Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/sources/unix/volume10/logo/part06/logoman.2

Text File | 1987-06-25 | 35.3 KB | 867 lines

\f3pendown\f1 \(em Command, no inputs.  Abbreviation: \f3pd\f1
.IN
This command tells the turtle to lower its pen, so that
later commands will draw lines when the turtle moves.
.OU
\f3penerase\f1 \(em Command, no inputs.  Abbreviation: \f3pe\f1
.IN
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 \f3penup\f1,
\f3pendown\f1, \f3penerase\f1, and \f3penreverse\f1
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 \f3penerase\f1 as \f3pendown\f1.)
.OU
\f3penreverse\f1 \(em Command, no inputs.  Abbreviation: \f3px\f1
.IN
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 \f3penup\f1,
\f3pendown\f1, \f3penerase\f1, and \f3penreverse\f1 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 \f3pendown\f1.)
.OU
\f3penmode\f1 \(em Operation, no inputs.
.IN
This operation applies to the floor or the display turtle.  It outputs one
of the words \f3penup\f1, \f3pendown\f1, \f3penerase\f1, or
\f3penreverse\f1, depending on the current state of the turtle's pen.
.OU
\f3lampon\f1 \(em Command, no inputs.  Abbreviation: \f3lon\f1
.IN
This command applies only to the floor turtle; it turns on the
headlamps on the front of the turtle.
.OU
\f3lampoff\f1 \(em Command, no inputs.  Abbreviation: \f3loff\f1
.IN
This command turns off the floor turtle's headlamps.
.OU
\f3hitoot\f1 \(em Command, one input.  Abbreviation: \f3hit\f1
.IN
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.
.OU
\f3lotoot\f1 \(em Command, one input.  Abbreviation: \f3lot\f1
.IN
This command sounds the floor turtle's horn at the lower of
its two pitches.  The input is the duration of the toot.
.OU
\f3ftouch\f1 \(em Operation (predicate), no inputs.  Abbreviation: \f3fto\f1
.IN
This operation can be used only with the floor turtle.  It has as its
output the word \f3true\f1 if the front of the turtle is touching an
obstacle; otherwise it has the word \f3false\f1 as its output.
.OU
\f3btouch\f1 \(em Operation (predicate), no inputs.  Abbreviation: \f3bto\f1
.IN
This operation can be used only with the floor turtle.  It has as its
output the word \f3true\f1 if the back of the turtle is touching an
obstacle; otherwise it has the word \f3false\f1 as its output.
.OU
\f3ltouch\f1 \(em Operation (predicate), no inputs.  Abbreviation: \f3lto\f1
.IN
This operation can be used only with the floor turtle.  It has as its
output the word \f3true\f1 if the left side of the turtle is touching an
obstacle; otherwise it has the word \f3false\f1 as its output.
.OU
\f3rtouch\f1 \(em Operation (predicate), no inputs.  Abbreviation: \f3rto\f1
.IN
This operation can be used only with the floor turtle.  It has as its
output the word \f3true\f1 if the right side of the turtle is touching an
obstacle; otherwise it has the word \f3false\f1 as its output.
.OU
\f3clearscreen\f1 \(em Command, no inputs.  Abbreviation: \f3cs\f1
.IN
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).
.OU
\f3wipeclean\f1 \(em Command, no inputs.  Abbreviation: \f3clean\f1
.IN
This command applies only to the display turtle.  It erases everything
on the TV screen, but does not change the turtle's position or heading.
.OU
\f3fullscreen\f1 \(em Command, no inputs.  Abbreviation: \f3full\f1
.IN
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 typing.  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, \f3fullscreen\f1 and
\f3splitscreen\f1 are equivalent; they make the entire screen available
for graphics, and text appears on the bottom line (Gigis) or superimposed
(ADMs), or somewhere.
.OU
\f3splitscreen\f1 \(em Command, no inputs.  Abbreviation: \f3split\f1
.IN
This command applies only to the Atari display turtle.  It restores the
normal text display at the bottom of the screen, undoing the
effect of the \f3full\f1 command.  On other displays, \f3fullscreen\f1 and
\f3splitscreen\f1 are equivalent; they make the entire screen available
for graphics, with text superimposed in a display-dependent area.
