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
;(Reciprocal 5)
;.2 
;(Reciprocal 2384)
;.000419463 

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
;(Square 5)
;25

;(DEFUN Cubed (y)
 (* y (* y y))
;Cubed
;(Cubed 5)
;125

;(DEFUN AVERAGE (W X Y Z)
  (/ (+ W X Y Z) 4)) 
;Average
;(Average 2 3 4 5)
;3.5

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.

Predicates

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)) 
;nil              

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)
;T

;(EQUAL 6732837 6732837)
;T

;(EQUAL 6732837 6732833)
;nil

;(EQUAL First Second)
;nil

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)
;T

;(ATOM thing)
;nil

;(ATOM (A B C D))
;nil

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))
;T

;(SETQ babble '(wd ihc wi kw))
;(LISTP babble)
;T
;(LISTP 'babble)
;nil

;(LISTP 'thing)
;nil

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

;(ATOM ())
;T


;(LISTP ())
;T

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

;(NULL good)
;nil

;(NULL (X Y Z))
;nil

;(NULL ())
;T

;(NULL nil)
;T

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

;(NUMBERP 56.887)
;T

;(NUMBERP fifty-six)
;nil

;(SETQ fifty-six '(56))
;(NUMBERP fifty-six)
;T

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))
;nil
    

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)
;nil

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)
;T

;(EVENP (- 806 35))
;nil

;(MINUSP 25)
;nil

;(MINUSP (-34 86))
;T

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,

;(FORWARD 50)

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))
;Line
;(Line)

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.

;(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:

;(Triangle)

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.

 
AAPL
$100.96
Apple Inc.
-0.83
MSFT
$47.52
Microsoft Corpora
+0.84
GOOG
$596.08
Google Inc.
+6.81

MacTech Search:
Community Search:

Software Updates via MacUpdate

WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Quicken 2015 2.0.4 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
iMazing 1.0 - Complete iOS device manage...
iMazing (formerly DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and... Read more
Xcode 6.0.1 - Integrated development env...
Apple Xcode is Apple Computer's integrated development environment (IDE) for OS X. The full Xcode package is free to ADC members and includes all the tools you need to create, debug, and optimize... Read more
Apple Safari 7.1 - Apple's Web brow...
Apple Safari in OS X Mavericks brings you all-new ways to find and enjoy the best of the web. It works with iCloud to give you a seamless browsing experience across all your devices. It looks out for... Read more

Latest Forum Discussions

See All

View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »
Tap Army Review
Tap Army Review By Jennifer Allen on September 19th, 2014 Our Rating: :: SHOOT EM ALLUniversal App - Designed for iPhone and iPad Mindless but fun, Tap Army is a lane-based shooter that should help you relieve some stress.   | Read more »
Monsters! Volcanoes! Loot! Epic Island f...
Monsters! Volcanoes! Loot! | Read more »
Plunder Pirates: Tips, Tricks, Strategie...
Ahoy There, Seadogs: Interested in knowing our thoughts on all this plundering and pirating? Check out our Plunder Pirates Review! Have you just downloaded the rather enjoyable pirate-em-up Plunder Pirates and are in need of some assistance? Never... | Read more »
Goat Simulator Review
Goat Simulator Review By Lee Hamlet on September 19th, 2014 Our Rating: :: THE GRUFFEST OF BILLY GOATSUniversal App - Designed for iPhone and iPad Unleash chaos as a grumpy goat in this humorous but short-lived casual game.   | Read more »
A New and Improved Wunderlist is Here fo...
A New and Improved Wunderlist is Here for iOS 8 Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Evernote Update for iOS 8 Adds Web Clipp...
Evernote Update for iOS 8 Adds Web Clipping, Quick Notes, and More Posted by Ellis Spice on September 19th, 2014 [ permalink ] | Read more »
Apple Names Ultimate Productivity Bundl...
Apple Names Ultimate Productivity Bundle by Readdle as the Essential Bundle on the App Store Posted by Jessica Fisher on September 19th, 2014 [ permalink | Read more »

Price Scanner via MacPrices.net

Mac Pros available for up to $260 off MSRP
Adorama has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: - 4-core Mac Pro: $2839.99, $160 off MSRP - 6-core Mac Pro: $3739.99, $260... Read more
13-inch 2.6GHz/256GB Retina MacBook Pros avai...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Previous-generation 15-inch 2.0GHz Retina Mac...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
21″ 2.7GHz iMac available for $1179, save $12...
Adorama has 21″ 2.7GHz Hawell iMacs on sale for $1179.99 including free shipping. Their price is $120 off MSRP. NY and NJ sales tax only. Read more
iOS 8 Adoption Rate Slower than iOS 7, 6, Hit...
Apple began pushing out iOS 8 updates to eligible devices around 1pm ET on September 17, 2014. However, unlike with iOS 7, which boasted a wide variety of differences from its predecessor iOS 6, in... Read more
LIkely Final Definitive OS X 10.9.5 Mavericks...
Apple has released what will almost certainly be the last incremental version number update of OS X 10.9 Mavericks (save for futire security updates) before OS X 10.10 Yosemite is released next month... Read more
Fingerprints, Apple Pay and Identity Theft Wa...
On Sep 9th, CEO Tim Cook unveiled Apple Pay, along with the new iPhone 6 and iWatch. Apple Pay is a newly developed technology that utilizes a near field communication (NFC) to enable customer... Read more
Amazon Introduces Two All-New Kindles
Amazon on Thursday introduced the 7th generation of its Kindle dedicated e-reader device: Kindle Voyage, its top-of-the-line e-reader, and the new $79 Kindle, with a 20% faster processor, twice the... Read more
Save up to $300 on the price of a new Mac wit...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
13-inch 2.8GHz Retina MacBook Pro available f...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more

Jobs Board

Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.