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***** THIS MANUAL IS NOT UPDATED TO REFLECT THE PORT TO WINDOWS *****
LSRHS Logo Manual
Introduction. Logo is a computer programming language which was
designed to be both simple to use and extremely powerful. It was designed
by a group of computer scientists at MIT and at Bolt, Beranek, and Newman.
Its structure is based largely on the LISP language, which is widely used
in Artificial Intelligence research, but the notation has been changed to
make Logo far easier for a beginner than LISP. This manual describes a
version of Logo for the PDP-11, originally written in C at the Boston
Children's Museum and extensively modified at the Lincoln-Sudbury Regional
High School.
The power of Logo comes primarily from the idea of the procedure. A
procedure is simply something the computer "knows" how to do; some pro-
cedures are built into Logo itself (these are called primitive procedures),
while others are defined by the programmer in terms of these simple pro-
cedures. Defined procedures can be used as part of the definition of other
procedures, so that a complicated program can be built in "layers" of com-
plexity. This layered structure is analogous to the description of a com-
plex machine in terms of building blocks: an automobile is made of a
chassis, a drive train, an electrical system, and so on. The drive train
contains an engine, a transmission, a clutch, an axle, and so on. The
transmission contains a housing, gears, and levers. A lever may include
connecting joints at the ends and at a pivot point. Eventually the
description of the automobile reaches the level of bolts and washers; these
correspond to the primitive procedures in Logo.
Starting_Logo. Use the shell command logo to start the Logo inter-
preter. When Logo is running it will print an asterisk (*) at the begin-
ning of a line to indicate that it is ready for you to type in a Logo in-
struction. The instruction may print something on the terminal, or draw a
line on a graphics display screen, or move a turtle around the floor. Then
another asterisk is typed and you may give another instruction. (If Logo
prints a "greater than" sign (>) instead of an asterisk, it is in procedure
definition mode, which will be described later. Type your system quit
character (control-G at Lincoln-Sudbury) to return to normal mode.)
If an argument is used with the shell command logo, the argument is
taken as the name of a procedure, which Logo runs before doing anything
else. You can therefore create a shell script which will start Logo, run a
procedure automatically, and (if you say "goodbye" at the end of the pro-
cedure) exit.
Syntax. Unlike most computer languages, Logo has an almost entirely
uniform syntax. That is, all of the different commands Logo understands
are represented using the same notation: the name of a procedure is fol-
lowed by its inputs, which may be constants (like numbers) or else may be
the results of using other procedures. Here is a simple example:
print "hello
In this Logo instruction, the primitive procedure print is used with a con-
stant input, the word hello. The quotation mark indicates that hello is
being used to represent the word itself; without the quotation mark it
would have been interpreted as the name of a procedure, just as print is
the name of a procedure. In Logo, the print procedure always requires ex-
actly one input, which is the thing to print. The input can be a word, as
1
in this example, or a list, which will be explained later. (A number is a
special case of a word, and a sentence is a special case of a list.) Here
is another example:
print first "hello
In this instruction, the primitive procedure first is used. This procedure
takes one input, a word, and has an output which is the first letter of the
word. The output from first is used as the input to print, so what is
printed is the letter h rather than the word hello as in the earlier exam-
ple.
Don't confuse the output from a procedure with what is printed by the
print command. In Logo, the word "output" is not used to refer to what is
printed by a program, just as the word "input" does not mean something you
type into the program. Instead, these words refer to objects (words or
lists) which are given to a procedure (inputs) or produced by a procedure
(outputs). A particular procedure has a fixed number of inputs and out-
puts. The number of inputs may be anything, including zero, whereas the
number of outputs is either zero or one. A procedure with an output (like
first) is called an operation; one without an output (like print) is called
a command.
Some operations only have two possible outputs: the word true and the
word false. Such a procedure is called a predicate. Predicates are used
to allow a program to carry out some instruction only if a particular con-
dition is met. By convention, predicates generally have names ending with
the letter "p".
Multiple_inputs_to_operations. Several Logo primitive procedures are
operations with two inputs. The arithmetic operations, like sum, are exam-
ples of this. A special extension to Logo syntax allows such an operation
to have more than two inputs by enclosing the operation and its inputs in
parentheses or braces:
(sum 2 5 13 8.5)
Association is right to left. At least two inputs must be given, except
for the list operation, which can take one input if parenthesized.
Multi-instruction_lines. It is possible to put more than one instruc-
tion on the same line when you are typing to Logo. To do this, type a
semicolon (;) between the instructions:
print "hello; print "goodbye
Later in this manual, the phrase "instruction line" will mean one or more
instructions on a line.
Multi-line_instructions. It is possible to continue an instruction on
a second line. To do this, end the first line with a backslash (\), im-
mediately followed by the RETURN key. If you are typing a quoted word, you
must end the word (with a space, for example) before using the backslash-
RETURN combination. Inside a quoted word, backslash-RETURN means to put an
actual RETURN as part of the word.
Comments. It is possible to include in an instruction line comments
which are meant for human readers of your program (including yourself, next
2
week), and which are not Logo instructions. To do this, begin the comment
with an exclamation point (!). Everything after the exclamation point on
the line will be ignored by Logo. For example:
print [Hi, there.] ! A friendly greeting.
However, the exclamation point does not begin a comment if it is part of a
word or list (see below). You should type a space before the exclamation
point, as in the example above, to make sure it will be interpreted as the
beginning of a comment.
Words. Every computer language deals with particular kinds of ob-
jects. Most languages, like FORTRAN or BASIC or Pascal, are best at deal-
ing with numbers. Logo is a list processing language, which is at its best
with more complicated data structures. The two main categories of object
are the word and the list.
A word is a string of characters. (A character is a letter, digit,
space, or punctuation mark. Things like TAB and RETURN on the terminal
keyboard are also characters, although they are not usually used as part of
Logo words.) A word can be any length, including zero. The way to indi-
cate a word as part of a Logo program is to use the quotation mark (") be-
fore the word. The word begins with the character after the quotation mark
and continues until a space, a tab, the end of the line, or one of these
characters:
( ) [ ] { } " ;
A quotation mark immediately followed by a space or one of the other word-
terminating characters indicates the empty word, which is a word of length
zero.
Please notice that, unlike most programming languages, Logo does not
use quotation marks in pairs to delimit strings of characters. The follow-
ing instruction is an error:
print "aardvark"
This is an error because the print command is followed by two words, the
word aardvark and an empty word which is indicated by the second quotation
mark. Since print only uses one input, the second word has no purpose, and
Logo gives the error message
There's something extra on the line.
print takes only one input.
