An Introductory Guide to the 6.001 Computer System
January 18, 2001
by Eric Grimson and Jacob Strauss
(original version by Jason A. Wilson and the Scheme Team)
Copyright (C) 1992, 1998, 2001 Massachusetts Institute of Technology
Department of Electrical Engineering and Computer Science.
Permission is granted to distribute verbatim copies of this manual provided
that the copyright notice and this permission notice are preserved on all
Table of contents
Please make special note of the following points:
We have put this information here in case you decide not to read this entire
guide. Generally, this manual is organized on a "learn-as-you-go" basis.
Anything that is not essential to doing a project or problem set is not
explained here; however, we will describe how you can find additional information
on-line. Although this extra information might not be necessary to complete
the problem sets, it will make the programming easier and so we urge you
The 6.001 Lab is reserved for the exclusive use of students in 6.001. You'll
find it convenient to work here because the Lab Assistants are available
to help you, and you can share the warmth and camaraderie of your classmates
while you work on problem sets. If you want to use your own computer, the
6.001 staff provides implementations of Scheme for Linux, Windows NT 4,
and Windows 95 (sorry, no Macs). See the course web page for software and
If you are working on a project (as opposed to an online problem set) read
the entire project BEFORE you begin your work. Often, a question you
have will be explained in a later section of the problem set.
Browse the 6.001 Web page regularly. It contains course handouts, announcements,
Scheme software, documentation, advice on where to get help, and other
useful information. The page is located at
Read section Logging
In, for information about logging in for the first time in the 6.001
The problem sets and projects are designed so that they should run on
all of these implementations, and you can move your work between them if
you find this useful. For example, you might start problem set at home,
then spend some time debugging it in the lab where the Lab Assistants can
help you, and then finish things up at home.
6.001 is not officially supported on Athena. The implementation
of Scheme on Athena that runs on Linux machines should be compatible with
the 6.001 implementations, although we do not guarantee that it can run
all the problem sets. Other versions of Scheme on Athena are likely
not to be compatible with 6.001.
The lab machines are set up as Athena machines in the following manner,
however. They will automatically save all your work in your Athena
home directory (in ~/u6001/work by default). You can perform most
Athena operations on these machines. Please note that this is not
a supported Athena platform, so don't bug I/S employees with problems.
Send all bug reports, comments, and questions about the lab setup to email@example.com.
The 6.001/.004 lab is located in room 34-501. The easiest way to get there
is by the elevator in building 36. Since the door outside the lab has an
electronic lock, you will want to bring along the combination, which is
given out in lecture. Make sure you also bring your textbook, this guide,
and a copy of the project if that is what you are planning to work on.
Lab Assistants (LAs) are an integral part of the 6.001 lab. They make sure
that everything runs smoothly by maintaining hardware and fixing certain
problems so that the lab is available to as many students as possible.
However, LAs are not in the lab just to maintain hardware; their real purpose
is to offer you assistance on problems that you encounter while trying
to do the problem sets. If you get stuck, check the blackboard for the
help queue status. If the queue is on, write the name of your machine in
the next available space. If the queue is off, look for an LA walking around
or using a computer to assist you.
Don't think that you must have a major problem to ask for help. Although
the LAs are not there to write your code for you, they can assist you when
you stop making progress on your problem set. The best way to make sure
that all of your questions get answered is to get to the lab early in the
week. The night before the problems set is due is not a good time to ask
an LA to explain a complicated issue. Instead, come to the lab early or
make an office appointment with your recitation instructor or TA if you
begin having difficulties. The lab will be very busy the night before the
If you notice hardware problems, you should contact one of the LAs on
duty. If one is not in the room, place a note by the computer that explains
what you think the problem is and when you noticed it. If an extreme problem
occurs (like a fire) when an LA is not around, contact someone at the equipment
desk located outside of the lab.
The 6.001 text book describes how to program in Scheme but it gives none
of the details of how these programs are entered into the computer, tested
and changed. This chapter describes the basic things that happen when you
use the computer.
Control of the computer begins with you. You can direct the computer by
moving the mouse or by typing at the keyboard. These keystrokes and mouse
actions are first inspected by a program called the "window manager" to
see if you are requesting that it perform some operation. The window manager
then passes your keystrokes to the application that has the focus
(the highlighted window has the focus). Ordinarily, the only application
that you see is Edwin, a text editor very similar to Emacs. You will usually
want Edwin to have the focus. You can tell the window manager to give Edwin
the focus by clicking the mouse on the border surrounding the Edwin window.
This should make the border a different color (whatever the default is
for your machine) and allow Edwin to receive your keystrokes.
A text editor is a program that allows you to compose text (programs,
memos, letters, books, etc.) much as you would with a typewriter. Unlike
a typewriter, Edwin will allow you to modify text that you have already
written so that you don't have to type it in all over again. Edwin also
allows you to work on more than one piece of text at a time, each in its
Be sure to save your files before quitting. Otherwise your work
will be lost.
