TweetFollow Us on Twitter

Debugging in Lisp
Volume Number:2
Issue Number:6
Column Tag:AI Applications

Debugging in Lisp

By Andy Cohen, AI Contributing Editor

[Since the last discussion on Macscheme, Semantic Microsystems has updated their product. They are currently shipping version 1.1 which includes access via Macscheme primitives to the Quickdraw routines in ROM. Version 1.1 is also compatible with The Mac+ and HFS. Registered owners of earlier versions can update by sending $20 and their original MacScheme disk to Sematic Microsystems. Additionally, Semantic Microsystems announced the first part of a new development system called Macscheme+Toolsmith. This product includes Pascal-like access to the Toolbox routines in ROM from Macscheme. It also allows the programmer to design an apparently free standing application without the MacScheme environment. A minimized run time system will be required, however, the application and the run time system may be sold without royalties to Sematic Microsystems. Macscheme+ToolSmith will be sold at $250, while Macscheme is still $125 (Which makes us wonder, why $20 for the update?!). We've been pleased with the MacScheme product and the developers' responses to help requests. This month we feature a simple description on using the debugger in MacScheme. - Andy Cohen, A.I. Contributing Editor]

Debugging in Lisp

by

Anne Hartheimer

Semantic Microsystems, Inc.

Most Lisp systems include good debugging tools, in part because Lisp has traditionally been an interactive, interpreted language. I am going to explain the sorts of debugging tools that are typically provided by Lisp systems, how to take advantage of them and, in particular, how to use the debugging aids provided by the MacScheme Lisp system.

What kinds of debugging help can a system give you? For starters, it can tell you that an error exists. It can also help you find the error that it has told you about. Of course, it can't detect all errors. For example, only you can tell if your program is doing the right thing or generating the output you desire. However, if you know that a program produces the wrong output, or at least questionable output, a debugging tool can help you figure out why. Once you have located the source of the error, Lisp debugging tools can help you fix your buggy program and test it conveniently.

There are several standard debugging tools. Compile-time and run-time error checks discover errors. Inspectors let you examine the scene of the crime. If you suspect that there might be something wrong with a particular procedure, then tracers, steppers, and the ability to insert breakpoints in a program can help you see what is happening as the program is executed. Lastly, editors and incremental compilers can make it easy to modify small parts of your program and move on to the next bug. (One of the reasons Lisp interpreters are popular is that they give you incremental compilation for free.)

MacScheme has all of these tools except a stepper. I'll be using MacScheme in the examples that follow.

Compile-time Checks

Error checks can be performed at compile-time or at run-time. A compiler checks for illegal syntax and some kinds of domain errors. (Compile-time checks for domain errors are usually called type-checks.) In MacScheme, when a compile-time error is detected, it is reported and the system performs a reset. For example, if is a reserved word, so it is not legal syntax to assign to it:

>>> (set! if 3)

ERROR:  Keywords of special forms may
        not be used as variables
(set! if 3)

If a procedure is known to the compiler, then the compiler can make sure it is given the right number of arguments. This is an example of a compile-time domain check.

>>> (cons 3 4 5)

ERROR:  Wrong number of arguments to
        integrable procedure
(cons 3 4 5)

MacScheme, like most Lisps, performs very few compile-time domain checks. This is because Lisp associates types with run-time objects instead of with variables, so compile-time checking is not possible in general. This puts Lisp at a disadvantage compared to Pascal because run-time domain checks are performed only on the pieces of code that are actually executed. If you have a program that contains several branches, you can not be sure that it is free of domain errors until you test each of the branches. In Pascal, on the other hand, many domain errors are caught at compile-time, placing less of a burden on testing. Lisp must make up for this disadvantage by having excellent run-time domain checking.

Run-time Checks

Interpretive Lisp systems excel at run-time error checks. An interpreter will detect undefined variables and domain errors such as passing the wrong number of arguments to a procedure, using a non-procedural object as a procedure, attempting to add two strings, etc. Most systems interrupt execution, print a helpful error message on the screen, and place you inside an inspector so you can examine the context of the error. This is a far cry from a core dump or a bomb window.