.OU
\f3textscreen\f1 \(em Command, no inputs.  Abbreviation: \f3text\f1
.IN
This command applies only to the display turtle.  It temporarily
removes the turtle display from the screen, making the entire
screen available for text display.  The commands \f3fullscreen\f1 and
\f3splitscreen\f1 will restore the graphics display.  Note:  On the Atari
display, the picture on the screen is remembered, so that when you return
to \f3fullscreen\f1 or \f3splitscreen\f1 mode, the picture returns to the
screen.  On other displays, the picture is forgotten, and you return to
an empty graphics screen.
.OU
\f3hideturtle\f1 \(em Command, no inputs.  Abbreviation: \f3ht\f1
.IN
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 faster 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 terminal cannot erase selectively.)
.OU
\f3showturtle\f1 \(em Command, no inputs.  Abbreviation: \f3st\f1
.IN
This command applies only to the display turtle.  It restores
the display of the turtle, after the \f3hideturtle\f1 command
has been used.  (Note:  On the Tektronix display, the turtle is
never visible.)
.OU
\f3shownp\f1 \(em Operation (predicate), no inputs.
.IN
This predicate applies only to the display turtle.  It outputs the word
\f3true\f1 if the turtle is visible on the TV screen, \f3false\f1 otherwise.
.OU
\f3pencolor\f1 \(em Command, one input.  Abbreviation: \f3penc\f1
.IN
This command applies only to the display turtle.  Its effect is different
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 \f3setcolor\f1 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.
.OU
\f3setcolor\f1 \(em Command, two inputs.  Abbreviation: \f3setc\f1
.IN
This command applies only to the Atari display turtle.  The first input
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
input.  If the first input is zero, the second input specifies the
background color for the color graphics display.  The second input
is either 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.
.OU
\f3setxy\f1 \(em Command, two inputs.
.IN
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.
.OU
\f3setheading\f1 \(em Command, one input.  Abbreviation: \f3seth\f1
.IN
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.
.OU
\f3towardsxy\f1 \(em Operation, two inputs.
.IN
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 \f3setheading\f1 if that is
what you want.
.OU
\f3xcor\f1 \(em Operation, no inputs.
.IN
The output is the turtle's current x (horizontal) coordinate.  The
operation works only with the display turtle.
.OU
\f3ycor\f1 \(em Operation, no inputs.
.IN
The output is the turtle's current y (vertical) coordinate.  This
operation works only with the display turtle.
.OU
\f3heading\f1 \(em Operation, no inputs.
.IN
The output is the turtle's current heading in degrees.  This operation
works only with the display turtle.
.OU
\f3getpen\f1 \(em Operation, no inputs.
.IN
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.
.OU
\f3setscrunch\f1 \(em Command, one input.  Abbreviation: \f3setscrun\f1
.IN
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 \f3setscrunch\f1 to deform the
turtle's motion 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.
.OU
\f3scrunch\f1 \(em Operation, no inputs.
.IN
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
\f3setscrunch\f1 command.
.OU
.ti +5
.UB "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.
.PP
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.
.EX
sum 3 2
.EC
However, for some arithmetic operations, Logo also recognizes the
more traditional \f2infix\f1 notation, with the operation between
the two inputs:
.EX
3 + 2
.EC
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 unambiguous.  For example, the first two of
these three instructions are equivalent, but the third is not:
.EX
if equalp count "hello 5 [print "Yes.]
if (count "hello) = 5 [print "Yes.]
if count "hello = 5 [print "Yes.]
.EC
The reason for the error message produced by the last of those three
instructions is that Logo interprets it as
.EX
if count equalp "hello 5 [print "Yes.]
.EC
That is, the equality test is done first, on the word \f3hello\f1
itself, rather than first taking the count of \f3hello\f1 as
was intended.
.sp 1
\f3sum\f1 \(em Operation, two inputs.  Infix: \f3+\f1
.IN
The output of this procedure is the sum of the two inputs.
.OU
\f3difference\f1 \(em Operation, two inputs.  Abbreviation: \f3diff\f1  Infix: \f3-\f1
.IN
The output of this procedure is the difference of the two inputs.