Once you have written your programs using Edwin, you can instruct Edwin
to send them to the Scheme interpreter and record the results. Although
almost any buffer can send expressions to the Scheme interpreter, the Scheme
interpreter writes its responses only into the `*scheme*' buffer.
The `*scheme*' buffer is the default buffer that Edwin displays
when you first log in.
Here is a short list of most of the buffers that you will working with:
We have already told you enough about communicating with the window manager
to get you through 6.001. Obviously, much more will be said about communicating
with Edwin and thus the Scheme interpreter.
This is the buffer where you get responses from the Scheme interpreter.
Also, you should evaluate test expressions in this buffer. Note, however,
you should really only use this buffer for evaluating test expressions.
Do the modifying of your code in a separate code buffer.
This buffer also gets all the responses from the Scheme interpreter. It
differs from the `*scheme*' buffer in that you are not meant to
type expressions here. Like a "black-box", think of it more as a diary
of what has taken place since you have logged in. You copy text out of
this buffer to make a transcript. You very rarely want to type things into
Project Code Buffers
These buffers contain the code that makes the projects do their magic.
Normally they are read-only signifying that you should not change them.
If it becomes necessary for you to change the code in one of these buffers,
copy only those parts which you need to change into your answer buffer.
Answer Code Buffers
This is where you spend time writing programs. Normally your answer buffers
end with `.scm' which tells Edwin that the contents of the buffer
is Scheme code. Usually you only need one answer buffer per project for
code but using additional buffers to store partial transcripts and comments
to yourself is very useful. You will also want your answer buffers to correspond
to a file in your directory.
Normally, pressing an alphanumeric key tells Edwin to insert a character
in the current buffer. However, you can press special keys in combination
with normal keys to give Edwin commands. Most of these keys are now standard
parts of keyboards. Thus, the Control key is located next
to the `a' key. It works like the shift key meaning that you should
hold it down while pressing another key. For example, when we say C-x
we mean: hold down the key labeled
CTRL while you press `x'.
The Meta key also works like the shift key. If this is not explicitly
labeled as Meta then it will typically be the Alt key,
located to the left of the keyboard. When we say M-x, we mean
hold down the Meta key while you press `x'. Sometimes
it is necessary to hold down both
CTRL and META at the
same time. When we want you to do this, we will say C-M-x.
Some commands take immediate effect while others wait for you to type
a response to a question posted in the mini-buffer. At this point, pressing
just one more key will sometimes cause the command to take effect. Otherwise,
Edwin expects you to type a few words into the mini-buffer and press RET
to tell Edwin when you are done. For more information, see section The
Like Meta and Control, there are other keys that we
refer to with special abbreviations.
Use your Athena username and password to log in. If you don't have an Athena
account yet (because you have Special Student status, are cross-registered,
etc), go to the Athena Accounts office at building N42's front desk. Their
phone number is 253-1325 and their hours are:
the tab key
the Return key
space or the space-bar
line-feed or C-j
the escape key
Monday, Wednesday, Friday
...................... 2:00pm - 5:00pm
Tuesday, Thursday ..............................
9:00am - 12:00pm
Athena login proceeds as normal. To run Scheme, do
Normally, you will just use the online tutor to deal with problem sets.
You may find it convenient, however, to use your own scheme environment
to experiment with your answer. In this case, you should be able
to cut-and-paste your answer from your Scheme environment into the tutor's
window for submission. In those cases where a problem set has supporting
code, we will provide a link from the tutor page that will enable you to
access that code.
Projects will involve more extensive coding that problem sets, and for
this we will provide a mechanism for letting you get access to that code.
For historical reasons, the command is M-x load-problem-set. Edwin
prompts you for the project number in the mini-buffer. Edwin should also
show a default project in parentheses. To select the default just type
Otherwise, enter the correct project number and then hit
If you are working on your own computer, and starting a new project,
you'll need to download the project files to the correct directory on your
own machine. After that, M-x load-problem-set will work as above.
You'll find the project files, together with instructions for downloading
them, on the course web site.
Edwin normally displays the `*scheme*' buffer, the mode line,
and the mini-buffer when it starts up. The mode line tells you useful information
like the name of the buffer above it and whether that buffer is read-only,
modified or unmodified. It has the following appearance:
--ch-Edwin: buffer (major minor)----pos-----------
The italicized fields have the following meaning:
The `*scheme*' buffer and any buffer that ends with `.scm'
have an additional field, the run-light, to tell you whether it
is currently evaluating a Scheme expression. For example:
is `**' if the text has been modified or `--' if the
buffer has not been modified since last being saved. A read-only buffer
contains `%%' in this field.
is the name of the buffer.
is the major mode of the buffer. For example, most buffers contain "Scheme"
(an optional field) is the minor mode of the buffer. For example, "Fill"
appears when a buffer is in auto-fill mode.
is information about the position of the point in the buffer. This field
may be either `All',`Top', `Bot', or a percentage.
