Switch back to spaces for indentation.

For better or worse, Python 3 won.  No need to be shitty about it, eh?

joy/joy.py

@@ -27,6 +27,7 @@ from builtins import input
 from traceback import print_exc
 from .parser import text_to_expression, ParseError, Symbol
 from .utils.stack import stack_to_string
+from .library import NotAnIntError, StackUnderflowError
 
 
 class UnknownSymbolError(KeyError): pass
@@ -131,6 +132,10 @@ def interp(stack=(), dictionary=None):
                 stack, _, dictionary = run(text, stack, dictionary)
             except UnknownSymbolError as sym:
                 print('Unknown:', sym)
+            except StackUnderflowError:
+                print('Not enough values on stack.')
+            except NotAnIntError:
+                print('Not an integer.')
             except:
                 print_exc()
             print(stack_to_string(stack))

joy/library.py

@@ -189,6 +189,10 @@ while == swap [nullary] cons dup dipd concat loop
 )
 
 
+class NotAnIntError(Exception): pass
+class StackUnderflowError(Exception): pass
+
+
 def FunctionWrapper(f):
     '''Set name attribute.'''
     if not f.__doc__:
@@ -215,7 +219,12 @@ def BinaryBuiltinWrapper(f):
     @FunctionWrapper
     @wraps(f)
     def inner(stack, expression, dictionary):
+        try:
             (a, (b, stack)) = stack
+        except ValueError:
+            raise StackUnderflowError
+        if not isinstance(a, int) or not isinstance(b, int):
+            raise NotAnIntError
         result = f(b, a)
         return (result, stack), expression, dictionary
     return inner