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Chapter 3

Command processor

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.

3.1  Current focus value and ##

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"
>

3.2  Command levels

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 auto-levels switch described below can be used to disable the automatic pushing of new levels.

The command processor's evaluation package and the value of the inspect-focus-value switch 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 switches 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 auto-levels switch 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).

3.3  Logistical commands

,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.

3.4  Module commands

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 switch 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).

3.5  Debugging commands

,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:

  1. If the current focus object is a continuation or a thread, then that continuation or thread's stack is displayed.

  2. 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.

  3. 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.

3.6  Switches

There are a number of binary switches that control the behavior of the command processor. They can be set using the ,set and ,unset commands.

,set switch [on | off | ?]
This sets the value of mode-switch switch. The second argument defaults to on. If the second argument is ? the value of switch is is displayed and not changed. Doing ,set ? will display a list of the switches and their current values.

,unset switch
,unset switch is the same as ,set switch off.

The switches are as follows:

batch
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.

auto-levels
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:

  • a longer prompt

  • retention of the continuation in effect at the point of errors

  • confusion among some newcomers

With auto-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.

inspect-focus-value
Enable or disable `inspection' mode, which is used for inspecting data structures and continuations. Inspection mode is described in section 3.7.

break-on-warnings
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
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
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
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.

3.7  Inspection mode

There is a data inspector available via the ,inspect and ,debug commands or by setting the inspect-focus-value switch. 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, by unsetting the inspect-focus-value switch, or by going to a command level where the inspect-focus-value is not set. 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:

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.

3.8  Command programs

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:

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.

3.9  Building images

,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 strings. 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 7.2) 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.

3.10  Resource query and control

.

,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.

3.11  Threads

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.

3.12  Quite obscure

,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.

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