.OU
\f3product\f1 \(em Operation, two inputs.  Infix: \f3*\f1
.IN
The output of this procedure is the product of the two inputs.
.OU
\f3quotient\f1 \(em Operation, two inputs.  Infix: \f3/\f1
.IN
The output of this procedure is the quotient of the two inputs.
.OU
\f3remainder\f1 \(em Operation, two inputs.  Abbreviation: \f3mod\f1  Infix: \f3\\\f1
.IN
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.
.OU
\f3maximum\f1 \(em Operation, two inputs.  Abbreviation: \f3max\f1
.IN
The output of this procedure is equal to whichever of the two inputs
is numerically greater.
.OU
\f3minimum\f1 \(em Operation, two inputs.  Abbreviation: \f3min\f1
.IN
The output of this procedure is equal to whichever of the two inputs
is numerically smaller.
.OU
\f3greaterp\f1 \(em Operation (predicate), two inputs.  Infix: \f3>\f1
.IN
The output of this procedure is the word \f3true\f1 if the first input
is numerically strictly greater than the second input.  Otherwise the
output is the word \f3false\f1.
.OU
\f3lessp\f1 \(em Operation (predicate), two inputs.  Infix: \f3<\f1
.IN
The output of this procedure is the word \f3true\f1 if the first input
is numerically strictly less than the second input.  Otherwise the
output is the word \f3false\f1.
.OU
\f3equalp\f1 \(em Operation (predicate), two inputs.  Infix: \f3=\f1
.IN
The two inputs to this procedure may be any Logo objects.  If they
are numbers, then the output is the word \f3true\f1 if they are
numerically equal, \f3false\f1 if they are numerically unequal.  If
either input is not a number, then the output is the same as for the
procedure \f3is\f1: it is \f3true\f1 if the two inputs are identical,
\f3false\f1 if not.  For example, the numbers \f32\f1 and \f32.0\f1 are
numerically equal, but not identical.
.OU
\f3numberp\f1 \(em Operation (predicate), one input.
.IN
The input may be any Logo object.  The output is the word \f3true\f1 if
the input is a number, \f3false\f1 if not.
.OU
\f3zerop\f1 \(em Operation (predicate), one input.
.IN
The input must be a number.  The output is the word \f3true\f1 if the
input is numerically equal to zero, \f3false\f1 otherwise.
.OU
\f3random\f1 \(em Operation, one input.  Abbreviation: \f3rnd\f1
.IN
The input must be a positive integer.  The output is a randomly
selected integer between 0 and one less than the input.
.OU
\f3sqrt\f1 \(em Operation, one input.
.IN
The input must be a nonnegative number.  The output is its square root.
.OU
\f3pow\f1 \(em Operation, two inputs.
.IN
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.
.OU
\f3sin\f1 \(em Operation, one input.
.IN
The input must be numeric.  The output is the sine of the input, taken
in degrees, not radians.
.OU
\f3cos\f1 \(em Operation, one input.
.IN
The input must be numeric.  The output is the cosine of the input, taken
in degrees, not radians.
.OU
\f3arctan\f1 \(em Operation, one input.  Abbreviation: \f3atan\f1
.IN
The input must be numeric.  The output is the arctangent, in degrees, of
the input.
.OU
.ti +5
.UB "Primitive procedures for conditional execution."
The predicates
(like \f3wordp\f1) 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 \f3if\f1:
.sp 1
\f3if\f1 \(em Command or operation, two or three inputs.
.IN
The first input to the \f3if\f1 procedure must be either the word \f3true\f1 or
the word \f3false\f1.  Typically, it is the output from a predicate.  The second
and (optional) third inputs are lists containing instruction lines.  The second
input is executed if the first input is \f3true\f1.  The third input, if any,
is executed if the first input is \f3false\f1:
.sp 1
.nf
\f3to greet :person
if equalp :person [Ronald Reagan] [print [Hi, turkey!]] \\
    [print sentence "Hi, :person]
end\f1
.fi
.sp 1
In that example, the first input to \f3if\f1 is the output from the expression
.br
\f3equalp :person [Ronald Reagan]\f1.