In order to include one of the word-terminating characters in a word,
you must precede it with a backslash (\). Do not confuse backslash with
the regular slash (/) character. For example, this instruction:
print "\(boo\)
will print the five characters (boo) as its result. The space character
may be included in a word by using a percent (%) instead of the space.
Therefore, the following are equivalent:
print "Hello%there.
print "Hello\ there.
3
To include a percent character or a backslash character in a word, you must
precede it with a backslash.
Numbers. A number is a special case of a word, in which the charac-
ters are all digits. (That definition isn't quite complete, and will be
expanded in the next paragraph.) A number need not be preceded with a quo-
tation mark. (This rule is possible because normally Logo interprets words
without quotation marks as the names of procedures, but there are no pro-
cedures whose names begin with digits.) If a quotation mark is not used,
then any nondigit terminates the word.
Actually, numbers may be written in scientific notation. That is,
they can include signs, decimal points, and a power of 10 by which the
number is multiplied. This exponent is indicated by the letter e followed
by the integer power of 10. The following numbers have the same value:
1000
"1000
1000.00
1e3
10.0e+2
"+1.0e3
10000e-1
Notice that if the number begins with a sign it must be quoted. A quoted
number still must begin with a digit or a sign, not with a decimal point or
a letter e. (The letter may be a capital E, by the way.) If a number is
quoted, it must end with one of the normal word-terminating characters. A
number which contains only digits (no decimal point or exponent) is called
an integer. Note that a number with a decimal point is not considered an
integer even if the digits after the decimal point are all zero.
Since a number is a word, the usual character-manipulating procedures
may be applied to it. For example,
print first 1024
prints the digit 1 which is the first character of the number. In addi-
tion, there are arithmetic procedures which apply specifically to numbers:
print sum 3 2
prints the number 5. These procedures will be listed later.
Lists. A word can be thought of as a list of characters; for example,
the word hello is a list of five letters. In Logo it is possible to mani-
pulate not only lists of characters but also lists of words, lists of lists
of words, and so on. This is a very powerful capability which allows very
complicated data structures to be manipulated easily. To indicate a list
in a program, you put square brackets ([ and ]) around the list, and
separate the list elements with spaces. For example:
print [This is a list of seven words.]
A list all of whose elements are words is called a sentence. Here is an
example of a list which is not a sentence:
4
print [[This is a list][of two sentences.]]
Within a bracketed list, square brackets delimit sub-lists (lists
which are elements of the main list). The quotation mark, parentheses, and
braces are not considered special within a bracketed list, unlike the rules
for quoted words. A list may extend over more than one line; that is, if
you have typed an open square bracket ([) and have not yet typed the match-
ing close square bracket, the Logo instruction is not ended by typing the
RETURN key.
Variables. A variable is an entity which has a name, which is a word,
and a thing (also called a value), which can be any Logo object. Variables
are used to "remember" a computed object for repeated or delayed use in a
program. In Logo, the most common way that a variable acquires a value is
that it is associated with an input to a user-written procedure. In the
following example, don't worry about the details of the format of the pro-
cedure, which will be explained later:
to pff :sentence
print first first :sentence
end
This is the definition of a command with one input. The name of the com-
mand is pff. It has one input because in the "title line" (the one start-
ing to pff) there is one variable name after the command name. The vari-
able whose name is sentence is associated with the first (and only, in this
case) input to pff. In the line starting with the word print, the notation
:sentence means "the thing of the variable whose name is sentence". (To
refer to the name itself, quote it as you would any word.) If this pro-
cedure is used in a Logo instruction like this:
pff [This is the poop.]
then the variable sentence has the value [This is the poop.].
It is also possible to assign a value to a variable by an explicit
Logo instruction. There is a primitive procedure to do this:
make - Command, two inputs.
The first input is the name of a variable (that is, it must be a
word); the second is any Logo object. The effect of the command is to
assign the second input as the value of the variable named by the
first input.
If you are accustomed to programming in a non-procedural language like
BASIC, you should strenuously avoid the temptation to overuse make; expli-
cit assignment is almost always the wrong thing to do in Logo. Total
abstention is the best policy for a Logo beginner.
In Logo, variables are dynamically scoped. That means that a variable
can "belong to" a particular procedure; such a variable can be used by that
procedure and by any procedure which is used by an instruction within the
procedure, but is not available to the procedure which invoked the owning
procedure. In other words, such a local variable comes into being when the
owning procedure starts running, and disappears when that procedure is fin-
ished. It is possible for a procedure with a local variable to use another
procedure with a local variable of the same name. In that case, the vari-
able belonging to the "inner" procedure is the one which is associated with
5
the name as long as it exists; when the inner procedure is finished, the
"hidden" variable belonging to the outer procedure is again available.
A variable which is associated with the input to a procedure is au-
tomatically local to that procedure. Other variables are normally global:
they are "permanent" and do not disappear when the procedure in which they
get their values finish. It is both possible and desirable to make such
variables local, by an explicit instruction to that effect:
local - Command, one input.
The input must be a word. A variable with that word as its name is
created, local to the currently running procedure (that is, local to
the procedure in which the local command is used).
The virtue of local variables is that they make procedures more independent
of one another than they would be if global variables were used. In other
words, if you use local variables consistently, then nothing that happens
in one procedure will change the values of variables used in another pro-
cedure. This makes it very much easier to find program errors.
Primitive_procedures_to_define_user_procedures. There are two ways to
define your own procedure. The first way, using the to command, is simple
to learn but limited in flexibility. The second way, using the edit com-
mand, is more complicated to learn, but makes it easy to make changes in
your procedures. The edit command uses the text editing program edt, just
as you might use it outside of Logo to edit a document you want to print.
Once you've learned the special editing commands in edt, it's easy to use.
The to command makes it possible to begin programming in Logo without hav-
ing learned how to use edt. It just lets you type in your procedure defin-
ition, without any provision for correcting errors or changing the defini-
tion of the procedure. It is fast and convenient for short procedures, but
limited.
The to command is unique, in Logo, in that its inputs are interpreted in a
special way. The inputs aren't evaluated: Logo doesn't run any procedures
you name, or look up the values of any variables, before carrying out the
to command. The example below should make this clearer.
to - Command, special form, see below.
This command takes a variable number of inputs. The first is the name
of a procedure to be defined. The rest, if any, must be preceded by
colons, and are the names of variables to be used as inputs to the
procedure. Logo responds to the to command by printing a "greater
than" sign (>) prompt, to show you that you are defining a procedure
rather than entering commands to be executed immediately. You type
the instruction lines which make up the definition. When you are done
with the definition, type the special word end on a line by itself.
