serializer)
                     (and serialize?
@@ -233,7 +246,7 @@ (define-syntax-rule (stem arg (... ...))
                (let ((conf (#,(id #'stem #'% #'stem) arg (... ...))))
                  (validate-configuration conf
                                          #,(id #'stem #'stem #'-fields))
-    	       conf)))))))
+                 conf))))))))
 
 (define no-serialization         ;syntactic keyword for 'define-configuration'
   '(no serialization))
@@ -241,26 +254,26 @@ (define no-serialization         ;syntactic keyword for 'define-configuration'
 (define-syntax define-configuration
   (lambda (s)
     (syntax-case s (no-serialization prefix)
-      ((_ stem (field (field-type def ...) doc custom-serializer ...) ...
+      ((_ stem (field field-type+def doc custom-serializer ...) ...
           (no-serialization))
        (define-configuration-helper
-         #f #f #'(_ stem (field (field-type def ...) doc custom-serializer ...)
+         #f #f #'(_ stem (field field-type+def doc custom-serializer ...)
                  ...)))
-      ((_ stem  (field (field-type def ...) doc custom-serializer ...) ...
+      ((_ stem  (field field-type+def doc custom-serializer ...) ...
           (prefix serializer-prefix))
        (define-configuration-helper
-         #t #'serializer-prefix #'(_ stem (field (field-type def ...)
+         #t #'serializer-prefix #'(_ stem (field field-type+def
                                                  doc custom-serializer ...)
                  ...)))
-      ((_ stem (field (field-type def ...) doc custom-serializer ...) ...)
+      ((_ stem (field field-type+def doc custom-serializer ...) ...)
        (define-configuration-helper
-         #t #f #'(_ stem (field (field-type def ...) doc custom-serializer ...)
+         #t #f #'(_ stem (field field-type+def doc custom-serializer ...)
                  ...))))))
 
 (define-syntax-rule (define-configuration/no-serialization
-                      stem (field (field-type def ...)
+                      stem (field field-type+def
                                   doc custom-serializer ...) ...)
-  (define-configuration stem (field (field-type def ...)
+  (define-configuration stem (field field-type+def
                                     doc custom-serializer ...) ...
     (no-serialization)))
 

tests/services/configuration.scm

@@ -27,6 +27,9 @@ (define-module (tests services configuration)
 
 (test-begin "services-configuration")
 
+(define (serialize-number field value)
+  (format #f "~a=~a" field value))
+
 
 ;;;
 ;;; define-configuration macro.
@@ -47,7 +50,6 @@ (define-configuration port-configuration-cs
   80
   (port-configuration-cs-port (port-configuration-cs)))
 
-(define serialize-number "")
 (define-configuration port-configuration-ndv
   (port (number) "The port number."))
 
@@ -101,15 +103,31 @@ (define-configuration configuration-with-prefix
 (define-maybe number)
 
 (define-configuration config-with-maybe-number
-  (port (maybe-number 80) "The port number."))
+  (port  (maybe-number 80) "")
-
+  (count maybe-number ""))
-(define (serialize-number field value)
-  (format #f "~a=~a" field value))
 
 (test-equal "maybe value serialization"
   "port=80"
   (serialize-maybe-number "port" 80))
 
+(define (config-with-maybe-number->string x)
+  (eval (gexp->approximate-sexp
+         (serialize-configuration x config-with-maybe-number-fields))
+        (current-module)))
+
+(test-equal "maybe value serialization of the instance"
+  "port=42count=43"
+  (config-with-maybe-number->string
+   (config-with-maybe-number
+    (port 42)
+    (count 43))))
+
+(test-equal "maybe value serialization of the instance, unspecified"
+  "port=42"
+  (config-with-maybe-number->string
+   (config-with-maybe-number
+    (port 42))))
+
 (define-maybe/no-serialization string)
 
 (define-configuration config-with-maybe-string/no-serialization