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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd"><!-- DO NOT EDIT THIS FILE-->
<!-- Edit the .tex version instead-->
<html>
<head>
<title>System Operations</title>
<link href="csug.css" rel="stylesheet" type="text/css">
</head>
<body>
<a name="g107"></a>
<a name="./system:h0"></a>
<h1>Chapter 12. System Operations<a name="CHPTSYSTEM"></a></h1>
<p>
This chapter describes operations for
handling exceptions, interrupts, environments,
compilation and evaluation, profiling,
controlling the operation of the system,
timing and statistics,
defining and setting parameters,
and
querying the operating system environment.
<p>
<h3><a name="g108"></a><a name="./system:h1"></a>Section 12.1. Exceptions<a name="SECTSYSTEMEXCEPTIONS"></a></h3>
<p>
<a name="./system:s0"></a><i>Chez Scheme</i> provides some extensions to the
Revised<sup>6</sup> Report exception-handling mechanism, including mechanisms
for producing formatted error messages, displaying conditions,
and redefining the base exception handler.
These extensions are described in this section.
<p>
<a name="./system:s1"></a><span class=formdef><b>procedure</b>: <tt>(warning <i>who</i> <i>msg</i> <i>irritant</i> ...)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>warning</tt> raises a continuable exception with condition type
<tt>&warning</tt> and should be used to describe situations for which the
<tt>&warning</tt> condition type is appropriate, typically a situation
that should not prevent the program from continuing but might result
in a more serious problem at some later point.
<p>
The continuation object with which the exception is raised also includes
a <tt>&who</tt> condition whose who field is <tt><i>who</i></tt> if <tt><i>who</i></tt> is
not <tt>#f</tt>, a <tt>&message</tt> condition whose message field is
<tt><i>msg</i></tt>, and an <tt>&irritants</tt> condition whose irritants field
is <tt>(<i>irritant</i> ...)</tt>.
<p>
<tt><i>who</i></tt> must be a string, a symbol, or <tt>#f</tt> identifying the procedure
or syntactic form reporting the warning upon whose behalf the warning is being
reported.
It is usually best to identify a procedure the programmer has called rather
than some other procedure the programmer may not be aware is involved in
carrying out the operation.
<tt><i>msg</i></tt> must be a string and should describe the exceptional situation.
The irritants may be any Scheme objects and should include values that may
have caused or been materially involved in the exceptional situation.
<p>
<a name="./system:s2"></a><span class=formdef><b>procedure</b>: <tt>(assertion-violationf <i>who</i> <i>msg</i> <i>irritant</i> ...)</tt></span>
<br>
<b>returns: </b>does not return
<br><span class=formdef><b>procedure</b>: <tt>(errorf <i>who</i> <i>msg</i> <i>irritant</i> ...)</tt></span>
<br>
<b>returns: </b>does not return
<br><span class=formdef><b>procedure</b>: <tt>(warningf <i>who</i> <i>msg</i> <i>irritant</i> ...)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><a name="./system:s3"></a>These procedures are like <tt>assertion-violation</tt>, <tt>error</tt>,
and <tt>warning</tt> except
that <tt><i>msg</i></tt> is assumed to be a format string, as if in a call to
<tt>format</tt> (Section <a href="./io.html#g83">9.13</a>), with
<tt><i>irritant</i> ...</tt> treated as the additional arguments to
<tt>format</tt>.
This allows programs to control the appearance of the error message, at
least when the default exception handler is in place.
<p>
For each of these procedures, the continuation object with which the exception
is raised includes a <tt>&format</tt> condition to signify that the string
contained in the condition object's <tt>&message</tt> condition is a
<tt>format</tt> string and the objects contained in the condition object's
<tt>&irritants</tt> condition should be treated as the additional
<tt>format</tt> arguments.
<p>
<a name="./system:s4"></a><span class=formdef><b>syntax</b>: <tt>&format</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(make-format-condition)</tt></span>
<br>
<b>returns: </b>a condition of type <tt>&format</tt>
<br><span class=formdef><b>procedure</b>: <tt>(format-condition? <i>obj</i>)</tt></span>
<br>
<b>returns: </b><tt>#t</tt> if <tt><i>obj</i></tt> is a condition of type <tt>&format</tt>, <tt>#f</tt> otherwise
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>
<p>Presence of this condition type within a compound condition indicates
that the string provided by the <tt>&message</tt> condition, if
present, is a <tt>format</tt> string and the list of objects provided by
the <tt>&irritants</tt> condition, if present, should be treated as
additional <tt>format</tt> arguments.
This condition type might be defined as follows.