--**-Edwin: *scheme* (REPL: listen)----ALL-----------
means that Edwin is ready to evaluate something. Edwin changes
to eval when it is busy evaluating an expression. If Edwin has
been evaluating for a long time and you would like to stop it, type C-c
C-c to interrupt the Scheme interpreter and get back to "listen" status.
The mini-buffer is the bottom line in the Edwin window. It is used for
communicating with Edwin when you request certain commands. For example,
when you visit a file, Edwin prompts you for the file's name in the mini-buffer.
The commands you will use most often will deal with editing your code.
We will briefly describe those commands in this chapter.
In order to modify a file or to create a new one, you must first instruct
Edwin to "visit" the file. Do this by typing C-x C-f
This will cause Edwin to create a new buffer and place the contents of
the file into it. If the file does not exist, Edwin will make an empty
buffer and allow you to edit that instead. If you didn't want to start
a new file, type C-x k RET to kill the buffer and try again.
Once you have a few buffers in memory, you will want to switch between
them quite often. Typing C-x b buffer-name allows you to
enter the name of the buffer that you would like to switch to. It will
also display a default buffer that you may select by pressing return.
C-x C-b switches you to the `*buffer list*' buffer.
This buffer isn't meant for editing text. Instead, you use `*buffer
list*' to select buffers from a list of all the buffers that Edwin
knows about. You may use the cursor keys to move to the line of the buffer
and press `f' to select it.
Other commands have a natural side effect of moving you to a new buffer.
For example C-x k will kill the current buffer and then place
you into the previous buffer. Remember that if you kill a buffer without
saving it, all of your changes will be lost. Edwin will warn you if you
try to kill an unsaved buffer, and give you a chance to back out of the
command. If you really want to kill the buffer without saving your changes,
The mini-buffer is the last line of the Edwin window where Edwin sometimes
expects you to type responses to questions that it asks. There are two
things that you should know about the mini-buffer. First of all, pressing
will exit the mini-buffer at any time. Second, pressing SPC or
while in the mini-buffer will generally cause Edwin to try to complete
what you have begun to type. If you have typed a unique prefix to a word,
then Edwin will complete that word. Otherwise, Edwin will list all of the
possible completions to your prefix. You may then type just enough characters
to give you the command, file, or buffer that you really want. This feature
of Edwin will save you countless keystrokes. To learn how Edwin can complete
Scheme procedure and variable names, see section
for Scheme Symbols.
In general, you should never change the buffers that contain read-only
code given to you in the problem sets, nor should you change the
buffer. If you want to change these buffers, you should copy the sections
you want into one of your buffers first (see section Using
the Region to Copy or Move Text). Otherwise, you will end up with tiny
changes that no one will be able to find. This will make your code much
more difficult to debug.
Once you are in the buffer you want to edit, typing characters causes text
to be inserted into the buffer at the point (the little reverse-video
box), but nothing is overwritten. If you would like to delete a character
or two use Back space to remove the character to the left of the
point and C-d to delete to the right of the point. If you want
to delete a whole line, use C-k to erase all the text right of
the point on the current line (C-y will bring the text back if
you make a mistake).
The cursor motion keys (the keys with triangles on them) allow you to
move the point to a new location and begin inserting characters there.
There are also commands that allow you to move the point around much faster
over larger areas. C-v will move the point down an entire screen
while M-v will move the point up by the same amount.
will move the point to the end of a line while
C-a will move it
to the beginning. Refer to the on-line documentation for more commands
to move the point around.
If you make a mistake while editing your code, type C-x u. Edwin
remembers about 8000 characters worth of changes that it will allow you
to undo. Consecutive repetitions of C-x u will cause Edwin to
undo older and older changes. Any command other than an undo breaks the
sequence. At this point C-x u will allow you to undo your undos,
known as redoing an undo.
If you really mess up, it is sometimes desirable to restore the buffer
to the way it was the last time you saved it. You can do this by typing
Many Edwin commands operate on a region of text. First you set up the region
and then you type a command to operate on it. You begin the region by setting
up what is known as the mark. To do this, move to the beginning
of the text that you want to define as the region and then press C-SPC.
Next move the point to where you want the region to end and invoke a command.
For example, C-w will delete the region and place it into what
is affectionately called the kill ring. You can get this text back
by typing C-y. Also, you can type M-w which will place
a copy of the text into the kill ring without deleting it. You can also
extract this copy with C-y. To find out where the mark is at any
time, press C-x C-x which swaps the point and the mark. Pressing
C-x again will swap the point and mark back.