The following example is from page 72 of The Little Lisper. vec+ is a function which takes two vectors and returns the vector that is their sum. (The Little Lisper represents vectors as lists instead of using Scheme's built-in vector data type.)

>>> (define vec+
      (lambda (vec1 vec2)
        (cond
         ((null? vec1) ())
         (t (cons
             (+ (car vec1) (car vec2))
             (vec+ (cdr vec1)
                   (cdr vec2)))))))
vec+
>>> (vec+ '(3 4 5) '(1 1 1))
(4 5 6)
>>> (vec+ '(1 2 3 4) '(0 2))

ERROR:  Bad argument to cdr
()

Entering debugger.  Enter ? for help.

Oops! Let's track down the cause of this error message. We first ask for the name of the procedure in which the error occured.

debug:> i ; "i" means "identify procedure"
vec+

So the error occurred in our vec+ routine. Let's look at the code for vec+.

debug:> c ; "c" means "show code"
(lambda (vec1 vec2)
  (cond ((null? vec1) ())
        (t
         (cons (+ (car vec1) (car vec2))
               (vec+ (cdr vec1)
                     (cdr vec2))))))

Since the error message told us that cdr was passed a bad argument of (), we should think about the calls to cdr in vec+. (In Scheme, cdr is defined only on non-null lists. When applied to a non-null list, it returns a list consisting of everything in its argument except the first element.) Let's check out the values of vec1 and vec2 since these are the two possible arguments to cdr that could be causing the error.

debug:> a ; "a" means "all variables"

vec1 = (3 4)
vec2 = ()

So vec2 is the problem. Looking more carefully at the code, we can see that we're testing to see if vec1 is null but not vec2. We need to add a test for vec2.

Inside the MacScheme debugger you can find out the name of the procedure in which the error occurred, the arguments that were passed to that procedure, the variables accessible at that point and their values, and the chain of procedures awaiting results. You can move back to the context of any procedure awaiting a result and look at the variables that are accessible to that procedure. You can evaluate arbitrary expressions in the environment of any of the procedures along the chain. You can modify arguments passed to those procedures, and then resume computation. You can modify the values of variables and procedure definitions within the environments of any of the procedures along the chain and resume computation. In addition, you can exit the MacScheme debugger, perform some other computation, and then decide that you want to do additional inspecting of the last bug you encountered. You can reenter the debugger and be back in the state of the most recent error.

Breakpoints, Tracers, and Steppers

Let's shift gears now, and consider the following situation. Suppose your program is producing incorrect output. The traditional approach is to insert print statements in it. The problem with inserting print statements is that it is difficult to know what information to print, and if you print too much the information is unwieldy. It is better to insert breakpoints into your program. In MacScheme, when (break) is evaluated, execution is interrupted and you are placed in the MacScheme debugger. You can now use the full power of the inspector to examine the context of the breakpoint. Instead of having to decide at compile-time what features will be of interest (as you had to do when you debugged by inserting print statements), you get to decide interactively what to look at. For breakpoints to be really useful, your inspector must let you resume the computation. Then you can move on to the next breakpoint that you have set if you decide that things look just fine in the context of the first breakpoint.

It's also useful to be able to induce a break manually to snoop around in computations that seem to be taking a long time or acting weirdly. MacScheme lets you do so by selecting Break from the menu. This interrupts a computation and places you in the MacScheme debugger. When you have finished looking around, you can type an "r" to resume the computation.

If you suspect that a particular routine embedded in your program is producing bad outputs or receiving bad inputs, a convenient way to test your hypothesis is to trace the routine. Whenever a traced routine is called, the MacScheme tracer prints its name and the arguments it was passed. When the routine returns, its name and the value it returns are printed.

>>> (define (fact n)
      (if (zero? n)
          0
          (* n (fact (- n 1)))))
fact
>>> (fact 3)
0
>>> (trace fact)
#!true
>>> (fact 3)
Computing (#<PROCEDURE fact> 3)
 Computing (#<PROCEDURE fact> 2)
  Computing (#<PROCEDURE fact> 1)
   Computing (#<PROCEDURE fact> 0)
   (#<PROCEDURE fact> 0) --> 0
  (#<PROCEDURE fact> 1) --> 0
 (#<PROCEDURE fact> 2) --> 0
(#<PROCEDURE fact> 3) --> 0
0
>>>

Here we see that the recursive calls are passing the correct arguments, but the wrong results are returned.