<p><tt>(define-condition-type &format &condition<br>
make-format-condition format-condition?)</tt>
<p><a name="./system:s5"></a><span class=formdef><b>syntax</b>: <tt>&source</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(make-source-condition <i>form</i>)</tt></span>
<br>
<b>returns: </b>a condition of type <tt>&source</tt>
<br><span class=formdef><b>procedure</b>: <tt>(source-condition? <i>obj</i>)</tt></span>
<br>
<b>returns: </b><tt>#t</tt> if <tt><i>obj</i></tt> is a condition of type <tt>&source</tt>, <tt>#f</tt> otherwise
<br><span class=formdef><b>procedure</b>: <tt>(source-condition-form <i>condition</i>)</tt></span>
<br>
<b>returns: </b>the contents of <tt><i>condition</i></tt>'s <tt>form</tt> field
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>
<p>This condition type can be included within a compound condition when a
source expression can be identified in situations in which a
<tt>&syntax</tt> condition would be inappropriate, such as when a
run-time assertion violation is detected.
The <tt>form</tt> argument should be an s-expression or syntax object
representing the source expression.
This condition type might be defined as follows.
<p><tt>(define-condition-type &source &condition<br>
make-source-condition source-condition?<br>
(form source-condition-form))</tt>
<p><a name="./system:s6"></a><span class=formdef><b>syntax</b>: <tt>&continuation</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(make-continuation-condition <i>continuation</i>)</tt></span>
<br>
<b>returns: </b>a condition of type <tt>&continuation</tt>
<br><span class=formdef><b>procedure</b>: <tt>(continuation-condition? <i>obj</i>)</tt></span>
<br>
<b>returns: </b><tt>#t</tt> if <tt><i>obj</i></tt> is a condition of type <tt>&continuation</tt>, <tt>#f</tt> otherwise
<br><span class=formdef><b>procedure</b>: <tt>(condition-continuation <i>condition</i>)</tt></span>
<br>
<b>returns: </b>the contents of <tt><i>condition</i></tt>'s <tt>continuation</tt> field
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>
<p>This condition type can be included within a compound condition to indicate
the current continuation at the point where the exception described by the
condition occurred.
The continuation of a failed <tt>assert</tt> or a call to
<tt>assertion-violation</tt>, <tt>assertion-violationf</tt>,
<tt>error</tt>, <tt>errorf</tt>, or <tt>syntax-error</tt> is now included
via this condition type in the conditions passed to <tt>raise</tt>.
The <tt>continuation</tt> argument should be a continuation.
This condition type might be defined as follows.
<p><tt>(define-condition-type &continuation &condition<br>
make-continuation-condition continuation-condition?<br>
(continuation condition-continuation))</tt>
<p><a name="./system:s7"></a><span class=formdef><b>procedure</b>: <tt>(display-condition <i>obj</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(display-condition <i>obj</i> <i>textual-output-port</i>)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>If <tt><i>textual-output-port</i></tt> is not supplied, it defaults to the current output port.
This procedure displays a message to the effect that an exception
has occurred with value <tt><i>obj</i></tt>.
If <tt><i>obj</i></tt> is a condition (Chapter <a href="http://scheme.com/tspl4/./exceptions.html#g147">11</a> of
<i>The Scheme Programming Language, 4th Edition</i>), it displays information encapsulated within the condition,
handling messages, <tt><i>who</i></tt> conditions, irritants, source information,
etc., as appropriate.
<p>
<a name="./system:s8"></a><span class=formdef><b>procedure</b>: <tt>(default-exception-handler <i>obj</i>)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>This procedure is the default value of the <tt>base-exception-handler</tt>
parameter called on a condition when no other exception handler has been
defined or when all dynamically established exception handlers have chosen
not to handle the condition.
It first displays <tt><i>obj</i></tt>, as if with <tt>display-condition</tt>, to the
console error port.
For non-serious warning conditions, it returns immediately after displaying
the condition.
<p>
For serious or other non-warning conditions, it
saves the condition in the parameter <tt>debug-condition</tt>, where
<tt>debug</tt> (Section <a href="./debug.html#g17">3.2</a>) can retrieve it and
allow it to be inspected.
If the <tt>debug-on-exception</tt> parameter is set to <tt>#f</tt> (the
default unless the <a name="./system:s9"></a><tt>--debug-on-exception</tt> command-line option is provided), the
handler prints a message instructing the user to type <tt>(debug)</tt> to
enter the debugger, then resets to the current café.
Otherwise, the handler invokes <tt>debug</tt> directly and resets if
<tt>debug</tt> returns.
<p>
If an I/O exception occurs while attempting to display the condition,
the default exception handler resets (as if by calling <tt>reset</tt>).
The intent is to avoid an infinite regression (ultimately ending
in exhaustion of memory) in which the process repeatedly recurs
back to the default exception handler trying to write to a console-error
port (typically stderr) that is no longer writable, e.g., due to
the other end of a pipe or socket having been closed.
