home-services: Add helper functions for service configurations.

* gnu/home-services/configuration.scm (helper functions): New variables.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add home-services/configuration.scm.

Signed-off-by: Oleg Pykhalov <go.wigust@gmail.com>

gnu/home-services/configuration.scm

@@ -0,0 +1,81 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2021 Andrew Tropin <andrew@trop.in>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(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)))

gnu/local.mk

@@ -76,6 +76,7 @@ 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			\