You should save your answer buffers periodically so that if the computer
crashes, you do not lose all your work. This is done by typing C-x
C-s. Also, be sure to save your buffers before you log out or quit
Generating all these files means that we will need some way to manage them.
Edwin provides a command, M-x dired, for showing a visual representation
of a directory of files. Dired creates a special buffer which you can not
edit in the normal way. Instead, you have an easy way to rename files,
to mark files for deletion, and to load files into buffers for editing.
When you are in the Dired buffer, typing
C-h m will display all
of the commands available there.
To select a file for editing, move the point to the line of a file and
then press `f'. This is like typing C-x C-f file-name.
Pressing `o' will do nearly the same thing except that the file
will be loaded into another text window and the Dired buffer will stay
where it is.
To delete individual files, move the cursor to the correct line and
press `d' and Edwin will place a `D' beside the file.
You can unmark the file by pressing `u' when you are on the line.
When you have marked all the files that you are sure you want to be deleted,
press `x' and answer yes. Now when you checkpoint your
disk or when you log out, Edwin will remove these files from your floppy
drive too. If you accidentally erase a file talk to an LA because sometimes
it is possible to recover lost files.
To delete backup copies of files with Dired, press `~' and
all the backup files will have a D placed beside their names. Now pressing
will cause Edwin to ask you if you would like to delete these files. Type
and then those files will be deleted. Again, the process isn't complete
until you checkpoint your disk or log out.
There are a few ways to evaluate code from a buffer. One way is to type
This command tells Edwin to send the entire buffer to the Scheme interpreter
to be evaluated. M-o is not defined in the
buffer. Another command, M-z, tells Edwin to only send the current
definition to Scheme in order to be interpreted. A definition in this case
is not necessarily an expression such as
(define (foobar baz) ...).
Instead, Edwin searches backwards until it finds an open parenthesis on
the left margin, searches forward to find its mate and evaluates the expression
in between. This makes it convenient for all sorts of "top-level" expressions.
A third way to evaluate expressions gives more control than either of the
first two methods. C-x C-e evaluates the expression just before
the point regardless of where it is in relation to all the parentheses.
Some people try to remember which definitions they have changed so that
they only evaluate those portions of their program. This is often more
trouble than it is worth because if you forget to update just a single
definition, then a bug that you thought you just fixed still appears and
you could be confused for a long time. If you always use M-o then
your code will be up-to-date with what is written when you test it.
The most sensible place to test your code is in the
buffer. Typing a test case (a Scheme expression meant to test portions
of your code) and then M-z or C-x C-e generates a nice
listing of trial expressions and their results. If you would like to repeat
a test case that you have evaluated already, M-p will allow you
to cycle through your most recent expressions. Every time you press
Edwin shows copies of older and older expressions that have been evaluated
in the `*scheme*' buffer. When this history runs out, Edwin just
starts over at the beginning. It is often the case that you would like
to repeat the last evaluated expression with different arguments. There
is a right and a wrong way to do this. The right way is to type M-p,
edit the expression, and then type C-x C-e. The wrong way is to
use the cursor keys to move up through the Scheme buffer and edit the previous
expression in its original context. It is much faster and easier to use
the history mechanism, and using it won't leave you with a confusing buffer,
unlike direct editing.
Note, however, that we urge you to only use this buffer for evaluating
test cases. Do the actual code development and modification in a
separate code buffer. This way you can cleanly separate your work
from its testing.
When testing your code you will find that often it doesn't work as expected.
Sometimes your code will stop dead in its tracks and produce an error message
on the screen, sometimes it will return an incorrect result, and sometimes
it won't return anything at all because it is caught in an infinite (or
at least an unreasonably long) loop. These mistakes in your code are called
and getting rid of them is an art-form known as debugging.
The first kind of bug is often the easiest to fix. Sometimes you have
just misspelled a variable or procedure name, or have misplaced a parenthesis.
Usually your typos are obvious when the computer points them out to you.
At other times the computer screams at you when your program is trying
to do an illegal operation (like trying to add two procedures!). In cases
like this, it is useful to enter the debugger and see what you can learn
about the bug. Questions you should ask yourself are:
These are useful questions to ask no matter what kind of bugs you have.
Remember that even if your procedure seems to work with one or two test
cases, you could still have errors in it. Make sure to test the boundary
conditions (if you don't your TA will). For example, maybe you forgot to
test your absolute value procedure with the number zero. Question: how
many fence posts do you need to buy to make 100 feet of fence with a fence
post every 10 feet? If you quickly answered 10, then you are especially
susceptible to fence-post errors. Otherwise, see how many of your friends
will fall for this one.
How did my procedure get into this state?
Do any variables have values that they clearly should not?
Are all the procedures that I use in this procedure working correctly?
Do I have the arguments in the correct order?
Am I using the correct variables?
Does my procedure have a logical error?
