TweetFollow Us on Twitter

Functions in Lisp
Volume Number:1
Issue Number:8
Column Tag:Lisp Listener

"Functions in Lisp"

By Andy Cohen, Human Factors Engineering, Hughes Aircraft, MacTutor Contributing Editor

Defining Procedures

As you may recall from the first installment of the Lisp Listener, a procedure is a description of an action or computation. A primitive is a predefined or "builtin" procedure (e.g. "+"). As in Forth, Lisp can have procedures which are defined by the programmer. DEFUN, from DEfine FUNction, is used for this purpose. The syntax for DEFUN in Experlisp is as follows:

(DEFUN FunctionName (symbols) 
          (All sorts of computations which may or may Not use the values 
represented by  the symbols))

The function name is exactly that. Whenever the name is used the defined procedure associated with that function name is performed. The symbols are values which may or may not be required by the procedures within the defined function. If required, the values must follow the function name. When given, these values are assigned to the symbol. This is similar to the way values are assigned to a symbol when using SETQ. It is easier to see how DEFUN works when observed within an example:

;(DEFUN Reciprocal (n)
 (/ 1 n))
;(Reciprocal 5)
;(Reciprocal 2384)

The word "Reciprocal" is the function name and the numbers following are the values for which the reciprocal (1/n) are found. After the list containing DEFUN is entered and the carriage return is pressed the function and it's title are assigned a location in memory. The function name is then printed in the Listener window.

;(DEFUN Square (x)
 (* x x))
;(Square 5)

;(DEFUN Cubed (y)
 (* y (* y y))
;(Cubed 5)

  (/ (+ W X Y Z) 4)) 
;(Average 2 3 4 5)

You might recognize "Average" from last month's Lisp Listener. One might imagine using defined functions inside other defined functions. If it was possible to have variables which have the same values in each procedure, then the version of Lisp used has what is called dynamic scoping. In this context the values of the variable are determined by the Lisp environment which is resident when the procedure is called. Experlisp, however, is lexically scoped. That means that variable values are local to each procedure. Two defined procedures can use the same labels for variables, but the values will not be considered as the same. Each variable is defined locally. This is in accordance to the Common Lisp standard. Lexical scoping makes it easier to debug someone elses'' programs. If you don't know what I mean yet, don't worry. This subject will come up again in more detail later.


If no values are required by the defined function then "nil" or an empty list must follow the function name.

;(DEFUN Line ()
 (Forward 50))

The empty list obviously contains no atoms (I'll describe the above function, "Line" later in the section on bunnies). It is synonymous to the special term nil, which is considered by Lisp as the opposite of T or True. Nil is used in many other contexts.

;(cddr '( one two)) 

In the above, the first cdr returns "two". The second cdr returns nothing, hence "nil". The values of true and false are returned by procedures called predicates. While nil represents a false condition, anything other then nil, including "T", is generally considered true. Please note that I used lowercase letters in the above. ExperLisp recognizes both upper and lowercase. I've been using uppercase only to make it clear within the text when I'm referring to Lisp

EQUAL is a predicate which checks the equality of two arguments. Note the arguments can be integers or symbols. If the two arguments are equal then "T" is returned. If they are not equal then "nil" is returned.

;(EQUAL try try)

;(EQUAL 6732837 6732837)

;(EQUAL 6732837 6732833)

;(EQUAL First Second)

ATOM checks to see if it's argument is a list or an atom. Remember, the single quote is used to indicate that what follows is a not evaluated as in the case of a list. Symbols are evaluated.

;(ATOM 'thing)

;(ATOM thing)

;(ATOM (A B C D))

In the first of the above 'thing is an atom due to the single quote. In the second, thing is considered a symbol. A symbol is evaluated and contains a value or values as a list. In the third, (A B C D) is obviously a list.

LISTP checks if it's argument is a list.

;(LISTP '( 23 45 65 12 1))

;(SETQ babble '(wd ihc wi kw))
;(LISTP babble)
;(LISTP 'babble)

;(LISTP 'thing)

One interesting observation is that nil is both an atom and a list, ()=nil. Therefore ATOM and LISTP both return true for nil.

