More robust version of (scheme lazy)

CHANGELOG.md

@@ -10,6 +10,7 @@ Features
 
 Bug Fixes
 
+- Improved `(scheme lazy)` to allow `force` and `make-promise` to accept an argument of any type. Improved representation of promises to more precisely differentiate them from other objects.
 - Add type checking to record type accessor functions. We now raise an error if the passed object is of the wrong record type.
 - Fix issues with expanding `cond-expand` expressions in libraries. Previously there would be issues with the expansion if the code needed to be within the context of a `begin`.
 

scheme/lazy.sld

@@ -17,9 +17,19 @@
   )
   (begin
 
+  ;; promise
+  ;; ( tag value/obj )
+
+  (define *promise-tag* '(promise))
+  (define (promise? obj)
+    (and (pair? obj)
+         (eq? *promise-tag* (car obj))))
+
   (define force
-      (lambda (object)
+      (lambda (obj)
-        (object)))
+        (if (promise? obj)
+            ((cdr obj))
+            obj)))
 
   (define-syntax delay
     (er-macro-transformer
@@ -32,20 +42,23 @@
        `(make-promise (lambda () ,(cadr expr))))))
 
   (define make-promise
-    (lambda (proc)
+    (lambda (obj)
+      (if (promise? obj)
+          obj
           (let ((result-ready? #f)
                 (result #f))
+            (cons
+              *promise-tag*
               (lambda ()
                 (if result-ready? 
                     result
-              (let ((x (proc)))
+                    (let ((x (if (procedure? obj)
+                                 (obj)
+                                 obj)))
                       (if result-ready?
                           result
                           (begin (set! result x)
                                  (set! result-ready? #t)
-                           result))))))))
+                                 result))))))))))
 
-  ;; Not a very satisfying implementation, but would need to change
-  ;; how promises are stored to do better
-  (define promise? procedure?)
 ))