Did I remember to evaluate all the changes that I made?
Did I do anything that would cause any old definitions to still be around?
Did I write this code?
Only experience can help you become a master debugger. Often the first
thing that a beginner does when he or she gets an error message is to type
to avoid entering the debugger. This violates a very important rule of
debugging: don't throw away information. So when you get an error, go ahead
and see what information you can gather.
If the debugger doesn't give you the needed information, sometimes it
is useful to put a display expression into your code to gather
information. Also, you may make your procedures robust so that when they
get illegal values, they give you information that the debugger wouldn't
give you. These two methods are especially useful with the never-ending-procedure
variety of bugs.
A more useful way to gather information is to use the procedure
displays its first argument which should be a string and then displays
the rest of its arguments which are any objects of special concern to debugging.
Then you are asked if you would like to enter the debugger. Here is an
(define (cube x)
(if (not (number? x))
(error "Argument should be a number instead of" x)
(* x x x)))
;cube --> #[compound-procedure 28 cube]
;Argument should be a number instead of hi
;Type D to debug error, Q to quit back to REP loop:
One of the best ways of debugging code is to get a lab assistant to help
you. Even if they can't immediately find your bug, they can probably tell
you whether what you are trying to do is a good idea. Another powerful
debugging technique is to completely rewrite some of your code in an improved
way. It is often easier to avoid bugs than to find them so use a clear
design instead of clever or tricky code that is sure to fail during the
next waning crescent. However, if you enjoy debugging code, feel free to
make lots of mistakes so that you can find them later.
For more information on using the debugger, see section
If you type an expression in `*scheme*' buffer and, instead of
evaluating it with C-x C-e, type M-s, Edwin will invoke
the Scheme stepper, which permits you to go through step through evaluation
of the expression element by element and see the evaluation of each subexpression.
In general, you use the debugger and stepper to home in on bugs from
two different "directions." If you have a program that signals an error,
you can just let the error occur and use the debugger to try to figure
out what happened; or you can step through the program up to the point
where you see the error happen and try to figure out what is causing it.
To exit from the stepper, simply kill the stepper buffer.
There are different forms of on-line documentation. Some on-line documentation
is meant to teach you skills. For example, the Edwin tutorial is an interactive
tutorial useful for when you first begin using Edwin. It is accessed by
typing C-h t.
Most of the on-line documentation is meant to be used as reference material.
This is available through web from the 6.001 home page. The documentation
files are included with the 6.001 distributions as HTML files that you
can read locally with a web browser if you are using your own computer
and are not connected to the network.
Here is a short summary of the on-line information available off the course
To print out a buffer type M-x print-buffer. Remember to get your
print-out right away or it may get lost in a huge heap of paper. The header
has the machine name on it.
This manual in its on-line incarnation.
`Scheme Reference Manual'
This is the MIT Scheme Reference Manual. It documents the special forms
and procedures that are available in MIT Scheme.
`Scheme User's Manual'
This has some details on MIT Scheme and Edwin.
The Revised Report on the Algorithmic Language Scheme is the official
(and not very readable) specification of the Scheme language.
Alternatively, from a top level window, use the Linux commands to print
will for example connect to your work directory, then print out the
Scheme file (note the .scm extension) titled project1.
The easiest way to get a transcript is to copy text from the
buffer and accumulate it into another buffer. You should remove the test
cases that you do not wish to keep and place your name and other information
including the problem set number and the exercise at the top. You might
also want to make special comments about some of the test cases. When you
are done with the entire problem set, you can print out this buffer. It's
a good idea to comment the transcript after each problem, while the details
are still in your mind. Incrementally building your final transcript takes
less effort than doing it all at the end.
To logout, make sure your have saved all of your files and then type
logout. Note that the standard quit command: C-x C-c also
works. If you have forgotten to save any of your buffers, Edwin will give
you a chance to do it now. If you still don't save all of the buffers,
Edwin will say "Modified buffers exist. Exit anyway?" If you don't want
to save these buffers then answer yes. Now Edwin will checkpoint
your floppy disk. Edwin will ask if you want to kill Scheme. Type yes
if you really want to exit Edwin and Scheme and return to the main Login
screen. Now you should take the floppy disk out of the floppy drive. If
you do leave something behind in the lab and can't find it later, check
the lost and found box near the front of the lab.
If you find any bugs, or problems dealing with the lab, send mail to:
You can send email to the entire 6001 staff via the address
For comments specific to the course you can either send mail to firstname.lastname@example.org
If you have any problems that aren't listed here, please pass them along
to us so that we can include them in the next printing of this document.
For directions on doing this, see section Feedback.
One cause of this is that Edwin is not receiving any keystrokes from the
window manager. If this is the case, the border around Edwin will be light
grey instead of dark grey. To fix this problem, click the left mouse button
when the cursor is in the border of the Edwin window.