;(ATOM ())

;(LISTP ())

When one needs to know if a list is empty, NULL does the job.

;(NULL good)

;(NULL (X Y Z))

;(NULL ())

;(NULL nil)

NUMBERP checks if the argument that follows is or represents a number rather than a string.

;(NUMBERP 56.887)

;(NUMBERP fifty-six)

;(SETQ fifty-six '(56))
;(NUMBERP fifty-six)

Now for a real slick one. MEMBER tests whether or not an argument is a part of a list. An easy demonstration follows:

;(MEMBER 'bananas (apples pears  bananas))
;(apples pears bananas)
;(MEMBER 'grapes (apples pears bananas))

When the argument is a member, then the contents of the list are given. If not then nil is returned. MEMBER also checks symbols of lists.

;(SETQ fruit '(apples grapes pears))
;(MEMBER 'grapes fruit)
; (apples grapes pears)
;(MEMBER 'banana   fruit)

EVENP tests to see if an integer is even and MINUSP checks if an integer is negative. ODDP and PLUSP are not needed since they are simply opposite of the first two.

;(EVENP 2)

;(EVENP (- 806 35))

;(MINUSP 25)

;(MINUSP (-34 86))

In the second and fourth examples above the lists contained within are calculated prior to MEMBERP evaluation. (806-35=771 & 34-86=-52. There's a few more simple predicates such as NOT, <, >, and ZEROP. I'll discuss them along with conditionals next month. Now for something completely different.

Bunny Graphics

If you've ever learned Logo, the concept of Bunny graphics should sound familiar. As mentioned last month, the Bunny is Expertelligence's version of the Turtle. All one needs to do in order to make a Bunny move is to tell it to. FORWARD X initially moves the Bunny upwards on the screen for 'X' display pixels. A negative number initially moves it down. When one enters the following in the Listener window,


the default graphics window (I'll discuss windows in more detail very soon in future installments) is then opened and the following is drawn:

RIGHT X aims the front of the line to the right by X degrees. If one then uses forward again the line moves in a different direction. For example:

 ;((RIGHT 50) (FORWARD 50))

or better yet

;(DEFUN Line 
 (RIGHT 50) (FORWARD 50))

After a line is moved, the end of the line remains where it was. If one made the Bunny move again the beginning of the new line would begin where the old left off. The original starting point is the graphics window default home position. This position is in the center of each graphics window when the window is first created. In order to return the Bunny to the original starting point one must use HOME.


The following produces a much neater triangle:

(DEFUN Triangle ()
         (Penup) (Left 45)
         (Forward 10) (Pendown)
        (Right 90) (Forward 25)
        (Right 90) (Forward 50)
        (Right 135) (Forward 71)
       (Right 135) (Forward 25))

After the above is typed into the edit buffer the "Compile All" selection should be chosen from the Menu Bar. The source code in the Edit Buffer quickly inverts to white letters on a black background as if the whole file was selected for a moment. The function name "Triangle is then printed in the Listener window. If the user enters the following in the Listener Window a different triangle is drawn in the default Graphics Window:


If you Look at the in Triangle you will see a couple more Bunny commands. LEFT does the same as RIGHT but in the opposite direction. PENUP raises the Bunny's pen so that when the Bunny moves no lines are drawn. PENDOWN returns the Bunny to the drawing orientation. The first line of code in "Triangle" puts the Bunny off the Home position so that the drawn triangle will be centered on the screen. As mentioned earlier, the orientation of the bunny remains. The last line of code in "Triangle left the Bunny aimed at about 1:00 rather than the initial position, 12:00. If we were to make "Triangle" execute ten times without eliminating the Graphics Window the following would result:

In getting "Triangle" to execute recompilation of the code in the edit buffer is not necessary. To get the above one can type the function name into a list ten times within the Listener window. The following however, is easier:

;(Dotimes (a 10) (Triangle))

DOTIMES is very similar to the FOR...NEXT looping routine in BASIC. I'll discuss it next month in a description of iteration and recursion in ExperLisp.

