From b8b129ebd8d017c957094f3d977a1c452d7d450f Mon Sep 17 00:00:00 2001
From: Eric Bavier <bavier@member.fsf.org>
Date: Tue, 20 Sep 2016 15:41:31 -0500
Subject: utils: Support defaults in substitute-keyword-arguments.

* guix/utils.scm (collect-default-args, expand-default-args): New
syntax.
(substitute-keyword-arguments): Allow default value declarations.
* tests/utils.scm (substitute-keyword-arguments): New test.
---
 guix/utils.scm  | 19 +++++++++++++++----
 tests/utils.scm | 20 ++++++++++++++++++++
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/guix/utils.scm b/guix/utils.scm
index ded31142aa..decadf64a6 100644
--- a/guix/utils.scm
+++ b/guix/utils.scm
@@ -375,13 +375,24 @@ keywords not already present in ARGS."
       (()
        args))))
 
+(define-syntax collect-default-args
+  (syntax-rules ()
+    ((_)
+     '())
+    ((_ (_ _) rest ...)
+     (collect-default-args rest ...))
+    ((_ (kw _ dflt) rest ...)
+     (cons* kw dflt (collect-default-args rest ...)))))
+
 (define-syntax substitute-keyword-arguments
   (syntax-rules ()
     "Return a new list of arguments where the value for keyword arg KW is
-replaced by EXP.  EXP is evaluated in a context where VAR is boud to the
-previous value of the keyword argument."
-    ((_ original-args ((kw var) exp) ...)
-     (let loop ((args    original-args)
+replaced by EXP.  EXP is evaluated in a context where VAR is bound to the
+previous value of the keyword argument, or DFLT if given."
+    ((_ original-args ((kw var dflt ...) exp) ...)
+     (let loop ((args (default-keyword-arguments
+                        original-args
+                        (collect-default-args (kw var dflt ...) ...)))
                 (before '()))
        (match args
          ((kw var rest (... ...))
diff --git a/tests/utils.scm b/tests/utils.scm
index 960928c842..bcfaa14faa 100644
--- a/tests/utils.scm
+++ b/tests/utils.scm
@@ -123,6 +123,26 @@
         (default-keyword-arguments '(#:bar 3) '(#:foo 2))
         (default-keyword-arguments '(#:foo 2 #:bar 3) '(#:bar 6))))
 
+(test-equal "substitute-keyword-arguments"
+  '((#:foo 3)
+    (#:foo 3)
+    (#:foo 3 #:bar (1 2))
+    (#:bar (1 2) #:foo 3)
+    (#:foo 3))
+  (list (substitute-keyword-arguments '(#:foo 2)
+          ((#:foo f) (1+ f)))
+        (substitute-keyword-arguments '()
+          ((#:foo f 2) (1+ f)))
+        (substitute-keyword-arguments '(#:foo 2 #:bar (2))
+          ((#:foo f) (1+ f))
+          ((#:bar b) (cons 1 b)))
+        (substitute-keyword-arguments '(#:foo 2)
+          ((#:foo _) 3)
+          ((#:bar b '(2)) (cons 1 b)))
+        (substitute-keyword-arguments '(#:foo 2)
+          ((#:foo f 1) (1+ f))
+          ((#:bar b) (cons 42 b)))))
+
 (test-assert "filtered-port, file"
   (let* ((file  (search-path %load-path "guix.scm"))
          (input (open-file file "r0b")))
-- 
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