<p>
<a name="./system:s10"></a><span class=formdef><b>global parameter</b>: <tt>debug-on-exception</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>The value of this parameter determines whether the default exception handler
immediately enters the debugger immediately when it receives a serious or
non-warning condition.
If the <a name="./system:s11"></a><tt>--debug-on-exception</tt>
command-line option (Section <a href="./use.html#g6">2.1</a>) has been provided, the
initial value of this parameter is <tt>#t</tt>.
Otherwise, the initial value is <tt>#f</tt>.
<p>
<a name="./system:s12"></a><span class=formdef><b>thread parameter</b>: <tt>base-exception-handler</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>The value of this parameter must be a procedure, and the procedure
should accept one argument.
The default value of <tt>base-exception-handler</tt> is
the procedure <tt>default-exception-handler</tt>.
<p>
The value of this parameter is invoked whenever no exception handler
established by a program has chosen to handle an exception.
<p>
<a name="./system:s13"></a><span class=formdef><b>thread parameter</b>: <tt>debug-condition</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>This parameter is used by the default exception handler to hold the
last serious or non-warning condition received by the handler, where
it can be inspected via the <tt>debug</tt> procedure
(Section <a href="./debug.html#g17">3.2</a>).
It can also be invoked by user code to store or retrieve a
condition.
<p>
<a name="./system:s14"></a><span class=formdef><b>thread parameter</b>: <tt>current-exception-state</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>current-exception-state</tt> may be used to get or set
the current exception state.
When called without arguments, <tt>current-exception-state</tt> returns
an <i>exception state</i> comprising the current stack of handlers established
by <tt>with-exception-handler</tt> and <tt>guard</tt>.
When called with a single argument, which must be an exception state,
<tt>current-exception-state</tt> sets the exception state.
<p>
<a name="./system:s15"></a><span class=formdef><b>procedure</b>: <tt>(create-exception-state)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(create-exception-state <i>procedure</i>)</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>create-exception-state</tt> creates an exception
state whose stack of exception handlers is empty except for, in effect,
an infinite number of occurrences of <i>handler</i> at its
base.
<tt><i>handler</i></tt> must be a procedure, and should accept one argument.
If not provided, <tt><i>handler</i></tt> defaults to a procedure equivalent
to the value of the following expression.
<p>
<p><tt>(lambda (x) ((base-exception-handler) x))</tt>
<p>
<h3><a name="g109"></a><a name="./system:h2"></a>Section 12.2. Interrupts<a name="SECTSYSTEMINTERRUPTS"></a></h3>
<p>
<a name="./system:s16"></a><i>Chez Scheme</i> allows programs to control
the action of the Scheme system when various events
occur, including an interrupt from the
keyboard, the expiration of an internal timer set by <tt>set-timer</tt>,
a breakpoint caused by a call to <tt>break</tt>, or a request from the
storage manager to initiate a garbage collection.
These mechanisms are described in this section, except for the
collect request mechanism, which is described in Section <a href="./smgmt.html#g125">13.1</a>.
<p>
Timer, keyboard, and collect-request interrupts are supported via a counter
that is decremented approximately once for each call to a nonleaf procedure.
(A leaf procedure is one that does not itself make any calls.)
When no timer is running, this counter is set to a default value (1000
in Version 9) when a program starts or after an interrupt occurs.
If a timer is set (via <tt>set-timer</tt>), the counter is set to the
minimum of the default value and the number of ticks to which the timer is
set.
When the counter reaches zero, the system looks to see if the timer
is set and has expired or if a keyboard or collect request interrupt
has occurred.
If so, the current procedure call is pended ("put on hold") while the
appropriate interrupt handler is invoked to handle the interrupt.
When (if) the interrupt handler returns, the pended call takes place.
Thus, timer, keyboard, and collect-request interrupts effectively occur
synchronously with respect to the procedure call mechanism, and
keyboard and collect request interrupts may be delayed by a number
of calls equal to the default timer value.
<p>
Calls to the break handler occur immediately
whenever <tt>break</tt> is called.
<p>
<a name="./system:s17"></a><span class=formdef><b>procedure</b>: <tt>(break <i>who</i> <i>msg</i> <i>irritant</i> ...)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(break <i>who</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(break)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>The arguments to <tt>break</tt> follow the protocol described above for
<tt>errorf</tt>.
The default break handler (see <tt>break-handler</tt>) displays a message and
invokes the <a name="./system:s18"></a>debugger.
The format string and objects may be omitted, in which case the
message issued by the default break handler identifies the break
using the <tt><i>who</i></tt> argument but provides no more information
about the break.
If the <tt><i>who</i></tt> argument is omitted as well, no message is generated.
