Wilfred / trifle

        # TODO: Move Frame to separate module to fix the cyclic import.

        from evaluator import Frame

        if frame.expression_index == 0:

            # We don't evaluate the if symbol.

            frame.expression_index += 1

            return None

        elif frame.expression_index == 1:

            # Evaluate the condition.

            stack.push(Frame(condition, environment))

            frame.expression_index += 1

            return None

        elif frame.expression_index == 2:

            # We've evaluated the condition, so either evaluate 'then'

            # or 'otherwise' depending on the return value.

            evalled_condition = frame.evalled[-1]

            if evalled_condition == TRUE:

                stack.push(Frame(then, environment))

                frame.expression_index += 1

                return None

            elif evalled_condition == FALSE:

                stack.push(Frame(otherwise, environment))

                frame.expression_index += 2

                return None

            else:

                return TrifleExceptionInstance(

                    wrong_type,

                    u"The first argument to if must be a boolean, but got: %s" %

                    evalled_condition.repr())

        else:

            # We've evaluated the condition and either 'then' or

            # 'otherwise', so pop this frame and return.

            return frame.evalled[-1]

class While(Special):

        from evaluator import Frame

        if frame.expression_index == 0:

            # We don't evaluate the while symbol.

            frame.expression_index += 1

            return None

        elif frame.expression_index == 1:

            # Evaluate the condition.

            stack.push(Frame(condition, env))

            frame.expression_index += 1

            return None

        elif frame.expression_index == 2:

            # We've evaluated the condition, so either evaluate the body, or return.

            evalled_condition = frame.evalled[-1]

            if evalled_condition == TRUE:

                stack.push(Frame(body, env, as_block=True))

                # Once we've evaluated the body, we should evaluate

                # the condition again.

                frame.expression_index = 1

                return None

            elif evalled_condition == FALSE:

                # while loops always return #null when done.

                return NULL

            else:

                return TrifleExceptionInstance(

                    wrong_type,

                    u"The first argument to while must be a boolean, but got: %s" %

                    evalled_condition.repr())

# todo: implement in prelude in terms of stdin and stdout