[bug#50208] [PATCH 2/5] home-services: Add helper functions for service configurations.

[Andrew Tropin]

* gnu/home-services/configuration.scm (helper functions): New variables.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add home-services/configuration.scm.
---
 gnu/home-services/configuration.scm | 63 +++++++++++++++++++++++++++++
 gnu/local.mk                        |  1 +
 2 files changed, 64 insertions(+)
 create mode 100644 gnu/home-services/configuration.scm

diff --git a/gnu/home-services/configuration.scm 
b/gnu/home-services/configuration.scm
new file mode 100644
index 0000000000..b7487fc49f
--- /dev/null
+++ b/gnu/home-services/configuration.scm
@@ -0,0 +1,63 @@
+(define-module (gnu home-services configuration)
+  #:use-module (gnu services configuration)
+  #:use-module (guix gexp)
+  #:use-module (srfi srfi-1)
+
+  #:export (filter-configuration-fields
+
+            interpose
+            list-of
+
+            list-of-strings?
+            alist?
+            string-or-gexp?
+           serialize-string-or-gexp
+           text-config?
+            serialize-text-config))
+
+(define* (filter-configuration-fields configuration-fields fields
+                                     #:optional negate?)
+  "Retrieve the fields listed in FIELDS from CONFIGURATION-FIELDS.
+If NEGATE? is @code{#t}, retrieve all fields except FIELDS."
+  (filter (lambda (field)
+            (let ((member? (member (configuration-field-name field) fields)))
+              (if (not negate?) member? (not member?))))
+          configuration-fields))
+
+
+(define* (interpose ls  #:optional (delimiter "\n") (grammar 'infix))
+  "Same as @code{string-join}, but without join and string, returns an
+DELIMITER interposed LS.  Support 'infix and 'suffix GRAMMAR values."
+  (when (not (member grammar '(infix suffix)))
+    (raise
+     (formatted-message
+      (G_ "The GRAMMAR value must be 'infix or 'suffix, but ~a provided.")
+      grammar)))
+  (fold-right (lambda (e acc)
+               (cons e
+                     (if (and (null? acc) (eq? grammar 'infix))
+                         acc
+                         (cons delimiter acc))))
+             '() ls))
+
+(define (list-of pred?)
+  "Return a procedure that takes a list and check if all the elements of
+the list result in @code{#t} when applying PRED? on them."
+    (lambda (x)
+      (if (list? x)
+          (every pred? x)
+          #f)))
+
+
+(define list-of-strings?
+  (list-of string?))
+
+(define alist? list?)
+
+(define (string-or-gexp? sg) (or (string? sg) (gexp? sg)))
+(define (serialize-string-or-gexp field-name val) "")
+
+(define (text-config? config)
+  (and (list? config) (every string-or-gexp? config)))
+(define (serialize-text-config field-name val)
+  #~(string-append #$@(interpose val "\n" 'suffix)))
diff --git a/gnu/local.mk b/gnu/local.mk
index e25ff3db53..e24da4716f 100644
--- a/gnu/local.mk
+++ b/gnu/local.mk
@@ -75,6 +75,7 @@ GNU_SYSTEM_MODULES =                          \
   %D%/home-services.scm                         \
   %D%/home-services/symlink-manager.scm         \
   %D%/home-services/fontutils.scm               \
+  %D%/home-services/configuration.scm           \
   %D%/image.scm                                        \
   %D%/packages.scm                             \
   %D%/packages/abduco.scm                      \
-- 
2.33.0

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