The default break handler returns normally if the debugger
exits normally.
<p>
<a name="./system:s19"></a><span class=formdef><b>thread parameter</b>: <tt>break-handler</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>The value of this parameter must be a procedure.
The current break handler is called by <tt>break</tt>, which passes
along its arguments.
See <tt>break</tt> for a description of the default break
handler.
The example below shows how to disable breaks.
<p>
<p><tt>(break-handler (lambda args (void)))</tt>
<p><a name="./system:s20"></a><span class=formdef><b>thread parameter</b>: <tt>keyboard-interrupt-handler</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>The value of this parameter must be a procedure.
The keyboard-interrupt handler is called (with no arguments) when
a keyboard interrupt occurs.
The default keyboard-interrupt handler invokes the interactive
<a name="./system:s21"></a>debugger.
If the debugger exits normally the interrupted computation is
resumed.
The example below shows how to install a keyboard-interrupt handler
that resets without invoking the debugger.
<p>
<p><tt>(keyboard-interrupt-handler <br>
(lambda ()<br>
(newline (console-output-port))<br>
(reset)))</tt>
<p><a name="./system:s22"></a><a name="desc:set-timer"></a>
<span class=formdef><b>procedure</b>: <tt>(set-timer <i>n</i>)</tt></span>
<br>
<b>returns: </b>previous current timer value
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><a name="./system:s23"></a><tt><i>n</i></tt> must be a nonnegative integer.
When <tt><i>n</i></tt> is nonzero, <tt>set-timer</tt> starts an internal timer with
an initial value of <tt><i>n</i></tt>.
When <tt><i>n</i></tt> ticks elapse, a timer interrupt occurs, resulting in
invocation of the timer interrupt handler.
Each tick corresponds roughly to one nonleaf procedure call (see the
introduction to this section); thus, ticks are not
uniform time units but instead depend heavily on how much work is done
by each procedure call.
<p>
When <tt><i>n</i></tt> is zero, <tt>set-timer</tt> turns the timer off.
<p>
The value returned in either case is the value of the timer before the
call to <tt>set-timer</tt>.
A return value of 0 should not be taken to imply that the timer was not on;
the return value may also be 0 if the timer was just about to fire when
the call to <tt>set-timer</tt> occurred.
<p>
The engine mechanism (Section <a href="./control.html#g46">6.4</a>) is built on top of the
timer interrupt so timer interrupts should not be used with engines.
<p>
<a name="./system:s24"></a><span class=formdef><b>thread parameter</b>: <tt>timer-interrupt-handler</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><a name="./system:s25"></a>The value of this parameter must be a procedure.
The timer interrupt handler is called by the system when the internal timer
(set by <tt>set-timer</tt>) expires.
The default handler raises an exception with condition type <tt>&assertion</tt>
to say that the handler has not
been defined; any program that uses the timer should redefine the
handler before setting the timer.
<p>
<a name="./system:s26"></a><span class=formdef><b>procedure</b>: <tt>(disable-interrupts)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(enable-interrupts)</tt></span>
<br>
<b>returns: </b>disable count
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>disable-interrupts</tt> disables the handling of interrupts,
including timer, keyboard, and collect request interrupts.
<tt>enable-interrupts</tt> re-enables these interrupts.
The system maintains a disable count that starts at zero; when zero,
interrupts are enabled.
Each call to <tt>disable-interrupts</tt> increments the count,
effectively disabling interrupts.
Each call to <tt>enable-interrupts</tt> decrements the count, if
not already zero, effectively enabling interrupts.
For example, two calls to <tt>disable-interrupts</tt> followed by one call to
<tt>enable-interrupts</tt> leaves interrupts disabled.
Calls to <tt>enable-interrupts</tt> when the count is already zero
(and interrupts are enabled) have no effect.
The value returned by either procedure is the number of calls to
<tt>enable-interrupts</tt> required to enable interrupts.
<p>
Great care should be exercised when using these procedures, since disabling
interrupts inhibits the normal processing of keyboard interrupts,
timer interrupts, and, perhaps most importantly, collect request interrupts.
Since garbage collection does not happen automatically when interrupts are
disabled, it is possible for the storage allocator to run out of space
unnecessarily should interrupts be disabled for a long period of time.
<p>
The <tt>with-interrupts-disabled</tt> syntactic form should be used instead of
these more primitive procedures whenever possible,
since <tt>with-interrupts-disabled</tt> ensures that interrupts are re-enabled
whenever a nonlocal exit occurs, such as when an exception is handled by
the default exception handler.