.sp 1
The \f3if\f1 procedure can be used as an operation, producing a value.  In this
case, the third input is required:
.sp 1
.nf
\f3print if equalp :person "Reagan ["Loser] ["Winner]\f1
.fi
.OU
\f3test\f1 \(em Command, one input.
.IN
The input must be the word \f3true\f1 or the word \f3false\f1.  The command
remembers its input for use in a later \f3iftrue\f1 or \f3iffalse\f1
command.  This is an alternative to \f3if\f1 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.
.OU
\f3iftrue\f1 \(em Command, one input.  Abbreviation: \f3ift\f1
.IN
The input must be an instruction list.  It is run if the most recent
\f3test\f1 command saved a \f3true\f1 value.
.OU
\f3iffalse\f1 \(em Command, one input.  Abbreviation: \f3iff\f1
.IN
The input must be an instruction list.  It is run if the most recent
\f3test\f1 command saved a \f3false\f1 value.
.OU
\f3both\f1 \(em Operation (predicate), two inputs.  Abbreviation: \f3and\f1
.IN
The two inputs must both be either \f3true\f1 or \f3false\f1.  The output
is \f3true\f1 if both inputs are \f3true\f1; otherwise the output is
\f3false\f1.
.OU
\f3either\f1 \(em Operation (predicate), two inputs.  Abbreviation: \f3or\f1
.IN
The two inputs must be either \f3true\f1 or \f3false\f1.  The output
is \f3true\f1 if at least one of the inputs is \f3true\f1; otherwise
the output is \f3false\f1.
.OU
\f3not\f1 \(em Operation (predicate), one input.
.IN
The input must be either \f3true\f1 or \f3false\f1.  The output is
\f3true\f1 if the input is \f3false\f1, and vice versa.
.OU
.ti +5
.UB "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 \f2opened\f1, 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.
.sp 1
\f3openread\f1 \(em Operation, one input.  Abbreviation: \f3openr\f1
.IN
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.
.OU
\f3fileread\f1 \(em Operation, one input.  Abbreviation: \f3fird\f1
.IN
The input must be a file descriptor, previously output by \f3openread\f1.  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.
.OU
\f3fileword\f1 \(em Operation, one input.  Abbreviation: \f3fiwd\f1
.IN
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.
.OU
\f3openwrite\f1 \(em Operation, one input.  Abbreviation: \f3openw\f1
.IN
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
cannot be opened, an error results.
.OU
.nf
\f3fileprint\f1 \(em Command, two inputs.  Abbreviation: \f3fip\f1
\f3filetype\f1 \(em Command, two inputs.  Abbreviation: \f3fity\f1
\f3filefprint\f1 \(em Command, two inputs.  Abbreviation: \f3fifp\f1
\f3fileftype\f1 \(em Command, two inputs.  Abbreviation: \f3fifty\f1
.fi
.IN
The first input must be a file descriptor previously output by
\f3openwrite\f1.  The second input is any object.  The second input
is printed (typed, fprinted, ftyped) into the file.
.OU
\f3close\f1 \(em Command, one input.
.IN
The input must be a file descriptor.  The file is closed.  This must
be done when you've finished reading or writing the file.
.sp 1
Sample program:
.sp 1
.nf
\f3make "fd openwrite "outfile
fileprint :fd "Hello.
close :fd\f1
.fi
.sp 1
This will create a file named \f3outfile\f1 containing the word \f3Hello.\f1
.OU
.ti +5
.UB "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
command, it can simply stop at the end of the procedure, or you can explicitly
make it stop before the end.
.sp 1
\f3output\f1 \(em Command, one input.  Abbreviation: \f3op\f1
.IN
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
procedure is an operation, while the \f3output\f1 primitive
procedure is a command used in that procedure.  Example:
.sp 1
.nf
\f3to nickname :person
if equalp :person [Peter Parker] [output "Spiderman]
if equalp :person [Lamont Cranston] [output "Shadow]
output first :person
end\f1
.fi
.OU
\f3stop\f1 \(em Command, no inputs.
.IN
This command is used in user procedures which are meant to be
commands.  It stops the user procedure.  (Note that it does not
stop all running procedures.  If user procedure A runs user procedure
B, a \f3stop\f1 command in procedure B returns to procedure A, which
continues after the point where procedure B was invoked.)
