fixing string-trim resulting in empty string

lib/chibi/strings.scm

@@ -81,10 +81,12 @@
                       (string-skip-right str pred))))
 
 (define (string-trim str . o)
-  (let ((pred (make-char-predicate (if (pair? o) (car o) #\space))))
+  (let* ((pred (if (pair? o) (car o) #\space))
-    (substring-cursor str
+         (left (string-skip str pred))
-                      (string-skip str pred)
+         (right (string-skip-right str pred)))
-                      (string-skip-right str pred))))
+    (if (string-cursor>=? left right)
+        ""
+        (substring-cursor str left right))))
 
 (define (string-mismatch prefix str)
   (let ((end1 (string-cursor-end prefix))

tests/string-tests.scm

@@ -59,6 +59,9 @@
 (test "abc" (string-trim "  abc"))
 (test "abc" (string-trim "abc  "))
 (test "abc" (string-trim "  abc  "))
+(test "" (string-trim ""))
+(test "" (string-trim " "))
+(test "" (string-trim "  "))
 
 (test #t (string-prefix? "abc" "abc"))
 (test #t (string-prefix? "abc" "abcde"))