For example:
*to twoprint :thing
>print :thing
>print :thing
>end
*
This example shows the definition of a procedure named twoprint, which
has one input, named thing. The procedure you are defining with the
to command must not already be defined.
6
edit - Command, zero or one input. Abbreviation: ed
The input to this command must be a word, which is the name of a pro-
cedure, or a list of words, each of which is the name of a procedure.
(Unlike the to command, but like all other Logo procedures, the edit
command evaluates its input, so you must use a quotation mark before
the procedure name, if only one is given, to indicate that it is the
name itself which is the input to edit; otherwise Logo would actually
run the procedure to calculate the input to edit.) The procedure you
name may or may not already be defined. Logo responds to the edit
command by running the text editor edt, editing the definition of the
procedure(s) named in its input. (If a procedure was not previously
defined, Logo creates an initial definition for it which contains only
a title line and the end line.) You then edit the definition(s) with
edt. When you write the file and leave edt, Logo will use the edited
file as the definition(s) of the procedure(s). You must not put any-
thing in the file except procedure definitions; in other words, every
nonempty line in the file must be between a "to" line and an "end"
line.
If the edit command is given with no input, edt is given the same file
as from the last time you used the edit command. This is a conveni-
ence for editing the same procedure(s) repeatedly.
If, while editing, you change your mind and want to leave edt without
redefining anything, use the command ESC ^Z instead of the normal ^Z.
This special way of leaving edt tells Logo not to redefine your pro-
cedures. You have the choice, before exiting edt, of writing or not
writing the temporary file which contains the definitions. If you
don't write the file, another edit command with no input will re-read
the previous contents of the temporary file; if you do, another edit
will re-read the new version.
If your Unix environment contains a variable named EDITOR, the con-
tents of that variable is used as the name of the text editor program
instead of the standard edt. The variable can contain a full path-
name, or just a program name if the program can be found in /bin or
/usr/bin. Your favorite editor may not have a facility like edt's ESC
^Z to abort redefinition.
show - Command, one input. Abbreviation: po
The input to this command is a word or a list of words. Each word
must be the name of a procedure. The command prints out the
definition(s) of the procedure(s) on your terminal. (The abbreviation
po stands for printout, which is the name used for this command in
some other versions of Logo.)
pots - Command, no inputs.
This command types at your terminal the title lines of all procedures
you've defined. The name is an abbreviation for "print out titles".
erase - Command, one input. Abbreviation: er
As for the show command, the input is either a word, naming one pro-
cedure, or a list of words, naming more than one. The named pro-
cedures are erased, so that they are no longer defined.
Primitive_procedures_to_manipulate_words_and_lists. There are primi-
tive procedures to print text objects on the terminal, to read them from
the terminal, to combine them into larger objects, to split them into
7
smaller objects, and to determine their size and nature:
print - Command, one input. Abbreviation: pr
The input, which may be a word or a list, is printed on the terminal,
followed by a new line character. (That is, the terminal is posi-
tioned at the beginning of a new line after printing the object.) If
the object is a list, any sub-lists are delimited by square brackets,
but the entire object is not delimited by brackets.
type - Command, one input.
The input, which may be a word or a list, is printed on the terminal,
without a new line character. (That is, the terminal remains posi-
tioned at the end of the object after printing it.) Brackets are used
as with the print command.
fprint - Command, one input. Abbreviation: fp
The input is printed as by the print command, except that if it is a
list (as opposed to a word) it is enclosed in square brackets. The
name of the command is short for "full print".
ftype - Command, one input. Abbreviation: fty
The input is printed as by the type command, except that if it is a
list, it is enclosed in square brackets.
readlist - Operation, no inputs. Abbreviation: rl
Logo waits for a line to be typed by the user. The contents of the
line are made into a list, as though typed within square brackets as
part of a Logo instruction. (The user should not actually type brack-
ets around the line, unless s/he desires a list of one element, which
is a list itself.) That list is the output from the operation.
request - Operation, no inputs.
A question mark is printed on the terminal as a prompt. Then Logo
waits for a line to be typed by the user, as for readlist.
word - Operation, two inputs.
The two inputs must be words. The output is a word which is the con-
catenation of the two inputs. There is no space or other separation
of the two inputs in the output.
sentence - Operation, two inputs. Abbreviation: se
The two inputs may be words or lists. The output is a list formed
from the two inputs in this way: if either input is a word, that word
becomes a member of the output list; if either input is a list, the
members of that input become members of the output. Here are some ex-
amples:
first input second input output
"hello "test [hello test]
"goodbye [cruel world] [goodbye cruel world]
[a b] [c d] [a b c d]
[] "garply [garply]
If an input is the empty list, as in the last example above, it con-
tributes nothing to the output.
list - Operation, two inputs.
The output is a list of two elements, namely, the two inputs. The in-
8
puts may be words or lists.
fput - Operation, two inputs.
The first input may be any Logo object; the second must be a list.
The output is a list which is identical with the second input except
that it has an extra first member, namely, the first input.
lput - Operation, two inputs.
The first input may be any Logo object; the second must be a list.
The output is a list which is identical with the second input except
that it has an extra last member, namely, the first input.
first - Operation, one input. Abbreviation: f
The input may be any non-empty Logo object. If the input is a list,
the output is its first member. If the input is a word, the output is
a single-letter word, namely the first letter of the input. If the
input is empty (a word or list of length zero) an error results.
last - Operation, one input. Abbreviation: l
The input may be any non-empty Logo object. If the input is a list,
the output is its last member. If the input is a word, the output is
a single-letter word, namely the last letter of the input. If the in-
put is empty (a word or list of length zero) an error results.
butfirst - Operation, one input. Abbreviation: bf
The input may be any non-empty Logo object. If the input is a list,
the output is a list equal to the input list with the first member re-
moved. (If the input list has only one member, the output is the emp-
ty list, a list of zero members.) If the input is a word, the output
is a word equal to the input word with the first letter removed. (If
the input is a single-letter word, the output is the empty word.) If
the input is empty, an error results.
butlast - Operation, one input. Abbreviation: bl
The input may be any non-empty Logo object. If the input is a list,
the output is a list equal to the input list with the last member re-
moved. (If the input list has only one member, the output is the emp-
ty list, a list of zero members.) If the input is a word, the output
is a word equal to the input word with the last letter removed. (If
the input is a single-letter word, the output is the empty word.) If
the input is empty, an error results.
count - Operation, one input.