.OU
\f3toplevel\f1 \(em Command, no inputs.
.IN
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.
.OU
.ti +5
.UB "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:
.IN
.nf
.sp 1
\f3pprop "bh "firstname "Brian
pprop "bh "lastname "Harvey\f1
.fi
.OU
The form of a property list is
.EX
[name1 val1 name2 val2 name3 val3]
.EC
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.
.sp 1
\f3pprop\f1 \(em Command, three inputs.
.IN
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 becomes the value
of the specified property of the specified name.
.OU
\f3gprop\f1 \(em Operation, two inputs.
.IN
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 property;
the output in that case is the empty list.
.OU
\f3remprop\f1 \(em Command, two inputs.
.IN
The inputs must be words, as for \f3gprop\f1.  The specified property
is removed from the specified name.
.OU
\f3plist\f1 \(em Operation, one input.
.IN
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 \f3plist\f1.
.OU
\f3pps\f1 \(em Command, no inputs.
.IN
All properties of all names are listed on your terminal.
.OU
.ti +5
.UB "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
variables.  This is different from stopping a procedure, which destroys
its local environment.  There are three ways a procedure can pause:  (1) You
can include the command \f3pause\f1 in the procedure definition, to make the
procedure pause at a particular place you choose in advance; (2) you can
decide to pause a procedure while it is running by typing the system
"interrupt" 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.
.PP
Note that when you type the system "quit" character (control-G at
Lincoln-Sudbury) Logo does not pause, but returns to toplevel.  All
information about the local state of your active procedures is lost.
.PP
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 "\f3-?\f1"
instead of just "\f3?\f1" as usual.  It is possible to pause within a
procedure within a pause; in this case your prompt is "\f3--?\f1" to
indicate two levels of pause.  This can be continued to higher levels.
.PP
To get out of a pause, there are three things you can do.  You can give the
command \f3toplevel\f1, which stops all pending procedures and returns to
interactive top level.  You can give the command \f3stop\f1 or the command
\f3output\f1 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 \f3continue\f1, which will resume
the procedure at the point where you paused.
.sp 1
\f3pause\f1 \(em Command, no inputs.
.IN
This command is meaningful only within a procedure.  It causes a pause.
.OU
\f3continue\f1 \(em Command, no inputs.  Abbreviation: \f3co\f1
.IN
This command is meaningful only when typed during an interactive pause.  It
continues the current procedure from where it was paused.
.OU
\f3errpause\f1 \(em Command, no inputs.
.IN
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.
.OU
\f3errquit\f1 \(em Command, no inputs.
.IN
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.
.OU
\f3setqpause\f1 \(em Command, no inputs.
.IN
This command tells Logo that from now on, the system quit character should
pause, and the system interrupt character should return to toplevel.  This
is the reverse of the usual interpretation.  This command 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.
.OU
\f3setipause\f1 \(em Command, no inputs.
.IN
This command tells Logo that from now on, the system interrupt character
should pause, and the system quit character should return to toplevel.  This
is the initial state of affairs when you start Logo.
.OU
.ti +5
.UB "Miscellaneous primitives."
The remaining primitives are one
of a kind, or very obscure, or both.
.sp 1
\f3goodbye\f1 \(em Command, no inputs.  Abbreviation: \f3bye\f1
.IN
This command is used to leave Logo.  It is the only way out, unless
there is a bug somewhere.
.OU
\f3thing\f1 \(em Operation, one input.
.IN
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:
.sp 1
.nf
\f3:foo
thing "foo\f1
.fi
.OU
\f3namep\f1 \(em Operation (predicate), one input.
.IN
The input must be a word.  The output is \f3true\f1 if that word is the
name of a variable which has a value assigned to it, \f3false\f1 otherwise.
.OU
\f3wait\f1 \(em Command, one input.
.IN
The input must be a positive integer.  Logo waits that many seconds
before continuing.
.OU
\f3trace\f1 \(em Command, no inputs.
.IN
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.
.OU
\f3untrace\f1 \(em Command, no inputs.
.IN
This command turns off the trace messages started by the \f3trace\f1
command.
.OU
\f3unix\f1 \(em Command, one input.