If we wanted to use a three dimensional bunny then the following would be added before "Triangle" in the Edit Buffer window:

(SETQ curbun (new3dbun))
(Pitch 30) (Yaw 45) (Roll 50)

Something like the following is drawn after the source code is recompiled and "(Triangle)" is entered into the Listener Window:

CURBUN is a special symbol in ExperLisp which always refers to the Bunny cursor. NEW3DBUN is a special term which always changes CURBUN. The default Bunny is 2 dimensional. If one wanted the Spherical Bunny then the following would be entered into the beginning of the first version of "Triangle":

(SETQ curbun (newspbun))

This would then produce what follows:

In order to have the above drawn in a different orientation, different Bunny direction would be required. Windows, two and three dimensional Bunny graphics and toolbox graphics use the same X,Y coordinate system. Home is 0,0. Dual negative coordinates are situated towards the upper left corner. Dual positive coordinates are situated towards the lower right corner. The range is +32767 to -32768 for each dimension. In ExperLisp one can sometimes use the third dimension, as in the 3D sample of "Triangle". Negative Z values are behind Home, while positive Z values are in front. The following illustrates the coordinate system in ExperLisp:

Compiler Information

The ExperLisp disk contains three essential files; Compiler, LispENV and Experlisp. Compiler is not actually the entire Lisp compiler. It contains the information needed in generating all of the higher level Lisp syntactics, such as the Bunny graphics. LispENV stands for Lisp Environment and it is simply a duplication of Compile. LispENV contains information on how the Macintosh memory was organized by the programmer and ExperLisp during the previous session. It also contains information on the system configuration such as the number of disk drives, the amount of memory, etc. Sometimes LispENV can be messed up (i.e. by changing the variable table). When this happens one might not be able to start ExperLisp. In this case LispENV should be removed from the disk. Afterward, when ExperLisp is opened, Compiler generates a new LispENV. Compiler is not needed on the disk unless the LispENV is ruined. Deleting it will provide 100K more space on the disk. Before eliminating it from the disk however, be sure you have a backup as it is an essential file. The Experlisp file contains the assembly language routines which represent the lower level Lisp routines like CAR and CDR. It also allows access to the Macintosh toolbox routines and contains the Listener Window. One opens the Experlisp file in starting a programming session with ExperLisp. Another file on the disk is automatically loaded and activated when Experlisp is booted. It is labeled ªlispinit. The contents of this file can be added to so that when one boots up ExperLisp a program can be automatically executed. It can also do automatic configurations. However the contents of ªlispinit should not be changed since it configures the Macintosh memory for Exper- Lisp.

Next month I'll discuss a few more predicate procedures. I also hope to start discussing iteration, recursion and conditionals. If there is enough room left over I might also begin discussing how to access the toolbox graphics.


Community Search:
MacTech Search:

Software Updates via MacUpdate

Macs Fan Control - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
Boom 2 1.5.2 - $14.99
Boom 2 is a system-wide volume booster and equalizer app that is designed especially for OS X 10.10 Yosemite. It comes with a smart interface, self-calibrates itself according to your Mac, offers... Read more
MacFamilyTree 8.1.3 - Create and explore...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more
WhiteCap 6.6 - Visual plug-in for iTunes...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
VOX 2.8.14 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Paparazzi! 1.0b2 - Make user-defined siz...
Paparazzi! is a small utility for OS X that makes screenshots of webpages. This very simple tool takes screenshots of websites which do not fit on one screen. You specify the desired width, minimal... Read more
Carbon Copy Cloner 4.1.13 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
TrailRunner 3.8.832 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more
VueScan 9.5.65 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Adobe Illustrator CC 2017 21.0.2 - Profe...
Illustrator CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Illustrator customer). Adobe Illustrator CC 2017 is the industry... Read more