The input may be any Logo object. If the input is a list, the output
is a number indicating the number of members in the list. (Note: only
top-level members are counted, not members of members. The count of
the list
[[This is] [a list]]
is 2, not 4.) If the input is a word, the output is the number of
letters (or other characters) in the word. Remember that in Logo a
number is just a particular kind of word, so the output from count can
be manipulated like any other Logo word.
emptyp - Operation (predicate), one input.
The input can be any Logo object. The output is the word true if the
input is of length zero (i.e., it is the empty word or the empty
9
list). The output is the word false otherwise.
wordp - Operation (predicate), one input.
The input can be any Logo object. The output is the word true if the
input is a word. The output is the word false if the input is a list.
sentencep - Operation (predicate), one input.
The input can be any Logo object. The output is the word true if the
input is a sentence, i.e., a list of words. The output is the word
false if the input is a word, or if any member of the input is a list.
is - Operation (predicate), two inputs.
The inputs can be any Logo objects. The output is the word true if
the two inputs are identical. That is, they must be of the same type
(both words or both lists), they must have the same count, and their
members (if lists) or their characters (if words) must be identical.
The output is the word false otherwise. (Note: this is an exception
to the convention that names of predicates end with the letter "p".)
memberp - Operation (predicate), two inputs.
If the second input is a word, the first input must be a word of
length one (a single character), and the output is true if and only if
the first input is contained in the second as a character. If the
second input is a list, the first input can be any Logo object, and
the output is true if and only if the first input is a member of the
second input. (Note that this is member, not subset.)
item - Operation, two inputs. Abbreviation: nth
The first input must be a positive integer less than or equal to the
count of the second input. If the second input is a word, the output
is a word of length one containing the selected character from the
word. (Items are numbered from 1, not 0.) If the second input is a
list, the output is the selected member of the list.
Primitive_procedures_for_turtles_and_graphics. An important part of
the Logo environment is a rich set of applications to which the computer
can be directed. The most important of these is turtle geometry, a way of
describing paths of motion in a plane which is well-suited to computer pro-
gramming. There are two ways to use the turtle procedures. First, you can
control a floor turtle, a small robot which can move one the floor or on a
table under computer control. Second, you can use a display turtle to draw
pictures on the TV screen of a graphics terminal.
Each computer center has a different variety of graphics hardware
available. Floor turtles are very different from display turtles, but also
each kind of display terminal has different characteristics. For example,
some terminals can draw in several colors; others can't. The following
descriptions of graphics primitives explain the "best" case for each one
and mention restrictions on some graphics devices.
The floor turtle can draw pictures on paper, because it has a pen at-
tached to its "belly": the underside of the turtle. Since it is a mechani-
cal device, however, it is not very precise; the pictures you get may not
be exactly like what your program specifies. A more interesting way to use
the floor turtle is to take advantage of its touch sensors. Switches under
the turtle's dome allow the computer to know when the turtle bumps into an
obstacle. You can use this information to write programs to get around
obstacles or to follow a maze.
10
The display turtle lives on the surface of a TV screen. It can draw
pictures more precisely than the floor turtle, since it does not measure
distances and angles mechanically. It is also faster than the floor tur-
tle. When using the display turtle, remember that it interprets commands
relative to its own position and direction, just as the floor turtle does.
The command left, for example, turns the turtle to its own left, which may
or may not be toward the left side of the TV screen.
turtle - Command, one input. Abbreviation: tur
The input is the name of a turtle. You can only control one turtle at
a time, so using this command a second time releases the turtle previ-
ously selected. The names of floor turtles are numbers like 0 and 1.
If you are using a graphics display terminal (not just a text display
trminal), you can control the display turtle by using the word display
(or the abbreviation dpy) as the turtle name. (As usual, the word
must be preceded by a quotation mark.) If you use a graphics primi-
tive without selecting a turtle, Logo assumes that you want to use the
display turtle. But once you select a floor turtle, you must say tur-
tle "display explicitly to switch to the display.
The word off as input to the turtle command releases a floor turtle,
if you have one, or turns off the graphics display if you have been
using the display turtle. This also happens when you leave Logo.
forward - Command, one input. Abbreviation: fd
The input is a number, the distance you would like the turtle to move.
For a floor turtle, the unit of distance is however far the turtle can
travel in 1/30 second. For a display turtle, the unit is one dot on
the TV screen. (Note: on some displays, one dot horizontally may not
be the same length as one dot vertically. The setscrunch command al-
lows you to control the relative sizes so that squares come out
square.) The turtle moves in whatever direction it is pointing when
you use the command.
back - Command, one input. Abbreviation: bk
The input is a number, a distance to move, as in the forward command.
The difference is that the turtle moves backward, i.e., in the direc-
tion exactly opposite to the way it's pointing.
left - Command, one input. Abbreviation: lt
The input is a number, the number of degrees of angle through which
the turtle should turn counterclockwise. This command does not change
the position of the turtle, but merely its heading (the direction in
which it points). The turn will be only approximately correct for the
floor turtle, because of mechanical errors. For the display turtle,
the angle will be perfectly reproducible, although it may not look
quite right on the screen because of the difference in size between
horizontal and vertical dots. Nevertheless, a display turtle program
will work in the sense that when the turtle is supposed to return to
its starting point, it will do so.
right - Command, one input. Abbreviation: rt
The input is a number; the turtle turns through the specified number
of degrees clockwise.
penup - Command, no inputs. Abbreviation: pu
This command tells the turtle to raise its pen from the paper, so that
it does not leave a trace when it moves. In the case of the display
11
turtle, there is no physical pen to move mechanically, but the effect
is the same: any forward or back commands after this point do not draw
a line. The floor turtle starts with its pen up; the display turtle
starts with its pen down. Note: the floor turtle will not move on the
carpet correctly with its pen down; put it on a smooth surface if you
want to draw pictures.
pendown - Command, no inputs. Abbreviation: pd
This command tells the turtle to lower its pen, so that later commands
will draw lines when the turtle moves.
penerase - Command, no inputs. Abbreviation: pe
This command tells the turtle to "lower its eraser", so that lines
previously drawn will be erased when retraced by the turtle. It only
works with the display turtle. The commands penup, pendown, penerase,
and penreverse are mutually exclusive; whichever was most recently
used is the one which affects the turtle. (Graphics terminals which
cannot selectively erase lines, such as Tektronix displays, will treat
penerase as pendown.)
penreverse - Command, no inputs. Abbreviation: px
This command tells the display turtle to lower its "reversing pen";
thereafter, when the turtle moves, it turns on any points which were
off, and turns off any points which were on. The commands penup, pen-
down, penerase, and penreverse are mutually exclusive; whichever was
most recently used is the one which affects the turtle. (Note:
Graphics terminals which cannot penreverse will treat this command as
pendown.)
penmode - Operation, no inputs.