Once you have determined the problem to be a routine that is receiving good inputs and producing bad outputs, and you have little idea why, you might want to step through the procedure to see what is going wrong. A stepper allows you to view the evaluation of each subexpression of a routine as it is happening. Some steppers let you use the full power of the inspector at each step along the way. You can view a stepper as the ability to insert breakpoints automatically between every expression of your program. A stepper is the one tool I've mentioned that MacScheme doesn't have. To explain why MacScheme does not have a stepper, we must look at the interaction between compiling and debugging.

Interpreters and Compilers

Interpreters are best for development and debugging because they work with the source code, which programmers understand. Compilers turn nice, readable source code into the gobbledygook of machine language--but that machine language sure runs fast. Most Lisp compilers also give up some run-time checking in order to get more speed. One of the main advantages of special-purpose Lisp machines over comparably priced conventional computers is that Lisp machines have special hardware to perform run-time checks very quickly.

MacScheme, like Smalltalk-80, uses a compromise. It compiles to byte code--the machine language for a hypothetical Lisp machine--and then interprets the byte code. This approach gives most of the speed of compiled code together with most of the nice debugging associated with interpreted code. The byte code is also more compact than either native code or source code if the source code is discarded after compilation, but MacScheme normally keeps the source code around to aid in debugging.

Though MacScheme keeps the source code for each user-defined procedure, it does not try to remember the correspondence between individual byte code instructions and source code subexpressions. There just isn't enough memory on a Macintosh to maintain the large tables that would be necessary, which is why MacScheme doesn't have a stepper. Smalltalk-80, on the other hand, does something very clever. Whenever you run its stepper, Smalltalk constructs those tables incrementally, by de-compiling the byte code if necessary.

Texas Instruments' PC Scheme for the IBM PC and TI Professional also uses byte code. Compared to MacScheme, PC Scheme emphasizes speed at some cost to debugging. For example, PC Scheme normally does not retain source code. Because the PC is slower than the Mac and can address less memory, this is a reasonable engineering compromise.

Conclusions

No set of debugging tools can make debugging easy, but they certainly can make it easier, faster, and more fun. We've looked at the help provided by compile-time and run-time error checking, inspectors, breakpoints, tracers and steppers. These tools were first developed for Lisp systems in the 1960's, yet are still hard to find in other languages. Lisp's supportive programming environment is one of the main reasons why so many Lisp programmers are fanatical about their favorite language.

References

Daniel Friedman and Matthias Felleisen. The Little Lisper. Second Edition. Chicago: Science Research Associates, Inc. (ISBN 0-574-21955-2) 1986.

Adele Goldberg. Smalltalk-80: The Interactive Programming Environment. Menlo-Park: Addison-Wesley Publishing Co. 1984.

How to Turn Off the Debugger

If the automatic placement into the debugger is confusing or it is just a pain to you, try the following procedure sent to us by the folks at Semantic Microsystems:

;;; (debugger #!false) turns off the debugger.
;;; (debugger #!true) turns on the debugger.
;;; (debugger) returns #!true if the debugger is on,
;;; otherwise it returns #!false

(define debugger
 (let ((state #!true)
 (real-debug debug)
 (fake-debug (lambda args (reset))))
 (lambda flags        ; flags is a list of arguments
 (if (null? flags)
 state            ; no arguments
 (begin
 (set! debug (if (car flags) real-debug fake-debug))
 (set! state (car flags))
 state)))))

Corrections

A couple of typos from the last column on MacScheme were identified by the author. The first was in one of the code samples using sort. The following is the correct code:

>>> (sort '("Even" "mathematicians" "are" "accustomed" "to" "treating" 
"functions" "as" "underprivileged" "objects")
 (lambda (x y)
 (or (<? (string-length x) (string-length y))
 (string<? x y))))

("as ...)