Stickman Surfer rides in with the tide t...
Stickson is back and this time he's taken up yet another extreme sport - surfing. Stickman Surfer is out this Thursday on both iOS and Android, so if you've been following the other Stickman adventures, you might be interested in picking this one... | Read more »
Z-Exemplar (Games)
Z-Exemplar 1.4 Device: iOS Universal Category: Games Price: $3.99, Version: 1.4 (iTunes) Description: | Read more »
5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »
Last week on PocketGamer
Welcome to a weekly feature looking back on the past seven days of coverage on our sister website, PocketGamer. It’s taken a while for 2017 to really get going, at least when it comes to the world of portable gaming. Thank goodness, then, for... | Read more »
ROME: Total War - Barbarian Invasion set...
To the delight of mobile strategy fans, Feral Interactive released ROME: Total War just a few months ago. Now the game's expansion, Barbarian Invasion is marching onto iPads as a standalone release. [Read more] | Read more »
Yuri (Games)
Yuri 1.0 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: It's night. Yuri opens his eyes. He wakes up in a strange forest.The small, courageous explorer rides on his bed on casters in this... | Read more »
Space schmup Xenoraid launches on the Ap...
10Tons Xenoraid is out today on the App Store, bringing some high-speed space action to your mobile gadgets just in time for the weekend. The company's last premium title, another sci-fi game titled Neon Chrome, did quite well for itself, so... | Read more »
Star Wars: Force Arena Beginner's G...
Star Wars: Force Arena joined the populous ranks of Star Wars games on mobile today. It's a two-lane MOBA starring many familiar faces from George Lucas's famed sci-fi franchise. As with most games of this nature, Force Arena can be a little obtuse... | Read more »
Mysterium: The Board Game (Games)
Mysterium: The Board Game 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: The official adaptation of the famous board game Mysterium! | Read more »
Sonny (Games)
Sonny 1.0.4 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.4 (iTunes) Description: Reimagined for iOS, cult-hit RPG Sonny brings challenging turn-based combat that requires strategy and mastery of each new skill to... | Read more »

Price Scanner via

CalcTape for macOS 1.2 Adding Machine App for...
schoettler Software has announced CalcTape 1.2, an update to their desktop calculator for macOS. When it comes to adding long columns of numbers, doing complex calculations or playing around with... Read more
New MacBooks And MacBook Pros WIth Kaby Lake...
Digitimes’ Joseph Tsai cites a Chinese-language Economic Daily News (EDN) report that unnamed market watchers are predicting Apple MacBook shipments to grow 10 percent in 2017, and projecting 15... Read more
New 2016 13-inch MacBook Pros on sale for up...
B&H Photo has the new 2016 13″ MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.9GHz/512GB Touch Bar MacBook Pro... Read more
New 15-inch Touch Bar MacBook Pros in stock a...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
Opera Announces Neon Concept Browser For Mac
Opera is inviting users to get a glimpse of what Opera for computers could become with its Opera Neon browser concept. Each Opera Neon feature is described as “an alternate reality” for the Opera... Read more
Tellini Releases TabView 3.0 Missing Tool fo...
Tellini has announced the release of TabView 3.0. TabView has been the first macOS viewer for PowerTab tablatures. PowerTab is a well-known and widely adopted tablature editor for Windows systems and... Read more
13-inch 1.6GHz/128GB MacBook Air on sale for... has the 1.6GHz/128GB 13″ MacBook Air on sale for $130 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (MMGF2LL/A): $869.99 $130 off MSRP Their price is the lowest... Read more
12-inch 32GB Space Gray iPad Pro on sale for...
B&H Photo has 12″ Space Gray 32GB WiFi Apple iPad Pros on sale for $55 off MSRP including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $744.44 $55 off... Read more
9-inch 32GB Space Gray iPad Pro on sale for $...
B&H Photo has the 9.7″ 32GB Space Gray Apple iPad Pro on sale for $549 for a limited time. Shipping is free, and B&H charges NY sales tax only. Read more
Apple iMacs on sale for up to $120 off MSRP,...
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $120 off MSRP, each including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2199 $100 off MSRP - 27″ 3.2GHz/1TB Fusion iMac... Read more

Jobs Board

*Apple* Retail - Multiple Positions (Multi-L...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Stamford, CT We have immediate job openings for several Desktop Support Technicians with one of our most well-known Read more
*Apple* macOS Systems Integration Administra...
…most exceptional support available in the industry. SCI is seeking an Junior Apple macOS systems integration administrator that will be responsible for providing Read more
*Apple* Premier Retailer - Service Technicia...
DescriptionSimply Mac is the largest premier retailer for Apple products and solutions. At Simply Mac we are all Apple , all the time. Same products. Same prices. Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.