This operation applies to the floor or the display turtle. It outputs
one of the words penup, pendown, penerase, or penreverse, depending on
the current state of the turtle's pen.
lampon - Command, no inputs. Abbreviation: lon
This command applies only to the floor turtle; it turns on the head-
lamps on the front of the turtle.
lampoff - Command, no inputs. Abbreviation: loff
This command turns off the floor turtle's headlamps.
hitoot - Command, one input. Abbreviation: hit
This command applies only to the floor turtle. It sounds the turtle's
horn at the higher of its two pitches. The input is a number which
indicates the number of quarter-seconds to toot the horn. Note: large
numbers are likely to lead to violent behavior on the part of other
computer users.
lotoot - Command, one input. Abbreviation: lot
This command sounds the floor turtle's horn at the lower of its two
pitches. The input is the duration of the toot.
ftouch - Operation (predicate), no inputs. Abbreviation: fto
This operation can be used only with the floor turtle. It has as its
output the word true if the front of the turtle is touching an obsta-
cle; otherwise it has the word false as its output.
btouch - Operation (predicate), no inputs. Abbreviation: bto
12
This operation can be used only with the floor turtle. It has as its
output the word true if the back of the turtle is touching an obsta-
cle; otherwise it has the word false as its output.
ltouch - Operation (predicate), no inputs. Abbreviation: lto
This operation can be used only with the floor turtle. It has as its
output the word true if the left side of the turtle is touching an
obstacle; otherwise it has the word false as its output.
rtouch - Operation (predicate), no inputs. Abbreviation: rto
This operation can be used only with the floor turtle. It has as its
output the word true if the right side of the turtle is touching an
obstacle; otherwise it has the word false as its output.
clearscreen - Command, no inputs. Abbreviation: cs
This command applies only to the display turtle. It erases everything
on the TV screen, and restores the turtle to its initial position and
heading (center of the screen, facing toward the top edge).
wipeclean - Command, no inputs. Abbreviation: clean
This command applies only to the display turtle. It erases everything
on the TV screen, but does not change the turtle's position or head-
ing.
fullscreen - Command, no inputs. Abbreviation: full
This command applies only to the Atari display turtle. It eliminates
the use of the bottom four lines of the screen to display the commands
you type; instead, the entire screen is available to show the picture
drawn by the turtle. However, you can no longer see what you're typ-
ing. The command may be used after the picture is already drawn; the
part "hidden" by the text at the bottom of the screen will become
visible. On other displays, fullscreen and splitscreen are
equivalent; they make the entire screen available for graphics, and
text appears on the bottom line (Gigis) or superimposed (ADMs), or
somewhere.
splitscreen - Command, no inputs. Abbreviation: split
This command applies only to the Atari display turtle. It restores
the normal text display at the bottom of the screen, undoing the ef-
fect of the full command. On other displays, fullscreen and
splitscreen are equivalent; they make the entire screen available for
graphics, with text superimposed in a display-dependent area.
textscreen - Command, no inputs. Abbreviation: text
This command applies only to the display turtle. It temporarily re-
moves the turtle display from the screen, making the entire screen
available for text display. The commands fullscreen and splitscreen
will restore the graphics display. Note: On the Atari display, the
picture on the screen is remembered, so that when you return to
fullscreen or splitscreen mode, the picture returns to the screen. On
other displays, the picture is forgotten, and you return to an empty
graphics screen.
hideturtle - Command, no inputs. Abbreviation: ht
This command applies only to the display turtle. It erases the
display of the turtle itself from the screen, so that only the lines
drawn when the turtle moves are visible. The display is faster when
the turtle is hidden (only slightly faster on the Atari, but much fas-
13
ter on other terminals). Also, once a graphics program is debugged,
it may be prettier to watch without the turtle visible. (Note: On
the Tektronix display, the turtle is never visible, because the termi-
nal cannot erase selectively.)
showturtle - Command, no inputs. Abbreviation: st
This command applies only to the display turtle. It restores the
display of the turtle, after the hideturtle command has been used.
(Note: On the Tektronix display, the turtle is never visible.)
shownp - Operation (predicate), no inputs.
This predicate applies only to the display turtle. It outputs the
word true if the turtle is visible on the TV screen, false otherwise.
pencolor - Command, one input. Abbreviation: penc
This command applies only to the display turtle. Its effect is dif-
ferent depending on how each type of terminal supports color. For the
Atari, the input must be an integer between 0 and 6. An input of 0
enters black-and-white display mode (which is the turtle's initial
mode), in which lines are as thin as possible but there is no control
of color. Any other input selects color mode, in which lines are
twice as thick, so the effective size of the screen is smaller, but
colors can be used. There are, in color mode, three possible pen
colors, numbered 1 to 3. There are 256 possible colors, but only
three can be on the screen at a time; the setcolor command is used to
decide which pen draws in which actual color. If the input is 4, 5,
or 6, the color is that of pen 1, 2, or 3, respectively, but lines are
drawn in "fill mode": for each point inked, all points to its right
are also inked until a point is reached which was already inked. On
the Gigi, there is only one mode, and there is no loss of resolution
in using color. The input must be between 0 and 7; 0 means black, 7
means white. The ADM, Tektronix, and Sun displays do not have multi-
color drawing.
setcolor - Command, two inputs. Abbreviation: setc
This command applies only to the Atari display turtle. The first in-
put must be an integer between 0 and 3. If the input is nonzero, the
second input specifies the color for the pen selected by the first in-
put. If the first input is zero, the second input specifies the back-
ground color for the color graphics display. The second input is ei-
ther an integer between 0 and 15, which is a color number, or a list
of two integers, in which case the first is a color number and the
second is an intensity number, an integer between 0 and 7.
setxy - Command, two inputs.
The two inputs must be numbers. The turtle is moved to the point on
the screen whose x (horizontal) coordinate is the first input, and
whose y (vertical) coordinate is the second input. The center of the
screen, where the turtle starts, has both coordinates zero. If the
pen is down, this command draws a line. This command applies only to
the display turtle.
setheading - Command, one input. Abbreviation: seth
The input must be a number. The turtle's heading is set to the input,
taken in degrees. Zero points straight up, as the turtle starts out;
positive headings are clockwise from zero. This command applies only
to the display turtle.