Often times you will lose and just want to return to some sort of steady
state so that you can continue to do your work. If all of a sudden you
do something that seems to trash half of your buffer, stop and relax. The
first thing that you should try is C-x u, the undo command. If
this fails to do any good, you might try performing a yank using
If neither of these seems to work, use the write command,
which will allow you to save the buffer under a different name. Now you
can load up the old version using C-x C-f. Next, visually compare
these two buffers to see if you can recover any recent changes. You might
also want to save your buffers often so that you can always restore to
a steady state by doing a M-x revert-buffer. If your problem is
that Edwin does not seem to be evaluating things properly, try typing C-c
C-c a few times. Look at the mode line in the `*scheme*'
buffer and make sure you see the word "listen" which means that Scheme
is ready to accept expressions. For really drastic problems, try saving
all of your buffers that don't seem screwed up and logging out. When you
log back in, things should be back to normal. If this doesn't help then
it is time to talk to an LA.
Edwin uses major modes to determine which keyboard commands are available
in a buffer. Edwin defines two major modes that facilitate the writing
and testing of Scheme programs. The REPL major mode is the default mode
of the `*scheme*' buffer and Scheme mode is normally used in buffers
of Scheme source code. REPL and Scheme mode are very similar to each other
and to the Emacs LISP major modes. REPL and Scheme modes include the editing
commands from fundamental mode and enhance these with more commands for
evaluating Scheme expressions.
The Scheme major mode is specialized for editing Scheme code. It adds indentation
and evaluation commands to the normal array of editing commands.
The following commands evaluate Scheme expressions:
indent the current line for Scheme.
indent the next expression.
TAB indents the current line to show the nesting of parentheses.
Pressing TAB anywhere on a line has the same effect: attractive
source files that make parenthesis-balancing clear.
Read and evaluate an expression in mini-buffer (eval-expression).
Evaluate the current definition (eval-defun).
Evaluate the expression preceding point, placing the result in the
Evaluate the buffer (eval-current-buffer).
Evaluate the current region (eval-region).
C-M-q is a great way to indent the current definition. Move
to the beginning of the current definition (using C-M-a) before
you use this command.
M-z is the work horse evaluation command in a REPL buffer,
but it is also useful in a Scheme buffer. M-z looks backwards
to find an expression that starts on the left margin and then evaluates
For more precise control over what is evaluated, use C-x C-e.
C-e evaluates the expression before the point regardless of where
it is in a parenthesized structure. Unlike M-z, it is not meant
for evaluating just top-level expressions (more importantly, it can evaluate
expressions that are not combinations).
M-o evaluates the buffer, which is useful because it insures
that all of your changes are evaluated. To make this command even more
useful, you should refrain from putting procedure definitions and operations
that are meant to test these procedures in the same source file. For example,
if you have the fibonacci definition in a file along with (fib 100000)
then evaluating the entire buffer will take too long. Instead, do all your
testing in the `*scheme*' buffer. To preserve your tests, you
can write them out as a separate file. Once you have fully tested a procedure,
you should save the transcript of your successful test cases into a separate
buffer and write it to disk. At the end of the problem set, you can collect
all of these transcripts into a single buffer and then print it out all
REPL is the major mode for communicating with an inferior read-eval-print
loop (the `*scheme*' buffer is normally in the REPL major mode).
All the editing and evaluation commands from Scheme mode are available
in addition to the ones listed below.
Expressions submitted for evaluation are saved in an expression history.
The history may be accessed with the following commands:
Stops an evaluation and returns to the top level. Can also be used like
when Edwin displays ;Type D to debug error, Q to quit back to REP loop.
M-p is the most frequently used history command. Imagine that
you have typed (+ 2 5) and then M-z. Now you wish to
evaluate the same expression using a zero instead of the five. Typing M-p
will recall the previous expression and place it into the current buffer
where the point is located. Now you can use the normal editing commands
to change the expression into the desired form. When you are done, you
can type M-z to evaluate your new expression. At this point, typing
twice would recall the first expression, (+ 2 5), while typing
just once would recall just the most recent expression,
(+ 2 0).
This chapter documents the special features that Edwin shares with Emacs
that allow you to edit Scheme code efficiently. These commands are almost
always available in any major mode meant for editing.
By convention, Edwin keys for dealing with balanced expressions are usually
characters. They tend to be analogous in function to the corresponding
and Meta- commands.
Cycle backward through the history.
Cycle forward through the history.
Search backward for a matching string.
Search forward for a matching string.
These commands fall into two classes. Some deal only with lists
(parenthetical groupings). They see nothing except parentheses, brackets,
braces, and escape characters that might be used to quote those.
The other commands deal with expressions or s-expressions. The
word `s-expression' is derived from symbolic expression, the ancient
term for any kind of expression in LISP. S-expressions are symbols, numbers,
string constants, and lists.
