yaml: Display arrays with a single atomic element on the same line.

* ccwl/yaml.scm (atom?): New function.
(display-dictionary-entry, scm->yaml): Display arrays with a single
atomic element on the same line.
* tests/yaml.scm ("strings with hyphen characters should be escaped",
"strings with asterisk characters should be escaped"): Update tests
with new behaviour.
("single element vectors must be serialized on the same line"): New
test.

2 files changed, 28 insertions, 5 deletions

diff --git a/ccwl/yaml.scm b/ccwl/yaml.scm
index 8a2b7a5..07ed913 100644
--- a/ccwl/yaml.scm
+++ b/ccwl/yaml.scm
@@ -34,6 +34,14 @@
   #:export (scm->yaml
             scm->yaml-string))
 
+(define (atom? x)
+  "Return @code{#t} if @var{x} is a primitive element that can be
+serialized to YAML. Else, return @code{#f}."
+  (or (symbol? x)
+      (boolean? x)
+      (number? x)
+      (string? x)))
+
 (define (display-atom atom port)
   "Display ATOM in PORT."
   (cond
@@ -68,9 +76,11 @@
      (display-atom key port)
      (display ":" port)
      (match value
-       ;; If value is an empty array or dictionary, display it on the
-       ;; same line.
-       ((or #() ())
+       ;; Display on the same line if value is
+       ;; - an empty array
+       ;; - an empty dictionary
+       ;; - an array with an atom as its only element
+       ((or #() () #((? atom? _)))
         (display " " port)
         (scm->yaml value port level))
        ;; Else, display it on the next line.
@@ -83,6 +93,12 @@
   "Convert SCM, an S-expression tree, to YAML and display to
 PORT. LEVEL is an internal recursion variable."
   (match scm
+    ;; Display arrays with a single atomic element on the same line.
+    (#((? atom? single-element))
+     (display "[" port)
+     (display-atom single-element port)
+     (display "]" port)
+     (newline port))
     (#(head tail ...)
      (display-array-element head port level)
      (for-each (lambda (element)
diff --git a/tests/yaml.scm b/tests/yaml.scm
index 545b49c..d3d01c2 100644
--- a/tests/yaml.scm
+++ b/tests/yaml.scm
@@ -31,12 +31,19 @@ bar: {}
 
 (test-equal "strings with hyphen characters should be escaped"
   "- \"-1\"
+- \"-2\"
 "
-  (scm->yaml-string #("-1")))
+  (scm->yaml-string #("-1" "-2")))
 
 (test-equal "strings with asterisk characters should be escaped"
   "- \"*foo\"
+- \"*bar\"
 "
-  (scm->yaml-string #("*foo")))
+  (scm->yaml-string #("*foo" "*bar")))
+
+(test-equal "single element vectors must be serialized on the same line"
+  "[foo]
+"
+  (scm->yaml-string #("foo")))
 
 (test-end "yaml")