.IN
The input must be a Unix shell command, which is carried out in a
forked shell.  (/bin/sh is used, not csh.)  Example:
.sp 1
.nf
\f3to whois :user
unix (sentence "grep (word "'^ :user ":') "/etc/inquir)
end\f1
.fi
.OU
\f3run\f1 \(em Command or operation, one input.
.IN
The input must be a list, containing a Logo instruction line.  The list is
run as if you typed it directly to Logo.  Example:
.sp 1
.nf
\f3to while :condition :cmd
if not run :condition [stop]
run :cmd
while :condition :cmd
end
.sp 1
make "number 1
while [:number < 5] [print :number; make "number :number+1]\f1
.fi
.sp 1
The \f3run\f1 procedure can be used as an operation, if its input is a Logo
expression which produces a value, instead of a complete instruction:
.sp 1
\f3print run [sum 2 3]\f1
.OU
\f3repeat\f1 \(em Command, two inputs.
.IN
The first input must be a positive number.  The second is an instruction list,
as for the \f3run\f1 command.  The list is run repeatedly, the number of times
specified by the first input:
.sp 1
\f3repeat 5 [print "hello]\f1
.OU
\f3repcount\f1 \(em Operation, no inputs.
.IN
This operation may be used only within the range of a \f3repeat\f1 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.
.OU
\f3break\f1 \(em Command, no inputs.
.IN
This command is only meaningful within the range of an \f3if\f1 command
within the range of a \f3repeat\f1 command.  It terminates the repeat
immediately.  If used in other contexts, the results may be strange.
.OU
\f3cbreak\f1 \(em Command, one input.
.IN
The input must be either the word \f3on\f1 or the word \f3off\f1.  If
the input is \f3on\f1, 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
\f3readchar\f1 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.
.OU
\f3readchar\f1 \(em Operation, no inputs.  Abbreviation: \f3rc\f1
.IN
This operation waits for you to type a single character at your terminal.  The
output is a word containing only that character.  This works only if you
have turned on cbreak mode; see above.
.OU
\f3keyp\f1 \(em Operation (predicate), no inputs.
.IN
This procedure outputs \f3true\f1 if there is a character waiting to
be read from the terminal, if you are in cbreak mode.  If not, it
outputs \f3true\f1 if there is an entire line waiting to be read.
.OU
\f3cleartext\f1 \(em Command, no inputs.  Abbreviation: \f3ct\f1
.IN
This command clears the screen.  It uses termlib to be terminal-independent.
.OU
\f3setcursorxy\f1 \(em Command, two inputs.  Abbreviation: \f3setcxy\f1
.IN
This command positions the terminal cursor to the column and row specified
by the two inputs, which must be integers.  It uses termlib to support a
wide variety of terminals, but not every terminal is capable of cursor
addressing.  There is a library procedure setcursor that takes one input,
a list of two numbers, column and row.
.OU
\f3oflush\f1 \(em Command, no inputs.
.IN
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 \f3oflush\f1 command tells Logo to
print whatever you've previously asked for right away, without waiting.
.OU
\f3help\f1 \(em Command, no inputs.
.IN
This command types at your terminal a brief message about Logo and
how to use it.
.OU
\f3describe\f1 \(em Command, one input.
.IN
The input must be the name of a Logo primitive procedure.  A brief
explanation of that primitive is typed at your terminal.
.OU
\f3go\f1 \(em Command, one input.
.IN
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 definition.  Note: there is always
a better way to do it.  If you have previously programmed in BASIC, your
only hope of ever really learning how to program computers is NEVER EVER
to use the \f3go\f1 command!
.OU
\f3debquit\f1 \(em Command, no inputs.
.IN
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.
.OU
\f3memtrace\f1 \(em Command, no inputs.
.IN
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 terminal.
.OU
\f3yaccdebug\f1 \(em Command, no inputs.
.IN
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.
.OU
.ti +5
.UB "The Logo library."
The directory /usr/lib/logo contains Logo
procedures which are available to all users.  They are not listed by
\f3pots\f1, 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 \f3setcursor\f1,
\f3listp\f1, \f3home\f1, \f3pos\f1, \f3setpos\f1, \f3towards\f1, \f3setx\f1,
and \f3sety\f1 in the library are provided for partial compatibility with
Apple Logo.