<p>
<a name="./system:s27"></a><span class=formdef><b>syntax</b>: <tt>(with-interrupts-disabled <i>body<sub>1</sub></i> <i>body<sub>2</sub></i> ...)</tt></span>
<br><span class=formdef><b>syntax</b>: <tt>(critical-section <i>body<sub>1</sub></i> <i>body<sub>2</sub></i> ...)</tt></span>
<br>
<b>returns: </b>the values of the body <tt><i>body<sub>1</sub></i> <i>body<sub>2</sub></i> ...</tt>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>with-interrupts-disabled</tt> evaluates the body
<tt><i>body<sub>1</sub></i> <i>body<sub>2</sub></i> ...</tt> with interrupts disabled.
That is, upon entry, interrupts are disabled, and
upon exit, interrupts are re-enabled.
Thus, <tt>with-interrupts-disabled</tt> allows the implementation of indivisible
operations in nonthreaded versions of <i>Chez Scheme</i> or within a single thread
in threaded versions of <i>Chez Scheme</i>.
<tt>critical-section</tt> is the same as <tt>with-interrupts-disabled</tt> and
is provided for backward compatibility.
<p>
<tt>with-interrupts-disabled</tt> can be defined as follows.
<p>
<p><tt>(define-syntax with-interrupts-disabled<br>
(syntax-rules ()<br>
[(_ b1 b2 ...)<br>
(dynamic-wind<br>
disable-interrupts<br>
(lambda () b1 b2 ...)<br>
enable-interrupts)]))</tt>
<p>The use of <tt>dynamic-wind</tt> ensures that interrupts are
disabled whenever the body of the <tt>with-interrupts-disabled</tt> expression
is active and re-enabled whenever it is not.
Since calls to <tt>disable-interrupts</tt> are counted (see the
discussion under <tt>disable-interrupts</tt> and
<tt>enable-interrupts</tt> above), <tt>with-interrupts-disabled</tt>
expressions may be nested with the desired effect.
<p>
<a name="./system:s28"></a><span class=formdef><b>procedure</b>: <tt>(register-signal-handler <i>sig</i> <i>procedure</i>)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>register-signal-handler</tt> is used to
establish a signal handler for a given low-level signal.
<tt><i>sig</i></tt> must be an exact integer identifying a valid signal, and
<tt><i>procedure</i></tt> should accept one argument.
See your host system's <tt><signal.h></tt> or documentation for a list
of valid signals and their numbers.
After a signal handler for a given signal has been registered, receipt
of the specified signal results in a call to the handler.
The handler is passed the signal number, allowing the same handler to
be used for different signals while differentiating among them.
<p>
Signals handled in this fashion are treated like keyboard interrupts in
that the handler is not called immediately when the signal is delivered
to the process, but rather at some procedure call boundary after the
signal is delivered.
It is generally not a good idea, therefore, to establish handlers for
memory faults, illegal instructions, and the like, since the code that
causes the fault or illegal instruction will continue to execute
(presumably erroneously) for some time before the handler is invoked.
<p>
<tt>register-signal-handler</tt> is supported only on Unix-based
systems.
<p>
<h3><a name="g110"></a><a name="./system:h3"></a>Section 12.3. Environments<a name="SECTMISCENVIRONMENTS"></a></h3>
<p>
Environments are first-class objects containing identifier bindings.
They are similar to modules but, unlike modules, may be manipulated
at run time.
Environments may be provided as optional arguments to <tt>eval</tt>,
<tt>expand</tt>, and the procedures that define, assign, or
reference top-level values.
<p>
There are several built-in environments, and new environments can
be created by copying existing environments or selected bindings
from existing environments.
<p>
Environments can be mutable or immutable.
A mutable environment can be extended with new bindings, its
existing bindings can be modified, and its variables can be assigned.
An immutable environment cannot be modified in any of these ways.
<p>
<a name="./system:s29"></a><span class=formdef><b>procedure</b>: <tt>(environment? <i>obj</i>)</tt></span>
<br>
<b>returns: </b><tt>#t</tt> if <tt><i>obj</i></tt> is an environment, otherwise <tt>#f</tt>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>
<p><tt>(environment? (interaction-environment)) <img src="math/csug/0.gif" alt="<graphic>"> #t<br>
(environment? 'interaction-environment) <img src="math/csug/0.gif" alt="<graphic>"> #f<br>
(environment? (copy-environment (scheme-environment))) <img src="math/csug/0.gif" alt="<graphic>"> #t<br>
(environment? (environment '(prefix (rnrs) $rnrs-))) <img src="math/csug/0.gif" alt="<graphic>"> #t</tt>
<p><a name="./system:s30"></a><span class=formdef><b>procedure</b>: <tt>(environment-mutable? <i>env</i>)</tt></span>
<br>
<b>returns: </b><tt>#t</tt> if <tt><i>env</i></tt> is mutable, otherwise <tt>#f</tt>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>
<p><tt>(environment-mutable? (interaction-environment)) <img src="math/csug/0.gif" alt="<graphic>"> #t<br>
(environment-mutable? (scheme-environment)) <img src="math/csug/0.gif" alt="<graphic>"> #f<br>
(environment-mutable? (copy-environment (scheme-environment))) <img src="math/csug/0.gif" alt="<graphic>"> #t<br>
(environment-mutable? (environment '(prefix (rnrs) $rnrs-))) <img src="math/csug/0.gif" alt="<graphic>"> #f</tt>
<p><a name="./system:s31"></a><span class=formdef><b>procedure</b>: <tt>(scheme-environment)</tt></span>
<br>
<b>returns: </b>an environment
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>scheme-environment</tt> returns an environment containing
the initial top-level bindings.
