Cleaner iteration of expression.

It's a destructive operation.  It breaks functional purity (but it could
be made functional by using a stack instead of a list for the stack.)

There's no point to returning the expression object now because it's
being mutated by the prepend() method (rather than being replaced by new
forms.)

implementations/Python/simplejoy.py

@@ -93,8 +93,7 @@ def joy(stack, expression, dictionary):
 
     '''
     expr = Expression(expression)
-    while expr:
-        term, expr = expr
+    for term in expr:
         if isinstance(term, Symbol):
             try:
                 func = dictionary[term]
@@ -164,6 +163,10 @@ tuples used in expressions where they would be redundant.)
 '''
 
 
+class StackUnderflowError(Exception):
+    pass
+
+
 def list_to_stack(el, stack=()):
     '''
     Convert a Python list (or other sequence) to a Joy stack::
@@ -269,7 +272,7 @@ class Expression:
         self.stack = []
 
     def __iter__(self):
-        return iter((self.__next__(), self))
+        return self
 
     def __next__(self):
         if self.current:
@@ -507,8 +510,7 @@ def run(text, stack, dictionary):
     :rtype: (stack, (), dictionary)
 
     '''
-    expr = text_to_expression(text)
-    return joy(stack, expr, dictionary)
+    return joy(stack, text_to_expression(text), dictionary)
 
 
 def interp(stack=(), dictionary=None):
@@ -1308,10 +1310,6 @@ class NotABoolError(Exception):
     pass
 
 
-class StackUnderflowError(Exception):
-    pass
-
-
 class UnknownSymbolError(KeyError):
     pass