make iterator-advance only take integers

1 files changed, 17 insertions, 10 deletions

diff --git a/generic-iterator.scm b/generic-iterator.scm
index 3c76400..ee5ac9e 100644
--- a/generic-iterator.scm
+++ b/generic-iterator.scm
@@ -29,17 +29,23 @@
   (comparison get-comparison-procedure)
   (private iterator-get-private))
 
+(define-syntax define-with-field-of-iterator
+  (syntax-rules ()
+    ((_ (name field args ...) emsg body ...)
+     (define (name iterator args ...)
+       (let ((field (field iterator)))
+         (if (not field)
+             (error emsg iterator args ...)
+             (begin body ...)))))))
+
 ;;; Define a function that invokes a field of the iterator on the data
 ;;; object inside the iterator and any other arguments supplied to the
 ;;; function.
 (define-syntax define-invoke-field-of-iterator
   (syntax-rules ()
-    ((define-invoker name field-accessor emsg args ...)
-     (define (name iterator args ...)
-       (let ((proc (field-accessor iterator)))
-         (if (not proc)
-             (error emsg iterator)
-             (proc args ...)))))))
+    ((_ name field-accessor emsg args ...)
+     (define-with-field-of-iterator (name field-accessor args ...) emsg
+       (field-accessor args ...)))))
 
 (define-invoke-field-of-iterator iterator-at-start?
   get-start-predicate
@@ -48,10 +54,11 @@
   get-end-predicate
   "no end predicate")
 
-(define-invoke-field-of-iterator iterator-advance
-  get-advance
-  "no procedure to move iterator"
-  spaces)
+(define-with-field-of-iterator (iterator-advance get-advance spaces)
+  "no advance procedure"
+  (if (not (integer? spaces))
+      (error "must advance an integer amount" spaces)
+      (get-advance spaces)))
 
 (define-invoke-field-of-iterator iterator-ref
   get-ref