This environment corresponds to the <tt>scheme</tt> module.
<p>
The environment returned by this procedure is immutable.
<p>
<p><tt>(define cons 3)<br>
(top-level-value 'cons (scheme-environment)) <img src="math/csug/0.gif" alt="<graphic>"> #<procedure cons><br>
(set-top-level-value! 'cons 3 (scheme-environment)) <img src="math/csug/0.gif" alt="<graphic>"> <i>exception</i></tt>
<p><a name="./system:s32"></a><span class=formdef><b>procedure</b>: <tt>(ieee-environment)</tt></span>
<br>
<b>returns: </b>an IEEE/ANSI standard compatibility environment
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>ieee-environment</tt> returns an environment containing
bindings for the keywords and variables whose meanings are
defined by the IEEE/ANSI Standard for Scheme [<a class=citation href="./bibliography.html#g171">25</a>].
<p>
The bindings for each of the identifiers in the IEEE environment are those
of the corresponding Revised<sup>6</sup> Report library, so this does not provide
full backward compatibility.
<p>
The environment returned by this procedure is immutable.
<p>
<p><tt>(define cons 3)<br>
(top-level-value 'cons (ieee-environment)) <img src="math/csug/0.gif" alt="<graphic>"> #<procedure cons><br>
(set-top-level-value! 'cons 3 (ieee-environment)) <img src="math/csug/0.gif" alt="<graphic>"> <i>exception</i></tt>
<p><a name="./system:s33"></a><span class=formdef><b>thread parameter</b>: <tt>interaction-environment</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>The original value of <tt>interaction-environment</tt> is the default
top-level environment.
It is initially set to a mutable copy of
<tt>(scheme-environment)</tt> and which may be extended or otherwise
altered by top-level definitions and assignments.
It may be set to any environment, mutable or not, to change the
default top-level evaluation environment.
<p>
An expression's top-level bindings resolve to the environment that is
in effect when the expression is expanded, and changing the value
of this parameter has no effect on running code.
Changes affect only code that is subsequently expanded, e.g., as the
result of a call to <tt>eval</tt>, <tt>load</tt>, or
<tt>compile-file</tt>.
<p>
<p><tt>(define cons 3)<br>
cons <img src="math/csug/0.gif" alt="<graphic>"> 3<br>
(top-level-value 'cons (interaction-environment)) <img src="math/csug/0.gif" alt="<graphic>"> 3
<br>
<br>
(interaction-environment (scheme-environment))<br>
cons <img src="math/csug/0.gif" alt="<graphic>"> #<procedure cons><br>
(set! cons 3) <img src="math/csug/0.gif" alt="<graphic>"> <i>exception: attempt to assign immutable variable</i><br>
(define cons 3) <img src="math/csug/0.gif" alt="<graphic>"> <i>exception: invalid definition in immutable environment</i></tt>
<p><a name="./system:s34"></a><span class=formdef><b>procedure</b>: <tt>(copy-environment <i>env</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(copy-environment <i>env</i> <i>mutable?</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(copy-environment <i>env</i> <i>mutable?</i> <i>syms</i>)</tt></span>
<br>
<b>returns: </b>a new environment
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>copy-environment</tt> returns a copy of <tt><i>env</i></tt>, i.e., a new
environment that contains the same bindings as <tt><i>env</i></tt>.
<p>
The environment is mutable if <tt><i>mutable?</i></tt> is omitted or true;
if <tt><i>mutable?</i></tt> is false, the environment is immutable.
<p>
The set of bindings copied from <tt><i>env</i></tt> to the new environment
is determined by <tt><i>syms</i></tt>, which defaults to the value of
<tt>(environment-symbols <i>env</i>)</tt>.
The binding, if any, for each element of <tt><i>syms</i></tt> is copied to the
new environment, and no other bindings are present in the new
environment.
<p>
In the current implementation, the storage space used by an environment
is never collected, so repeated use of <tt>copy-environment</tt> will
eventually cause the system to run out of memory.