The second error was in the code defining the procedure make-counter. The code should have included the argument n as follows:

(define make-counter
 (lambda (n)
 (lambda ()
 (set! n (+ n 1))
 n)))

There was also an error in the third reference. The reference was supposed to read Joseph Stoy's Denotational Semantics of Programming Languages. Our thanks to Will Clinger of the Tektronix Computer Research Laboratory (and also of Semantic Microsystems).

 
AAPL
$108.00
Apple Inc.
+1.02
MSFT
$46.95
Microsoft Corpora
+0.90
GOOG
$559.08
Google Inc.
+8.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Vitamin-R 2.20b1 - Personal productivity...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Dropbox 2.10.44 - Cloud synchronization...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Sandvox 2.9.2 - Easily build eye-catchin...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
Cocktail 8.0.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LibreOffice 4.3.3.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
Iridient Developer (was RAW Developer) is a powerful image conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more

Latest Forum Discussions

See All

Monster Flash Review
Monster Flash Review By Jordan Minor on October 31st, 2014 Our Rating: :: ALONE IN THE DARKUniversal App - Designed for iPhone and iPad Solid shooting and a surprising amount of spooky tension make Monster Flash a great portable... | Read more »
Retry Review
Retry Review By Rob Thomas on October 31st, 2014 Our Rating: :: SOARING HIGHUniversal App - Designed for iPhone and iPad Flappy who? Let Retry wash all those bad bird-related memories away on a cool retro-flavored flight… right... | Read more »
Dementia: Book of the Dead Review
Dementia: Book of the Dead Review By Lee Hamlet on October 31st, 2014 Our Rating: :: A TOUGH READUniversal App - Designed for iPhone and iPad A witch hunter is sent after a demonic book in the spooky but short-lived Dementia: Book... | Read more »
Card Dungeon, the Semi-Board Game Roguel...
Card Dungeon, the Semi-Board Game Roguelike, Has Been Renovated Posted by Jessica Fisher on October 31st, 2014 [ permalink ] | Read more »
Logitech Protection + Power iPhone5/5S C...
Made by: Logitech Price: $99.99 Hardware/iOS Integration Rating: 3 out of 5 stars Usability Rating: 0.5 out of 5 stars Reuse Value Rating: 0.75 out of 5 stars Build Quality Rating: 0.75 out of 5 stars Overall Rating: 1.25 out of 5 stars | Read more »
This Is Not a Test Goes Free, Permanentl...
This Is Not a Test Goes Free, Permanently Posted by Jessica Fisher on October 31st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swap Heroes Review
Swap Heroes Review By Campbell Bird on October 31st, 2014 Our Rating: :: STRATEGIC SWAPPINGUniversal App - Designed for iPhone and iPad Rotate a cast of heroes to fend of waves of monsters in this difficult, puzzle rpg.   | Read more »
Night Sky Pro™ (Reference)
Night Sky Pro™ 3.0.1 Device: iOS Universal Category: Reference Price: $2.99, Version: 3.0.1 (iTunes) Description: Night Sky Pro™Wonder No More™ Night Sky Pro™ is the ultimate stargazing experience. From the creators of the original... | Read more »
Audio Defence : Zombie Arena (Games)
Audio Defence : Zombie Arena 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A zombie shooter audio game. Made from gut-wrenching 3D binaural sound, for a new kind of weird immersion. You... | Read more »
RPG Asdivine Hearts (Games)
RPG Asdivine Hearts 1.1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.1.0 (iTunes) Description: SPECIAL PRICE50% OFF (USD 7.99 -> USD 3.99)!!! Travel alongside four companions and a cat in the adventure of a... | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 2014 13-inch R...
The Apple Store is now offering Apple Certified Refurbished 2014 13″ Retina MacBook Pros for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
**Job Summary** Every day, business customers come to the Apple Store to discover what powerful, easy-to-use Apple products can do for them. As a Business Leader, Read more
Sr. Manager, *Apple* Deployment Programs fo...
**Job Summary** Apple is seeking candidates for a new position on the Education Content and Technology team. iPad and Mac is in the hands of millions of teachers and Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.