14
towardsxy - Operation, two inputs.
This operation applies only to the display turtle. The two inputs
must be numbers, which are the x and y coordinates of a point on the
TV screen. The output is a number which is the heading to which the
turtle must be set, in order to point towards that point from its
current position. Note: this operation does not actually move or turn
the turtle. You must use it as the input to setheading if that is
what you want.
xcor - Operation, no inputs.
The output is the turtle's current x (horizontal) coordinate. The
operation works only with the display turtle.
ycor - Operation, no inputs.
The output is the turtle's current y (vertical) coordinate. This
operation works only with the display turtle.
heading - Operation, no inputs.
The output is the turtle's current heading in degrees. This operation
works only with the display turtle.
getpen - Operation, no inputs.
The output is the turtle's current pen color, or (on the Atari) zero
if in black-and-white mode. This operation works only with the
display turtle.
setscrunch - Command, one input. Abbreviation: setscrun
This command is used only for display turtles. The input must be a
number. The vertical component of turtle motion is multiplied by this
number before each motion is taken. If squares come out too wide on
your screen, you should increase the number; if too tall, you should
decrease it. (You can also use setscrunch to deform the turtle's mo-
tion on purpose, so for example a circle program will draw an ellipse
instead.) The initial scrunch value depends on the terminal you are
using: for the Atari and the Gigi, it is around 0.8 (your particular
computer center will adjust this for the particular TV monitors you
use), but for the ADM, Tektronix, and Sun, it is 1.0 because these
terminals display the same size steps horizontally and vertically.
scrunch - Operation, no inputs.
This operation is used only for display turtles. It outputs a number,
which is the scrunch factor (or aspect ratio) by which vertical motion
is multiplied before it is displayed. This number is changed using
the setscrunch command.
Primitive_procedures_for_arithmetic. Several procedures are available
for arithmetic operations on numbers. In all cases, the inputs to these
procedures must be numbers, except as otherwise indicated in the individual
descriptions.
In general, procedures are used in Logo by typing first the name of
the procedure, then its inputs. This is true of arithmetic procedures
also, e.g.
sum 3 2
However, for some arithmetic operations, Logo also recognizes the more
traditional infix notation, with the operation between the two inputs:
15
3 + 2
Be warned, though, that the use of infix forms makes it difficult for Logo
to know how to group operations, unless parentheses are used. If you stick
to the standard (in Logo) prefix notation, the grouping is always unambigu-
ous. For example, the first two of these three instructions are
equivalent, but the third is not:
if equalp count "hello 5 [print "Yes.]
if (count "hello) = 5 [print "Yes.]
if count "hello = 5 [print "Yes.]
The reason for the error message produced by the last of those three in-
structions is that Logo interprets it as
if count equalp "hello 5 [print "Yes.]
That is, the equality test is done first, on the word hello itself, rather
than first taking the count of hello as was intended.
sum - Operation, two inputs. Infix: +
The output of this procedure is the sum of the two inputs.
difference - Operation, two inputs. Abbreviation: diff Infix: -
The output of this procedure is the difference of the two inputs.
product - Operation, two inputs. Infix: *
The output of this procedure is the product of the two inputs.
quotient - Operation, two inputs. Infix: /
The output of this procedure is the quotient of the two inputs. If
both inputs are integers, the output is also an integer; the remainder
of the division is lost. If either input is not an integer, the quo-
tient can include a fractional part. Therefore, these two are not the
same:
quotient 2 3
quotient 2.0 3
remainder - Operation, two inputs. Abbreviation: mod Infix: \
The inputs to this procedure must be integers. The output is also an
integer, and is the remainder of dividing the first input by the
second.
maximum - Operation, two inputs. Abbreviation: max
The output of this procedure is equal to whichever of the two inputs
is numerically greater.
minimum - Operation, two inputs. Abbreviation: min
The output of this procedure is equal to whichever of the two inputs
is numerically smaller.
greaterp - Operation (predicate), two inputs. Infix: >
The output of this procedure is the word true if the first input is
numerically strictly greater than the second input. Otherwise the
output is the word false.
16
lessp - Operation (predicate), two inputs. Infix: <
The output of this procedure is the word true if the first input is
numerically strictly less than the second input. Otherwise the output
is the word false.
equalp - Operation (predicate), two inputs. Infix: =
The two inputs to this procedure may be any Logo objects. If they are
numbers, then the output is the word true if they are numerically
equal, false if they are numerically unequal. If either input is not
a number, then the output is the same as for the procedure is: it is
true if the two inputs are identical, false if not. For example, the
numbers 2 and 2.0 are numerically equal, but not identical.
numberp - Operation (predicate), one input.
The input may be any Logo object. The output is the word true if the
input is a number, false if not.
zerop - Operation (predicate), one input.
The input must be a number. The output is the word true if the input
is numerically equal to zero, false otherwise.
random - Operation, no inputs.
The output from this procedure is an integer between 0 and 9, i.e., a
single digit. It is chosen randomly, so the output may be different
each time the procedure is used.
rnd - Operation, one input.
The input must be a positive integer. The output is a randomly
selected integer between 0 and one less than the input.
sqrt - Operation, one input.
The input must be a nonnegative number. The output is its square
root.
pow - Operation, two inputs.
The inputs must be numbers. If the first is negative, the second must
be an integer. The output is the first number raised to the power of
the second input.
sin - Operation, one input.
The input must be numeric. The output is the sine of the input, taken
in degrees, not radians.
cos - Operation, one input.
The input must be numeric. The output is the cosine of the input,
taken in degrees, not radians.
arctan - Operation, one input. Abbreviation: atan
The input must be numeric. The output is the arctangent, in degrees,
of the input.
Primitive_procedures_for_conditional_execution. The predicates (like
wordp) which we've mentioned above can be used to carry out some command
only if a condition is met. The basic command for the purpose is if:
if - Command or operation, two or three inputs.
The first input to the if procedure must be either the word true or
the word false. Typically, it is the output from a predicate. The
17
second and (optional) third inputs are lists containing instruction
lines. The second input is executed if the first input is true. The
third input, if any, is executed if the first input is false:
to greet :person
if equalp :person [Ronald Reagan] [print [Hi, turkey!]] \
[print sentence "Hi, :person]
end
In that example, the first input to if is the output from the expres-
sion
equalp :person [Ronald Reagan].
