made debugger handle errors in printing *DEBUG-CONDITION*

[sbcl.git] / doc / beyond-ansi.sgml

<chapter id="beyond-ansi"><title>Beyond the &ANSI; Standard</>

<para>Besides &ANSI;, we have other stuff..</para>

<sect1 id="non-conformance"><title>Non-Conformance with the &ANSI; Standard</>

<para>&SBCL; is derived from code which was written before the &ANSI;
standard, and some incompatibilities remain.</para>

<para>The &ANSI; standard defines constructs like
<function>defstruct</>, <function>defun</>, and <function>declaim</>
so that they can be implemented as macros which expand into ordinary
code wrapped in <function>eval-when</> forms. However, the pre-&ANSI;
&CMUCL; implementation handled these (and some related functions like
<function>proclaim</>) as special cases in the compiler, with subtly
(or sometimes not-so-subtly) different semantics. Much of this
weirdness has been removed in the years since the &ANSI; standard was
released, but bits and pieces remain, so that e.g., as of &SBCL; 0.6.3
compiling the function

<programlisting>(defun foo () (defstruct bar))</>

will cause the class <type>BAR</> to be defined, even when the
function is not executed. These remaining nonconforming behaviors are
considered bugs, and clean patches will be gratefully accepted, but as
long as they don't cause as many problems in practice as other known
issues, they tend not to be actively fixed.</para>

<para>More than any other &Lisp; system I am aware of, &SBCL; (and its
parent &CMUCL;) store and use a lot of compile-time static type
information. By and large they conform to the standard in doing so,
but in one regard they do not &mdash; they consider <function>defun</>s to,
in effect, implicitly <function>proclaim</> type information about the
signature of the function being defined. Thus, if you compile and load

<programlisting>(defun foo-p (x)
  (error "stub, foo-p ~s isn't implemented yet!" x))
(defun foolike-p (x)
  (or (foo-p x) (foo-p (car x))))</programlisting>

everything will appear to work correctly, but if you subsequently
redefine <function>foo-p</>

<programlisting>(defun foo-p (x) (or (null x) (symbolp (car x))))</>

and call

<programlisting>(foolike-p nil)</>

you will not get the correct result, but an error,

<screen>debugger invoked on SB-DEBUG::*DEBUG-CONDITION* of type
SB-KERNEL:SIMPLE-CONTROL-ERROR:
  A function with declared result type NIL returned:
  FOO-P</screen>

because when &SBCL; compiled <function>foolike-p</>, &SBCL; thought it
knew that <function>foo-p</> would never return. More insidious
problems are quite possible when &SBCL; thinks it can optimize away e.g.
particular branches of a <function>case</> because of what it's proved
to itself about the function's return type. This will probably be
fixed in the foreseeable future, either with a quick fix, or ideally
in conjunction with some related fixes to generalize the principle
that declarations are assertions (see below). But for now it remains a
gross violation of the &ANSI; spec (and reasonable user
expectations).</para>

<para>The &CMUCL; <function>defstruct</> implementation treated
structure accessors and other <function>defstruct</>-related functions
(e.g. predicates) as having some special properties, not quite like
ordinary functions. This specialness has been reduced in &SBCL;, but
some still remains. In particular, redefining a structure accessor
function may magically cause the entire structure class to be deleted.
This, too, will probably be fixed in the foreseeable future.</para>

<para>The CLOS implementation used in &SBCL; is based on the
<application>Portable Common Loops</> (PCL) reference implementation
from Xerox. Unfortunately, PCL seems never to have quite conformed to
the final CLOS specification. Moreover, despite the "Portable" in its
name, it wasn't quite portable. Various implementation-specific hacks
were made to make it run on &CMUCL;, and then more hacks were added to
make it less inefficient. The result is a system with mostly tolerable
performance which mostly conforms to the standard, but which has a few
remaining weirdnesses which seem to be hard to fix. The most important
remaining weirdness is that the <type>CL:CLASS</> class is not the
same as the <type>SB-PCL:CLASS</> type used internally in PCL; and
there are several other symbols maintained in parallel (e.g.
<type>SB-PCL:FIND-CLASS</> vs. <type>CL:FIND-CLASS</>). So far, any
problems this has caused have had workarounds involving consistently
using the SB-PCL versions or the CL versions of the class hierarchy.
This is admittedly ugly, but it may not be fixed in the foreseeable
future, since the required cleanup looks nontrivial, and we don't have
anyone sufficiently motivated to do it.</para>

</sect1>

<sect1 id="idiosyncrasies"><title>Idiosyncrasies</>

<para>Declarations are generally treated as assertions. This general
principle, and its implications, and the bugs which still keep the
compiler from quite satisfying this principle, are discussed in the
<link linkend="compiler">chapter on the compiler</link>.</para>

