This chapter details Scheme 48’s command processor, which incorporates
both a read-eval-print loop and an interactive debugger.
At the > prompt, you can type either a Scheme form
(expression or definition) or a command beginning with a comma.
In
inspection mode (see section 3.7)
the prompt changes to : and commands
no longer need to be preceded by a comma; input beginning with
a letter or digit is assumed to be a command, not an expression.
In inspection mode the command processor prints out a
menu of selectable components for the current object of interest.
The command processor keeps track of a current focus value.
This value is normally the last value returned by a command.
If a command returns multiple values the focus object is a list of the
values.
The focus value is not changed if a command returns no values or
a distinguished ‘unspecific’ value.
Examples of forms that return this unspecific value are definitions,
uses of set!, and (if #f 0).
It prints as #{Unspecific}.
The reader used by the command processor reads ## as a special
expression that evaluates to the current focus object.
> (list 'a 'b)
'(a b)
> (car ##)
'a
> (symbol->string ##)
"a"
> (if #f 0)
#{Unspecific}
> ##
"a"
>
If an error, keyboard interrupt, or other breakpoint occurs, or the
,push command is used, the command
processor invokes a recursive copy of itself, preserving the dynamic state of
the program when the breakpoint occured.
The recursive invocation creates a new command level.
The command levels form a stack with the current level at the top.
The command prompt indicates the number of stopped levels below the
current one: > or : for the
base level and n> or n: for all other levels,
where n is the command-level nesting depth.
The levels setting
described below can be used to disable the automatic pushing of new levels.
The command processor’s evaluation package and the value of the
current focus value are local to each command level.
They are preserved when a new level is pushed and restored when
it is discarded.
The settings of all other settings are shared by all command levels.
-
- ⟨eof⟩
Discards the current command level and resumes running the level down.
⟨eof⟩ is usually
control-D at a Unix shell or control-C control-D using
the Emacs cmuscheme48 library.
- ,pop
The same as ⟨eof⟩.
- ,proceed [exp ...]
Proceed after an interrupt or error, resuming the next command
level down, delivering the values of exp ... to the continuation.
Interrupt continuations discard any returned values.
,Pop and ,proceed have the same effect after an interrupt
but behave differently after errors.
,Proceed restarts the erroneous computation from the point where the
error occurred (although not all errors are proceedable) while
,pop (and ⟨eof⟩) discards it and prompts for
a new command.
- ,push
Pushes a new command level on above the current one.
This is useful if the levels setting has been used
to disable the automatic pushing of new levels for errors and interrupts.
- ,reset [number]
Pops down to a given level and restarts that level.
Number defaults to zero, ,reset restarts the command
processor, discarding all existing levels.
Whenever moving to an existing level, either by sending
an ⟨eof⟩
or by using ,reset or the other commands listed above,
the command processor runs all of the dynamic-wind “after” thunks
belonging to stopped computations on the discarded level(s).
-
- ,load filename ...
Loads the named Scheme source file(s).
Easier to type than (load "filename") because you don’t have to
shift to type the parentheses or quote marks.
(However, it is still possible to specify a filename as a Scheme
string literal, with quote marks—you’ll need this for filenames
containing whitespace.)
Also, it works in
any package, unlike (load "filename"), which will work only
work in packages in which the variable load is defined appropriately.
- ,exit [exp]
Exits back out to shell (or executive or whatever invoked Scheme 48
in the first place).
Exp should evaluate to an integer. The
integer is returned to the calling program.
The default value of exp is zero, which, on Unix,
is generally interpreted as success.
There are many commands related to modules.
Only the most commonly used module commands are described here;
documentation for the
rest can be found in section 4.8.
There is also
a brief description of modules, structures, and packages in section 2.6 below.
-
- ,open structure ...
Makes the bindings in the structures visible in the current package.
The packages associated with the structures will be loaded if
this has not already been done (the ask-before-loading setting
can be used disable the automatic loading of packages).
- ,config [command]
Executes command in the config package, which includes
the module configuration language.
For example, use
,config ,load filename
to load a file containing module definitions.
If no command is given, the config package becomes the
execution package for future commands.
- ,user [command]
This is similar to the ,config. It
moves to or executes a command in the user package (which is the
default package when the Scheme 48 command processor starts).
-
- ,preview
Somewhat like a backtrace, but because of tail recursion you see
less than you might in debuggers for some other languages.
The stack to display is chosen as follows:
If the current focus object is a continuation or a thread,
then that continuation or thread’s stack is displayed.
Otherwise, if the current command level was initiated because of
a breakpoint in the next level down, then the stack at that
breakpoint is displayed.
Otherwise, there is no stack to display and a message is printed
to that effect.
One line is printed out for each continuation on the chosen stack,
going from top to bottom.
- ,run exp
Evaluate exp, printing the result(s) and making them
(or a list of them, if exp returns multiple results)
the new focus object.
The ,run command is useful when writing
command programs, which are described in section 3.8 below.
- ,trace name ...
Start tracing calls to the named procedure or procedures.
With no arguments, displays all procedures currently traced.