<p>
<p><tt>(define e (copy-environment (scheme-environment)))<br>
(eval '(define cons +) e)<br>
(eval '(cons 3 4) e) <img src="math/csug/0.gif" alt="<graphic>"> 7<br>
(eval '(cons 3 4) (scheme-environment)) <img src="math/csug/0.gif" alt="<graphic>"> (3 . 4)</tt>
<p><a name="./system:s35"></a><span class=formdef><b>procedure</b>: <tt>(environment-symbols <i>env</i>)</tt></span>
<br>
<b>returns: </b>a list of symbols
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>This procedure returns a list of symbols representing the identifiers
bound in environment <tt><i>env</i></tt>.
It is primarily useful in building the list of symbols to be copied
from one environment to another.
<p>
<p><tt>(define listless-environment<br>
(copy-environment<br>
(scheme-environment)<br>
#t<br>
(remq 'list (environment-symbols (scheme-environment)))))<br>
(eval '(let ([x (cons 3 4)]) x) listless-environment) <img src="math/csug/0.gif" alt="<graphic>"> (3 . 4)<br>
(eval '(list 3 4) listless-environment) <img src="math/csug/0.gif" alt="<graphic>"> <i>exception</i></tt>
<p><a name="./system:s36"></a><span class=formdef><b>procedure</b>: <tt>(apropos-list <i>s</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(apropos-list <i>s</i> <i>env</i>)</tt></span>
<br>
<b>returns: </b>see below
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p>This procedure returns a selected list of symbols and pairs.
Each symbol in the list represents an identifier bound in <tt><i>env</i></tt>.
Each pair represents a set of identifiers exported by a
predefined library or a library previously defined or loaded
into the system.
The car of the pair is the library name, and the cdr is a list
of symbols.
If <tt><i>s</i></tt> is a string, only entries whose names have <tt><i>s</i></tt> as a
substring are included, and if <tt><i>s</i></tt> is a symbol, only those whose names
have the name of <tt><i>s</i></tt> as a substring are selected.
If no environment is provided, it defaults to the value of
<tt>interaction-environment</tt>.
<p>
<p><tt>(library (a) (export a-vector-sortof) (import (rnrs))<br>
(define a-vector-sortof '(vector 1 2 3)))<br>
(apropos-list 'vector-sort) <img src="math/csug/0.gif" alt="<graphic>"><br>
(vector-sort vector-sort!<br>
((a) a-vector-sortof)<br>
((chezscheme) vector-sort vector-sort!)<br>
((rnrs) vector-sort vector-sort!)<br>
((rnrs sorting) vector-sort vector-sort!)<br>
((scheme) vector-sort vector-sort!))</tt>
<p><a name="./system:s37"></a><span class=formdef><b>procedure</b>: <tt>(apropos <i>s</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(apropos <i>s</i> <i>env</i>)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>apropos</tt> is like <tt>apropos-list</tt> except the information is
displayed to the current output port, as shown in the following
transcript.
<p>
<p><tt>> (library (a) (export a-vector-sortof) (import (rnrs))<br>
(define a-vector-sortof '(vector 1 2 3)))<br>
> (apropos 'vector-sort)<br>
interaction environment:<br>
vector-sort, vector-sort!<br>
(a):<br>
a-vector-sortof<br>
(chezscheme):<br>
vector-sort, vector-sort!<br>
(rnrs):<br>
vector-sort, vector-sort!<br>
(rnrs sorting):<br>
vector-sort, vector-sort!<br>
(scheme):<br>
vector-sort, vector-sort!</tt>
<p>
<h3><a name="g111"></a><a name="./system:h4"></a>Section 12.4. Compilation, Evaluation, and Loading<a name="SECTMISCCOMPILEEVAL"></a></h3>
<p>
<a name="./system:s38"></a><span class=formdef><b>procedure</b>: <tt>(eval <i>obj</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(eval <i>obj</i> <i>env</i>)</tt></span>
<br>
<b>returns: </b>value of the Scheme form represented by <tt><i>obj</i></tt>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>eval</tt> treats <tt><i>obj</i></tt> as the representation of an expression.
It evaluates the expression in environment <tt><i>env</i></tt> and returns
its value.
If no environment is provided, it defaults to the environment
returned by <tt>interaction-environment</tt>.
<p>
Single-argument <tt>eval</tt> is a <i>Chez Scheme</i> extension.
<i>Chez Scheme</i> also permits <tt><i>obj</i></tt> to be the representation of a
nonexpression form, i.e., a definition, whenever the environment
is mutable.
<i>Chez Scheme</i> further allows <tt><i>obj</i></tt> to be an annotation
(Section <a href="./syntax.html#g106">11.11</a>), and the default evaluators
make use of annotations to incorporate source-file
information in error messages and associate source-file
information with compiled code.