The if procedure can be used as an operation, producing a value. In
this case, the third input is required:
print if equalp :person "Reagan ["Loser] ["Winner]
test - Command, one input.
The input must be the word true or the word false. The command
remembers its input for use in a later iftrue or iffalse command.
This is an alternative to if which is useful if several instructions
are to be made conditional on the same condition. The remembered
truth value is local to the current procedure, if any.
iftrue - Command, one input. Abbreviation: ift
The input must be an instruction list. It is run if the most recent
test command saved a true value.
iffalse - Command, one input. Abbreviation: iff
The input must be an instruction list. It is run if the most recent
test command saved a false value.
both - Operation (predicate), two inputs. Abbreviation: and
The two inputs must both be either true or false. The output is true
if both inputs are true; otherwise the output is false.
either - Operation (predicate), two inputs. Abbreviation: or
The two inputs must be either true or false. The output is true if at
least one of the inputs is true; otherwise the output is false.
not - Operation (predicate), one input.
The input must be either true or false. The output is true if the in-
put is false, and vice versa.
Primitive_procedures_for_file_input_and_output. In the Unix operating
system, there are two steps in reading or writing files: first, the file
must be opened, thereby associating a "file descriptor" (an integer) with
the file name; second, the file descriptor is used to specify the file for
each read or write operation. Logo has primitive procedures for each of
these steps.
openread - Operation, one input. Abbreviation: openr
The input to this procedure is a word, which must be a Unix filename.
It can contain slashes to indicate directory names. If the file can
be opened for reading, the output from the procedure is a file
descriptor, which should be stored in a variable for use in reading
the file. If the file cannot be opened, an error results.
18
fileread - Operation, one input. Abbreviation: fird
The input must be a file descriptor, previously output by openread.
The procedure reads one line from the file. The output is the line,
in the form of a list. (That is, the output is the file line as if
enclosed in square brackets in a program.) If the end of the file has
been reached, the output is the empty word. If the file line contains
mismatched brackets, trouble may result.
fileword - Operation, one input. Abbreviation: fiwd
The input must be a file descriptor, open for reading. The procedure
reads one line from the file. The output is that line, in the form of
a single word, including spaces and punctuation characters. If the
end of the file has been reached, the output is the empty list.
openwrite - Operation, one input. Abbreviation: openw
The input must be a Unix filename. The file is opened for writing
(replacing any previous version), if allowed, and the output is a file
descriptor, for use by file printing commands below. If the file can-
not be opened, an error results.
fileprint - Command, two inputs. Abbreviation: fip
filetype - Command, two inputs. Abbreviation: fity
filefprint - Command, two inputs. Abbreviation: fifp
fileftype - Command, two inputs. Abbreviation: fifty
The first input must be a file descriptor previously output by
openwrite. The second input is any object. The second input is
printed (typed, fprinted, ftyped) into the file.
close - Command, one input.
The input must be a file descriptor. The file is closed. This must
be done when you've finished reading or writing the file.
Sample program:
make "fd openwrite "outfile
fileprint :fd "Hello.
close :fd
This will create a file named outfile containing the word Hello.
Primitive_procedures_for_procedure_exit. A procedure written by a
user, in Logo, can be a command or an operation. If it is an operation,
you must, in the procedure, say what its output should be. If it is a com-
mand, it can simply stop at the end of the procedure, or you can explicitly
make it stop before the end.
output - Command, one input. Abbreviation: op
This command is used in a user procedure which is meant to be an
operation. The input to this command becomes the output from the user
procedure. Please don't be confused by the fact that the user pro-
cedure is an operation, while the output primitive procedure is a com-
mand used in that procedure. Example:
to nickname :person
if equalp :person [Peter Parker] [output "Spiderman]
if equalp :person [Lamont Cranston] [output "Shadow]
output first :person
end
19
stop - Command, no inputs.
This command is used in user procedures which are meant to be com-
mands. It stops the user procedure. (Note that it does not stop all
running procedures. If user procedure A runs user procedure B, a stop
command in procedure B returns to procedure A, which continues after
the point where procedure B was invoked.)
toplevel - Command, no inputs. Abbreviation: top
This command stops all running procedures. The user at the terminal
is prompted to type another command. This can be used when a user
procedure discovers some error condition and wants to abort the entire
program, for example.
Property_lists. It is possible to associate with any name a list of
"properties". A property list contains property names and property values.
For example:
pprop "bh "firstname "Brian
pprop "bh "lastname "Harvey
The form of a property list is
[name1 val1 name2 val2 name3 val3]
Although this data structure could be created using other Logo primitives,
special property list primitives are provided because they are faster. The
property lists do not share storage with Logo variables, so you can change
the value of any property without having to recopy the entire property list
as you would ordinarily. The following primitives manipulate property
lists.
pprop - Command, three inputs.
The first input, which must be a word, is a name with which a property
list is associated. The second input, which must be a word, is the
name of a property. The third input can be any Logo object. It be-
comes the value of the specified property of the specified name.
gprop - Operation, two inputs.
Both inputs must be words. The first is a name, and the second is a
property name. The output is the value of the indicated property of
the indicated object. It is not an error if there is no such proper-
ty; the output in that case is the empty list.
remprop - Command, two inputs.
The inputs must be words, as for gprop. The specified property is re-
moved from the specified name.
plist - Operation, one input.
The input must be a word, which is a name. The output is the property
list of the specified name. Note: the output is actually a copy of
the property list. The real property list is not a Logo list. Any
later changes to the properties of the specified name will not change
the list which was output by an earlier plist.
pps - Command, no inputs.
All properties of all names are listed on your terminal.
20
Pausing. When you are debugging a complicated Logo program, it is
very helpful to be able to stop in the middle of a procedure, so that you
can give interactive commands to examine its inputs and other local vari-
ables. This is different from stopping a procedure, which destroys its lo-
cal environment. There are three ways a procedure can pause: (1) You can
include the command pause in the procedure definition, to make the pro-
cedure pause at a particular place you choose in advance; (2) you can de-
cide to pause a procedure while it is running by typing the system "inter-
rupt" character (this is control-C at Lincoln-Sudbury but is different on
other systems); or (3) you can arrange for an error in the processing of
the procedure to pause instead of stopping as it usually does.
Note that when you type the system "quit" character (control-G at
Lincoln-Sudbury) Logo does not pause, but returns to toplevel. All infor-
mation about the local state of your active procedures is lost.
When you are paused, Logo accepts instructions from your terminal as
it does at toplevel, but local variables can be examined or modified. To
let you know that you are paused, Logo prompts with the characters "-*" in-
stead of just "*" as usual. It is possible to pause within a procedure
within a pause; in this case your prompt is "--*" to indicate two levels of
pause. This can be continued to higher levels.