These commands can be used to find an unbalanced parenthesis quite easily.
To find an extra close parenthesis, move to the the beginning of the buffer
and type an open parenthesis. Then move backwards one character and type
f. The point should move to the definition with the extra parenthesis.
Move to the beginning of the current definition (beginning-of-defun).
Move to the end of the current definition (end-of-defun).
Move forward over an expression (forward-sexp).
Move backward over an expression (backward-sexp).
Kill expression forward (kill-sexp).
Move up and backward in list structure (backward-up-list).
Move down and forward in list structure (down-list).
Move forward over a list (forward-list).
Move backward over a list (backward-list).
Transpose expressions (transpose-sexps).
Put mark after following expression (mark-sexp).
In a Scheme mode buffer, lines are indented according to their nesting
in parentheses. To indent a line, press TAB. In Scheme or REPL
mode, TAB aligns the line according to its depth in parentheses.
No matter where in the line you are when you type TAB, it aligns
the line as a whole.
There are several commands that will re-indent several lines of code.
Indent current line "appropriately" for the current mode.
Perform RET followed by TAB (newline-and-indent).
Join current line with line above (delete-indentation). This cancels
out the effect of LFD.
Split line at point; text on the line after point becomes a new line indented
to the same column that it now starts in (split-line).
Move (forward or back) to the first non-blank character on the current
Indent several lines to same column (indent-region).
Shift block of lines rigidly right or left (indent-rigidly).
Indent from point to under an indentation point in the previous line.
You can re-indent the contents of a single expression by positioning the
point before the beginning of it and typing C-M-q (indent-sexp).
The indentation of the current line is not changed; therefore, only the
relative indentation within the list, and not its position, is changed.
To correct the position as well, type a
TAB before the C-M-q.
Re-indent all the lines within one expression (indent-sexp).
Shift an entire expression rigidly sideways so that its first line is properly
Re-indent all lines in the region (indent-region).
C-M-q is a great way to indent the current definition. Move
to the beginning of the current definition using C-M-a before
you use this command.
If the relative indentation within an expression is correct but the
indentation of its beginning is not, go to the line the expression begins
on and type C-u TAB. When TAB is given a numeric argument,
it moves all the lines in the grouping starting on the current line sideways
the same amount that the current line moves. It is clever, though, and
does not move lines that start inside strings.
Another way to specify the range to be re-indented is with the point
and mark. The command C-M-\ (indent-region) applies TAB
to every line whose first character is between the point and mark.
Completion maximizes the entropy of your keystrokes. Whenever the computer
can figure out what you mean to type, you may tell it to type the rest
for you. Usually completion happens in the mini-buffer. But one kind of
completion is available in Scheme and REPL buffers: completion for Scheme
The command M-TAB takes the partial Scheme variable name before
point to be an abbreviation, and compares it against all bound variables
in the REPL environment. Any additional characters that they all have in
common are inserted at the point.
If the partial name in the buffer is not a unique prefix, a list of
all possible completions is displayed in another window. At this point
you can type enough characters to make the symbol unique and press
C-u M-TAB works like M-TAB except that it also completes
When an error occurs in your code, you will be asked whether you would
like to enter the debugger. The debugger creates two buffers in which debugging
information is presented. The contents of the buffers will change based
on the commands you enter.
Each line beginning with `S' represents either a subproblem or a stack
frame. A subproblem line may be followed by one or more indented lines
(beginning with the letter `R') which represent reductions associated with
that subproblem. To obtain a more complete description of a subproblem
or reduction, click the mouse on the desired line or move the cursor to
the line using the arrow keys (or C-n and C-p). The description
buffer will display the additional information.
Select a subproblem or reduction and display information in the description
Move the cursor down the list of subproblems and reductions and display
info in the description buffer.
Move the cursor up the list of subproblems and reductions and display info
in the description buffer.
Show the environment structure.
Quit the debugger, destroying its window.
Invoke the standard restarts.
Display info on current item in the description buffer.
Display help information.
The description buffer contains three major regions which contain information
associated with the selected subproblem or reduction. The first region
contains a pretty printed version of the expression. The second region
contains a representation of the environment. The variables in the frames
are listed along with there values. The bottom of the description buffer
contains a region for evaluating expressions in the environment of the
selected subproblem or reduction (similar to the `*scheme*' buffer).
This is the only portion of the buffer where editing is possible. Evaluating
expressions here can be useful in gathering information about the circumstances
of the bug.
Typing e creates a new buffer in which you may browse through
the current environment. In this new buffer, you can use the mouse, the
arrows, or C-n and C-p to select lines and view different
environments. The environments listed are the same as those in the description
buffer. If the selected environment structure is too large to display (if
there are more than environment-package-limit items in the environment)
an appropriate message is displayed. To display the environment in this
case, set the
environment-package-limit variable to #f.