<p>
In <i>Chez Scheme</i>, <tt>eval</tt> is actually a wrapper that simply
passes its arguments to the current evaluator.
(See <tt>current-eval</tt>.)
The default evaluator is <tt>compile</tt>, which expands the
expression via the current expander (see
<tt>current-expand</tt>), compiles it,
executes the resulting code, and returns its value.
If the environment argument, <tt><i>env</i></tt>, is present,
<tt>compile</tt> passes it along to the current expander,
which is <tt>sc-expand</tt> by default.
<p>
<a name="./system:s39"></a><span class=formdef><b>thread parameter</b>: <tt>current-eval</tt></span>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>current-eval</tt> determines the evaluation procedure used by the
procedures <a name="./system:s40"></a><tt>eval</tt>, <tt>load</tt>, and
<tt>new-cafe</tt>.
<tt>current-eval</tt> is initially bound to the value of
<a name="./system:s41"></a><tt>compile</tt>.
(In <i>Petite Chez Scheme</i>, it is initially bound to the value of
<a name="./system:s42"></a><tt>interpret</tt>.)
The evaluation procedureshould expect one or two arguments: an object
to evaluate and an optional environment.
The second argument might be an annotation
(Section <a href="./syntax.html#g106">11.11</a>).
<p>
<p><tt>(current-eval interpret)<br>
(+ 1 1) <img src="math/csug/0.gif" alt="<graphic>"> 2
<br>
<br>
(current-eval (lambda (x . ignore) x))<br>
(+ 1 1) <img src="math/csug/0.gif" alt="<graphic>"> (+ 1 1)</tt>
<p><a name="./system:s43"></a><span class=formdef><b>procedure</b>: <tt>(compile <i>obj</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(compile <i>obj</i> <i>env</i>)</tt></span>
<br>
<b>returns: </b>value of the Scheme form represented by <tt><i>obj</i></tt>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt><i>obj</i></tt>, which can be an annotation (Section <a href="./syntax.html#g106">11.11</a>)
or unannotated value, is treated as a Scheme expression, expanded with the
current expander (the value of <tt>current-expand</tt>) in the specified
environment (or the interaction environment, if no environment
is provided), compiled to machine code, and executed.
<tt>compile</tt> is the default value of the <tt>current-eval</tt>
parameter.
<p>
<a name="./system:s44"></a><span class=formdef><b>procedure</b>: <tt>(interpret <i>obj</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(interpret <i>obj</i> <i>env</i>)</tt></span>
<br>
<b>returns: </b>value of the Scheme form represented by <tt><i>obj</i></tt>
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt>interpret</tt> is like <tt>compile</tt>, except that the expression
is interpreted rather than compiled.
<tt>interpret</tt> may be used as a replacement for <tt>compile</tt>,
with the following caveats:
<p>
<ul>
<li>Interpreted code runs significantly slower.
<p>
<li>Inspector information is not generated for
interpreted code, so the inspector is not as useful for interpreted
code as it is for compiled code.
<p>
<li>Foreign procedure expressions cannot be
interpreted, so the interpreter invokes the compiler for all
foreign procedure expressions (this is done transparently).
</ul>
<p>
<p>
<tt>interpret</tt> is sometimes faster than <tt>compile</tt> when the
form to be evaluated is short running, since it avoids some of the
work done by <tt>compile</tt> prior to evaluation.
<p>
<a name="./system:s45"></a><span class=formdef><b>procedure</b>: <tt>(load <i>path</i>)</tt></span>
<br><span class=formdef><b>procedure</b>: <tt>(load <i>path</i> <i>eval-proc</i>)</tt></span>
<br>
<b>returns: </b>unspecified
<br>
<b>libraries: </b><tt>(chezscheme)</tt>
<p><tt><i>path</i></tt> must be a string.
<tt>load</tt> reads and evaluates the contents of the file specified by
<tt><i>path</i></tt>.
The file may contain source or object code.
By default, <tt>load</tt> employs <tt>eval</tt> to evaluate each source
expression found in a source file.
If <tt><i>eval-proc</i></tt> is specified, <tt>load</tt> uses this procedure instead.
<tt><i>eval-proc</i></tt> must accept one argument, the expression to evaluate.
The expression passed to <tt><i>eval-proc</i></tt> might be an annotation
(Section <a href="./syntax.html#g106">11.11</a>) or an unannotated value.
<p>
The <tt><i>eval-proc</i></tt> argument
facilitates the implementation of embedded Scheme-like languages
and the use of alternate
evaluation mechanisms to be used for Scheme programs.
<tt><i>eval-proc</i></tt> can be put to other uses as well.
For example,
<p>