To get out of a pause, there are three things you can do. You can
give the command toplevel, which stops all pending procedures and returns
to interactive top level. You can give the command stop or the command
output with an input, which will terminate the current procedure (without
or with an output respectively) and return to its calling procedure. Or
you can give the command continue, which will resume the procedure at the
point where you paused.
pause - Command, no inputs.
This command is meaningful only within a procedure. It causes a
pause.
continue - Command, no inputs. Abbreviation: co
This command is meaningful only when typed during an interactive
pause. It continues the current procedure from where it was paused.
errpause - Command, no inputs.
This command tells Logo that any errors which happen during procedure
execution from now on should cause a pause, rather than a return to
toplevel.
errquit - Command, no inputs.
This command tells Logo that any errors which happen during procedure
execution from now on should return to toplevel, rather than pausing.
This is the initial state of affairs when you start Logo.
setqpause - Command, no inputs.
This command tells Logo that from now on, the system quit character
should pause, and the system interrupt character should return to to-
plevel. This is the reverse of the usual interpretation. This com-
mand is provided for people whose systems or keyboards make one of
these characters easier to type than the other. In particular, under
Eunice there is only an interrupt character, not a quit character.
setipause - Command, no inputs.
21
This command tells Logo that from now on, the system interrupt charac-
ter should pause, and the system quit character should return to to-
plevel. This is the initial state of affairs when you start Logo.
Miscellaneous_primitives. The remaining primitives are one of a kind,
or very obscure, or both.
goodbye - Command, no inputs. Abbreviation: bye
This command is used to leave Logo. It is the only way out, unless
there is a bug somewhere.
thing - Operation, one input.
The input must be a word, and must be the name of a variable. The
output is the value of the variable. These are equivalent:
:foo
thing "foo
namep - Operation (predicate), one input.
The input must be a word. The output is true if that word is the name
of a variable which has a value assigned to it, false otherwise.
wait - Command, one input.
The input must be a positive integer. Logo waits that many seconds
before continuing.
trace - Command, no inputs.
This command is used for debugging your Logo programs. After you use
this command, every time a user-defined procedure starts or stops, a
message is typed at your terminal naming the procedure and its inputs
or its output, if any. The message is indented according to the depth
in procedure calls.
untrace - Command, no inputs.
This command turns off the trace messages started by the trace com-
mand.
unix - Command, one input.
The input must be a Unix shell command, which is carried out in a
forked shell. (/bin/sh is used, not csh.) Example:
to whois :user
unix (sentence "grep (word "'^ :user ":') "/etc/inquir)
end
run - Command or operation, one input.
The input must be a list, containing a Logo instruction line. The
list is run as if you typed it directly to Logo. Example:
to while :condition :cmd
if not run :condition [stop]
run :cmd
while :condition :cmd
end
make "number 1
while [:number < 5] [print :number; make "number :number+1]
22
The run procedure can be used as an operation, if its input is a Logo
expression which produces a value, instead of a complete instruction:
print run [sum 2 3]
repeat - Command, two inputs.
The first input must be a positive number. The second is an instruc-
tion list, as for the run command. The list is run repeatedly, the
number of times specified by the first input:
repeat 5 [print "hello]
repcount - Operation, no inputs.
This operation may be used only within the range of a repeat command.
It outputs the number of repetitions which have been done, including
the current one. That is, it outputs 1 the first time through, 2 the
second time, and so on.
break - Command, no inputs.
This command is only meaningful within the range of an if command
within the range of a repeat command. It terminates the repeat im-
mediately. If used in other contexts, the results may be strange.
cbreak - Command, one input.
The input must be either the word on or the word off. If the input is
on, your terminal is placed in cbreak mode, which means that what you
type is made available to your program every character, rather than
every line. This must be done before the readchar procedure, below,
will work. This facility is good for writing video game programs or
text editors. While in cbreak mode, echo is turned off also.
readchar - Operation, no inputs. Abbreviation: rc
This operation waits for you to type a single character at your termi-
nal. The output is a word containing only that character. This works
only if you have turned on cbreak mode; see above.
keyp - Operation (predicate), no inputs.
This procedure outputs true if there is a character waiting to be read
from the terminal, if you are in cbreak mode. If not, it outputs true
if there is an entire line waiting to be read.
oflush - Command, no inputs.
Normally, when you tell Logo to print something, the printing is not
done right away. Instead, Logo remembers everything you tell it to
print, and the printing is done all at once the next time Logo is
waiting for you to type something. This arrangement makes Logo much
faster than it would be if everything were printed immediately. The
oflush command tells Logo to print whatever you've previously asked
for right away, without waiting.
help - Command, no inputs.
This command types at your terminal a brief message about Logo and how
to use it.
describe - Command, one input.
The input must be the name of a Logo primitive procedure. A brief ex-
planation of that primitive is typed at your terminal.
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go - Command, one input.
This command can be used only inside a procedure. The input must be a
number. The same number must appear at the beginning of some line in
the same procedure. (This line number is otherwise ignored.) The next
command executed will be the one on the indicated line in the defini-
tion. Note: there is always a better way to do it. If you have pre-
viously programmed in BASIC, your only hope of ever really learning
how to program computers is NEVER EVER to use the go command!
debquit - Command, no inputs.
This command is meant to be used only for debugging Logo itself. It
is explained here only for completeness. After this command is used,
the QUIT signal is not caught by Logo, so it will cause a core dump.
memtrace - Command, no inputs.
This command is meant to be used only for debugging Logo itself. It
is explained here only for completeness. After this command is used,
every allocation or deallocation of memory, and every character parsed
by the interpreter, types an incomprehensible message at your termi-
nal.
yaccdebug - Command, no inputs.
This command is meant to be used only for debugging Logo itself. It
is explained here only for completeness. After this command is used,
every state transition in the yacc parser types an incomprehensible
message at your terminal.
The_Logo_library. The directory /usr/lib/logo contains Logo pro-
cedures which are available to all users. They are not listed by pots, but
can be thought of as pseudo-primitives which happen to be written in Logo.
Some of these procedures are only useful in conjunction with the teaching
units used in Introduction to Computers, but others are generally useful.
This manual does not fully document the Logo library, because it changes
too often. Look through the /usr/lib/logo directory yourself if you want.
The procedures listp, home, pos, setpos, towards, setx, and sety in the li-
brary are provided for partial compatibility with Apple Logo.
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