This process is initiated by the command M-x set-variable. You
can not use
set! to set the variable because it is an editor variable
and does not exist in the current scheme environment. At the bottom of
the new buffer is a region for evaluating expressions similar to that of
the description buffer.
Type q or to quit the debugger, killing its primary buffer
and any others that it has created.
NOTE: The debugger creates description buffers in which debugging information
is presented. These buffers are given names beginning with spaces so that
they do not appear in the buffer list; they are automatically deleted when
you quit the debugger using q. If you wish to keep one of these
buffers, rename it using M-x rename-buffer: once it has been renamed,
it will not be deleted automatically.
Understanding the concepts of reduction and subproblem is
essential to good use of the debugging tools. The Scheme interpreter evaluates
an expression by reducing it to a simpler expression. In general,
Scheme's evaluation rules designate that evaluation proceeds from one expression
to the next by either starting to work on a
subexpression of the
given expression, or by reducing the entire expression to a new
(simpler, or reduced) form. Thus, a history of the successive forms processed
during the evaluation of an expression will show a sequence of subproblems,
where each subproblem may consist of a sequence of reductions.
For example, both (+ 5 6) and (+ 7 9) are subproblems
of the following combination:
(* (+ 5 6) (+ 7 9))
If (prime? n) is true, then (cons 'prime n) is the reduction
for the following expression:
(if (prime? n)
(cons 'prime n)
(cons 'not-prime n))
This is because the entire subproblem of the if combination can
be reduced to the problem (cons 'prime n), once we know
that (prime? n) is true; the (cons 'not-prime n) can
be ignored, because it will never be needed. On the other hand, if
n) were false, then (cons 'not-prime n) would be the reduction
for the if combination.
The subproblem level is a number representing how far back in
the history of the current computation a particular evaluation is. Consider
(define (factorial n)
(if (< n 2)
(* n (factorial (- n 1)))))
If we stop factorial in the middle of evaluating (- n 1),
the (- n 1) is at subproblem level 0. Following the history of
the computation "upwards," (factorial (- n 1)) is at subproblem
level 1, and (* n (factorial (- n 1))) is at subproblem level
2. These expressions all have reduction number 0. Continuing upwards,
the if combination has reduction number 1.
Moving backwards in the history of a computation, subproblem levels
and reduction numbers increase, starting from zero at the expression currently
being evaluated. Reduction numbers increase until the next subproblem,
where they start over at zero. The best way to get a feel for subproblem
levels and reduction numbers is to experiment with the debugger.
A buffer is a block of text that you may examine and change. Whenever
you edit in Edwin, you change the contents of a buffer. To preserve a buffer
for another time you will use Edwin, you must
save the buffer to
a file on disk. Then you must remember to checkpoint your floppy
A command is an instruction you give Edwin through a special key
combination. For example, use the C-x C-f command to copy a file
from a disk into a new buffer.
The cursor is a solid rectangle visible on the screen whenever the computer
is waiting for you to type something. If you type an ordinary character
(letter, number, or punctuation), it will be inserted at the current cursor
position, and the cursor will move past it.
An extended command is a command used by name, rather than by a
special key. To use an extended command, press M-x, followed by
the name of the command. Your typing will appear at the bottom of the screen,
in the mini-buffer (see below). When you've typed the full name of the
extended command, press RET.
A file is a block of text stored on a disk. A file may be copied
from a disk into a buffer. A file is created by saving the contents of
a buffer on a disk.
The several most recently-deleted regions are kept in the kill ring.
Killed text can be retrieved with a yank.
The mark is an invisible point in a buffer. Many commands set it
to the buffer location where they were performed. Each buffer has its own
mark. C-x C-x exchanges the point and the mark, thus revealing
The mini-buffer is that last line of the Edwin window. It is used
by Edwin to display informative messages and to prompt you for various
inputs, for example, file names, confirmation, etc.
Every buffer has one major mode, and maybe some minor modes.
determine special Edwin behavior particular to the language you are using
(e.g., Scheme or English). Minor modes add special features regardless
of the language you are using in the buffer.
The current cursor location in each buffer is called the point.
The cursor is just a reflection of the point in the current buffer.
Text between the point and the mark is called the
To save is to create a disk file from a buffer. The file and the
buffer are identical, but only the file remains when you log out; the buffer
is erased. However if you don't checkpoint your files onto your floppy
disk, your files will be erased when someone else logs into the machine.
A window has two meanings. When referring just to Edwin, it is a
rectangular area on the screen sometimes called a text window. When
referring to a program in general, it is an area of the screen that contains
text or graphics and is manipulated by the window manager. Each text window
displays part of some buffer. Because most buffers are too big to fit in
one window, only part of the buffer is visible at any time; you can see
other parts by using Edwin commands to scroll through the buffer.