Christopher Fry
Harlequin, Ltd.
1 Cambridge Center
Cambridge, Mass. USA
cfry@harlequin.com
Henry Lieberman
Media Laboratory
Massachusetts Institute of Technology
Cambridge, Mass. USA
lieber@media.mit.edu
ZStep 94 is a reversible, WYSIWYG, animated, source code debugger that brings
programming into the safety conscious '90s. It provides safety and efficiency
options not found on the used car dealer's lot.
A basic piece of safety equipment missing from most programming environments is
the rear-view mirror. Since many traffic hazards sneak up on you from behind,
the ability to see where you've just been is essential. ZStep 94 keeps a
complete history of the computation, and can run the program both forward and
backward. There is fine control over the level of detail, essential in those
tight parallel parking situations. In conventional environments, even if some
rear view is provided by stack inspection, the view is obscured by fuzzy dice
and tree-shaped air fresheners such as the loss of intermediate values in the
computation and the warning that objects in the mirror may be closer than they
appear. ZStep 94 tries to assure that the view of past states resembles as
closely as possible the original state.
ZStep 94 installs airbags for the programming environment. An error is
"fail-soft", and doesn't throw the programmer out of the programming
environment, or into a special breakpoint loop. The recorded history of the
computation, including partial results, is still available for inspection, and
the program can be run forward or backward from any point before the error
occurred. Even if the car is damaged, you can still examine the undamaged
portions or the site of impact. The driver can walk away from the scene of the
accident without a scratch.
Further, the history keeping acts like an airplane's "black box", providing the
information necessary for post-accident analysis and providing the basis for
improving safety measures in the future. A video camera pointed out the side
window recording the history is useful to understand those unfortunate side
effects that you sometimes observe during the trip, just as the motorist who
taped the Rodney King incident.
ZStep 94 keeps a first aid kit in the glove compartment for easy access to code
repair tools. There is a convenient cigarette lighter interface to a data
inspector.
When a bug happens, the programmer is usually placed in the role of an
ambulance driver racing to the scene of an accident. S/he's asleep at the wheel
in the hospital garage when the car radio blurts out "Car 54, where are you?
There's been a crash in the city. Victim last seen heading down Main ();
Street". The poor driver of the '57 Chevy wagon ambulance has a map of the
city, a full tank of Jolt, and flames painted on hood indicating a long history
of being burned.
The first problem is to find the scene of the accident. In fact, that's
the problem. The static representation of the code, the map, doesn't
help a whole lot because the dynamic execution goes the wrong way down one way
streets, finds blind alleys that are too small to see on the map and discovers
traffic jams that can't be represented on static media. The task of the
ambulance driver might not be much easier even if the driver happens to be the
city planner, the same person who laid out the streets in the first place.
For programming, the navigation problem is understanding the relationship
between the static code and the dynamic process of execution. ZStep 94 displays
the static code in the user's editor buffer, exactly as it was typed, including
ccomments and formatting. Expressions to be evaluated and the stepper's state
are highlighted, and a floating window follows the focus of execution and
displays the current value. This is like a car with transparent floorboards
enabling you to see the source code painted on the street. Street signs at
every corner display the value of the executed source code that you just drove
over.
Care is taken to make sure that relevant information appears on the screen when
and where it is needed and in the appropriate static and dynamic context. ZStep
94 provides one-click access from expressions in the code to their values and
graphical output, and one-click access from graphical objects to the code that
drew them.
This reduces air drag caused by the programmer moving the mouse to another part
of the screen, and air turbulence caused by the programmer turning his or her
head to see information displayed in separate windows. This allows ZStep 94 to
get more mileage *out a very expensive resource -
hacker power.
For a more academic treatment of ZStep 94, read and attend [4]. For a less
academic treatment, come to the demo!
[2] Marc Eisenstadt and Mike Brayshaw, The Transparent Prolog Machine: An
Execution Model and Graphical Debugger for Logic Programming, Journal of Logic
Programming 5(4), p.1-66, 1988.
[3] Henry Lieberman, Steps Toward Better Debugging Tools for Lisp, ACM
Symposium on Lisp and Functional Programming, Austin, Texas, August 1984
[4] Henry Lieberman and Christopher Fry, Bridging the Gulf Between Code and
Behavior in Programming, CHI '95.
[5] Stewart Watt, Froglet: A Source-Level Stepper for Lisp, Human Cognition
Research Laboratory, Open University, Milton Keynes, England, 1994.
Abstract
Programming is dangerous. As programmers, we are still driving the equivalent
of a '57 Chevy: the chrome plated bumpers on our programming environment might
look good while it's cruising down the road, but it's not very efficient with
[mental] fuel, and it's all too likely to crumple in a crash. No seat belts, no
anti-lock brakes, and the rear view mirror is obstructed by the fuzzy dice. SAFETY EQUIPMENT
All programs are accident-prone. It took lawsuits and bad publicity from
such debacles as the Corvair and Pinto to wake up the car industry to designing
cars to expect the possibility of crashes and protect the driver. Most current
programming environments are like the old Corvair; when an error occurs, the
programmer is left staring at nothing but the flaming wreckage of an error
message and perhaps a core dump. Every program crash totals the vehicle, and
nothing can be learned from the experience except to buy a new car and start
over. NAVIGATION TOOLS
The goal of driving isn't just to make the car go; it's to get where
you're going. ZStep 94 replaces the '57 Chevy door map pocket with a modern GPS
satellite positioning system that displays a dynamic "you are here" cursor. SPEED CONTROL
We replace the '57 Chevy 3-speed manual gearbox with a 4-speed manual
and automatic transmission, in both forward and reverse, permitting a
variety of execution speeds. The fine control helps you find that one empty
parking space in the Haystacks-Calhoun parking garage, yet lets you zoom out so
you don't miss the Forrest Ave. exit on the Trees Interstate Highway. Optional
equipment includes cruise control for programmers too busy necking to click the
mouse.FUEL EFFICIENCY
The mental energy of the programmer is the fuel that runs programming
environments. ZStep 94 features aerodynamic styling that conserves programmer
energy. References
[1] Ron Baecker, Two Systems Which Produce Animated Representations of the
Execution of Computer Programs, SigCSE Bulletin, February 1975. FOOTNOTES
* Estimated Mileage: 33.3 expressions/hour for city hacking, 45
exp/hr on the Information Superhighway. However, your mileage may vary.
Return to text