new TRY-INPUT-FIELD-VALUE returns #f, if INPUT-FIELD-VALUE fails

scheme/httpd/surflets/packages.scm

@@ -166,6 +166,7 @@
 	  make-submit-button
 	  make-reset-button
 	  make-image-button
+	  try-input-field-value
 	  input-field-value
 	  input-field-binding
 
@@ -192,6 +193,8 @@
 	define-record-types
 	weak				;MAKE-WEAK-POINTER
 	locks
+	let-opt				;:OPTIONAL
+	handle-fatal-error
 	scsh
 	scheme)
   (files servlets))

scheme/httpd/surflets/surflets.scm

@@ -586,6 +586,20 @@
      (else
       (error "no such input-field" input-field bindings)))))
 
+;; Trys to get a value for INPUT-FIELD in BINDINGS. If this fails
+;; (i.e. INPUT-FIELD-VALUE returns an error), the default-value is
+;; returned. The default-value defaults to #f. NOTE: If you do this
+;; with input-fields whose valid values may be the same as the default
+;; value, you cannot determine by the result if there was such a value
+;; or not. Keep in mind, that INPUT-FIELD-VALUE returns also an error,
+;; if there was not such an input field.
+(define (try-input-field-value input-field bindings . maybe-default)
+  (let ((default (:optional maybe-default #f)))
+    (with-fatal-error-handler
+     (lambda (condition more)
+       default)
+     (input-field-value input-field bindings))))
+
 (define (real-input-field-binding input-field bindings)
   (assoc (input-field-name input-field) bindings))