This affects the binding of name, not the behavior of the
procedure that is its current value. Name is redefined
to be a procedure that prints a message,
calls the original value of name, prints another
message, and finally passes along the value(s) returned by the
original procedure.
- ,untrace name ...
Stop tracing calls to the named procedure or procedures.
With no argument, stop tracing all calls to all procedures.
- ,condition
The ,condition command displays the condition object
describing the error or interrupt that initiated the current
command level. The condition object becomes the current focus
value. This is particularly useful in conjunction with
the inspector. For example, if a procedure is passed the wrong number of
arguments, do ,condition followed by
,inspect to inspect the
procedure and its arguments.
- ,bound? name
Display the binding of name, if there is one, and otherwise
prints ‘Not bound’.
- ,expand form
- ,expand-all form
Show macro expansion of form, if any.
,expand performs a single macro expansion while
,expand-all fully expands all macros in form.
- ,where procedure
Display name of file containing procedure’s source code.
There are a number of settings that control the behavior of the
command processor; most of them are booleans.
They can be set using the ,set and ,unset commands.
-
- ,set setting [on | off | literal | ?]
This sets the value of setting setting.
For a boolean setting, the second argument must be on or
off; it then defaults to on.
Otherwise, the value must be a literal, typically a posititive number.
If the second argument is ? the value of setting is
is displayed and not changed.
Doing ,set ? will display a list of the setting and
their current values.
- ,unset setting
,unset setting is the same as
,set setting off.
The settings are as follows:
-
- batch (boolean)
In ‘batch mode’ any error or interrupt that comes up will cause
Scheme 48 to exit immediately with a non-zero exit status. Also,
the command processor doesn’t print prompts. Batch mode is
off by default.
- levels (boolean)
Enables or disables the automatic pushing of a new command level when
an error, interrupt, or other breakpoint occurs.
When enabled (the default), breakpoints push a new command level,
and ⟨eof⟩ (see above)
or ,reset is required to return to top level. The effects of
pushed command levels include:
With levels disabled one must issue a
,push command immediately
following an error in order to retain the error continuation for
debugging purposes; otherwise the continuation is lost as soon as
the focus object changes. If you don’t know anything about the
available debugging tools, then levels might as well be disabled.
- break-on-warnings (boolean)
Enter a new command level when a warning is produced, just as
when an error occurs. Normally warnings only result in a displayed
message and the program does not stop executing.
- ask-before-loading (boolean)
If on, the system will ask before loading modules that are arguments
to the ,open command. Ask-before-loading is off by
default.
> ,set ask-before-loading
will ask before loading modules
> ,open random
Load structure random (y/n)? y
>
- load-noisily (boolean)
When on, the system will print out the names of modules and files
as they are loaded. load-noisily is off by default.
> ,set load-noisily
will notify when loading modules and files
> ,open random
[random /usr/local/lib/scheme48/big/random.scm]
>
- inline-values (boolean)
This controls whether or not the compiler is allowed to substitute
variables’ values in-line.
When inline-values mode is on,
some Scheme procedures will be substituted in-line; when it is off,
none will.
Section 2.4
has more information.
- inspector-menu-limit (positive integer)
This controls how many items the displayed portion of the inspector
menu contains at most. (See Section 3.7.)
- inspector-writing-depth (positive integer)
This controls the maximum depth to which the inspector
prints values. (See Section 3.7.)
- inspector-writing-length (positive integer)
This controls the maximum length to which the inspector
prints values. (See Section 3.7.)
- condition-writing-depth (positive integer)
This controls the maximum depth to which conditions
are printed.
- condition-writing-length (positive integer)
This controls the maximum length to which conditions
are printed.
- trace-writing-length (positive integer)
This controls the maximum length to which tracing
prints procedure calls.
There is a data inspector available via the ,inspect and
,debug commands.
The inspector is particularly useful with procedures, continuations,
and records.
The command processor can be taken out of inspection mode by
using the q command.
When in inspection mode, input that begins with
a letter or digit is read as a command, not as an expression.
To see the value of a variable or number, do (begin exp)
or use the ,run exp command.
In inspection mode the command processor prints out a
menu of selectable components for the current focus object.
To inspect a particular component, just type the corresponding number in
the menu.
That component becomes the new focus object.
For example:
> ,inspect '(a (b c) d)
(a (b c) d)
[0] a
[1] (b c)
[2] d
: 1
(b c)
[0] b
[1] c
:
When a new focus object is selected the previous one is pushed onto a
stack.
You can pop the stack, reverting to the previous object, with
the u command, or use the stack command to move to
an earlier object.
Commands useful when in inspection mode:
u (up) pop object stack
m (more) print more of a long menu
(...) evaluate a form and select result
q quit
template select a closure or continuation’s template
(Templates are the static components of procedures; these are found
inside of procedures and continuations, and contain the quoted
constants and top-level variables referred to by byte-compiled code.)
d (down) move to the next continuation
(current object must be a continuation)
menu print the selection menu for the focus object
Multiple selection commands (u, d, and menu indexes)
may be put on a single line.