<note><para>It's not an idiosyncrasy yet, since we haven't done
it, but someday soon &SBCL; may become a compiler-only implementation.
That is, essentially, <function>eval</> will be defined to create
a lambda expression, call <function>compile</> on the lambda
expression to create a compiled function, and then
<function>funcall</> the resulting function. This would allow
a variety of simplifications in the implementation, while introducing
some other complexities. It remains to be seen when it will be
possible to try this, or whether it will work well when it's tried,
but it looks appealing right now.</para></note>

</sect1>

<sect1 id="extensions"><title>Extensions</>

<para>&SBCL; is derived from &CMUCL;, which implements many extensions to the
&ANSI; standard. &SBCL; doesn't support as many extensions as &CMUCL;, but
it still has quite a few.</para>

<sect2><title>Things Which Might Be in the Next &ANSI; Standard</>

<para>&SBCL; provides extensive support for 
calling external C code, described 
<link linkend="ffi">in its own chapter</link>.</para>

<para>&SBCL; provides additional garbage collection functionality not
specified by &ANSI;. Weak pointers allow references to objects to be
maintained without keeping them from being GCed. And "finalization"
hooks are available to cause code to be executed when an object is
GCed.</para> <!-- FIXME: Actually documenting these would be good.:-| -->

<para>&SBCL; does not currently provide Gray streams, but may do so in
the near future. (It has unmaintained code inherited from &CMUCL; to
do so.) <!-- FIXME: Add citation to Gray streams.-->
</para>

</sect2>

<sect2><title>Support for Unix</>

<para>The UNIX command line can be read from the variable
<varname>sb-ext:*posix-argv*</>. The UNIX environment can be queried with the
<function>sb-ext:posix-getenv</> function.</para>

<para>The &SBCL; system can be terminated with <function>sb-ext:quit</>,
optionally returning a specified numeric value to the calling Unix
process. The normal Unix idiom of terminating on end of file on input
is also supported.</para>

</sect2>

<sect2><title>Tools to Help Developers</title>

<para>&SBCL; provides a profiler and other extensions to the &ANSI;
<function>trace</> facility. See the online function documentation for
<function>trace</> for more information.</para>

<para>The debugger supports a number of options. Its documentation is
accessed by typing <userinput>help</> at the debugger prompt.</para>

<para>Documentation for <function>inspect</> is accessed by typing
<userinput>help</> at the <function>inspect</> prompt.</para>

</sect2>

<sect2><title>Interface to Low-Level &SBCL; Implementation</title>

<para>&SBCL; has the ability to save its state as a file for later
execution. This functionality is important for its bootstrapping
process, and is also provided as an extension to the user See the
documentation for <function>sb-ext:save-lisp-and-die</> for more
information.</para>

<note><para>&SBCL; has inherited from &CMUCL; various hooks to allow
the user to tweak and monitor the garbage collection process. These
are somewhat stale code, and their interface might need to be cleaned
up. If you have urgent need of them, look at the code in
<filename>src/code/gc.lisp</filename> and bring it up on the
developers' mailing list.</para></note>

<note><para>&SBCL; has various hooks inherited from &CMUCL;, like
<function>sb-ext:float-denormalized-p</>, to allow a program to take
advantage of &IEEE; floating point arithmetic properties which aren't
conveniently or efficiently expressible using the &ANSI; standard. These
look good, and their interface looks good, but &IEEE; support is
slightly broken due to a stupid decision to remove some support for
infinities (because it wasn't in the &ANSI; spec and it didn't occur to
me that it was in the &IEEE; spec). If you need this stuff, take a look
at the ecode and bring it up on the developers' mailing
list.</para></note>

</sect2>

<sect2><title>Efficiency Hacks</title>

<para>The <function>sb-ext:purify</function> function causes &SBCL;
first to collect all garbage, then to mark all uncollected objects as
permanent, never again attempting to collect them as garbage. (This
can cause a large increase in efficiency when using a primitive
garbage collector, but is less important with modern generational
garbage collectors.)</para>

<para>The <function>sb-ext:truly-the</> operator does what the
<function>cl:the</> operator does in a more conventional
implementation of &CommonLisp;, declaring the type of its argument
without any runtime checks. (Ordinarily in &SBCL;, any type declaration
is treated as an assertion and checked at runtime.)</para>

<para>The <function>sb-ext:freeze-type</> declaration declares that a
type will never change, which can make type testing
(<function>typep</>, etc.) more efficient for structure types.</para>

<para>The <function>sb-ext:constant-function</> declaration specifies
that a function will always return the same value for the same
arguments. This is appropriate for functions like <function>sqrt</>.
It is not appropriate for functions like <function>aref</>, which can
change their return values when the underlying data are
changed.</para>

</sect2>

</sect1>

</chapter>