All ordinary commands are available when in inspection mode.
Similarly, the inspection commands can be used when not in inspection
mode.
For example:
> (list 'a '(b c) 'd)
'(a (b c) d)
> ,1
'(b c)
> ,menu
[0] b
[1] c
>
If the current command level was initiated because of
a breakpoint in the next level down, then
,debug will invoke the inspector on the
continuation at the point of the error.
The u and d (up and down)
commands then make the inspected-value stack look like a conventional stack
debugger, with continuations playing the role of stack frames. D goes
to older or deeper continuations (frames), and u goes back up to more
recent ones.
The exec package contains procedures that are used
to execute the command processor’s commands.
A command ,foo is executed by applying the value of
the identifier foo in the exec package to
the (suitably parsed) command arguments.
-
- ,exec [command]
Evaluate command in the exec package.
For example, use
,exec ,load filename
to load a file containing commands.
If no command is given, the exec package becomes the
execution package for future commands.
The required argument types are as follows:
filenames should be strings
other names and identifiers should be symbols
expressions should be s-expressions
commands (as for ,config and ,exec itself)
should be lists of the form
(command-name argument ...)
where command-name is a symbol.
For example, the following two commands are equivalent:
,config ,load my-file.scm
,exec (config '(load "my-file.scm"))
The file scheme/vm/load-vm.scm in the source directory contains an
example of an exec program.
-
- ,dump filename [identification]
Writes the current heap out to a file, which can then be run using the
virtual machine. The new image file includes the command processor.
If present, identification
should be a string (written with double quotes); this string will
be part of the greeting message as the image starts up.
- ,build exp filename [option ...]
Like ,dump, except that the image file contains the value of
exp, which should be a procedure of one argument, instead of
the command processor. When
filename is resumed, that procedure will be invoked on the VM’s
-a arguments, which are passed as a list of OS strings (see
section 5.15. The
procedure should return an integer which is
returned to the program that invoked the VM. The command
processor and debugging system are not included in the image
(unless you go to some effort to preserve them, such as retaining
a continuation).
If no-warnings appears as an option after the file
name, no warnings about undefined external bindings
(see Section 8.2) will be printed upon
resuming the image. This is useful when the definitions of
external bindings appear in shared objects that are only loaded
after the resumption of the image.
Doing ,flush before building an image will reduce the amount
of debugging information in the image, making for a smaller
image file, but if an error occurs, the error message may be less
helpful. Doing ,flush source maps before loading any programs
used in the image will make it still smaller.
See section 3.10
for more information.
.
-
- ,time exp
Measure execution time.
- ,collect
Invoke the garbage collector. Ordinarily this happens
automatically, but the command tells how much space is available
before and after the collection.
- ,keep kind
- ,flush kind
These control the amount of debugging information retained after
compiling procedures. This information can consume a fair amount
of space. kind is one of the following:
maps - environment maps (local variable names, for inspector)
source - source code for continuations (displayed by inspector)
names - procedure names (as displayed by write and in error
messages)
files - source file names
These commands refer to future compilations only, not to procedures
that already exist. To have any effect, they must be done before
programs are loaded. The default is to keep all four types.
- ,flush
The flush command with no argument deletes the database of names
of initial procedures. Doing ,flush before a ,build or
,dump
will make the resulting image significantly smaller, but will
compromise the information content of many error
messages.
Each command level has its own set of threads. These threads are suspended
when a new level is entered and resumed when the owning level again becomes
the current level.
A thread that raises an error is not resumed unless
explicitly restarted using the ,proceed command.
In addition to any threads spawned by the user, each level has a thread
that runs the command processor on that level.
A new command-processor thread is started if the current one
dies or is terminated.
When a command level is abandoned for a lower level, or when
a level is restarted using ,reset, all of the threads on that
level are terminated and any dynamic-wind “after” thunks are run.
The following commands are useful when debugging multithreaded programs:
-
- ,resume [number]
Pops out to a given level and resumes running all threads at that level.
Number defaults to zero.
- ,threads
Invokes the inspector on a list of the threads running at the
next lower command level.
- ,exit-when-done [exp]
Waits until all user threads have completed and then
exits back out to shell (or executive or whatever invoked Scheme 48
in the first place).
Exp should evaluate to an integer which is then
returned to the calling program.
-
- ,go exp
This is like ,exit exp except that the evaluation of exp
is tail-recursive with respect to the command processor. This
means that the command processor itself can probably be GC’ed,
should a garbage collection occur in the execution of exp.
If an error occurs Scheme 48 will exit with a non-zero value.
- ,translate from to
For load and the ,load command
(but not for open-{in|out}put-file), file
names beginning with the string from will be changed so that the
initial from is replaced by the string to. E.g.
,translate /usr/gjc/ /zu/gjc/
will cause (load "/usr/gjc/foo.scm") to have the same effect as
(load "/zu/gjc/foo.scm").
- ,from-file filename form ... ,end
This is used by the cmuscheme48 Emacs library to indicate the file
from which the forms came. Filename is then used by the
command processor to determine the package in which the forms
are to be evaluated.