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FAQ: Lisp Frequently Asked Questions 2/7 [Monthly posting]

Frequently asked questions about Lisp -- General Questions
Archive-name: lisp-faq/part2
Last-Modified: Tue Feb 20 13:29:08 1996 by Mark Kantrowitz
Version: 1.54
Maintainer: Mark Kantrowitz and Barry Margolin <>
Size: 49548 bytes, 999 lines

;;; ****************************************************************
;;; Answers to Frequently Asked Questions about Lisp ***************
;;; ****************************************************************
;;; Written by Mark Kantrowitz and Barry Margolin
;;; lisp_2.faq 

This post contains Part 2 of the Lisp FAQ.

If you think of questions that are appropriate for this FAQ, or would
like to improve an answer, please send email to us at

Topics Covered (Part 2):

  [2-1]   Is there a GNU-Emacs interface to Lisp?
  [2-2]   When should I use a hash table instead of an association list?
  [2-3]   What is the equivalent of EXPLODE and IMPLODE in Common Lisp?
  [2-4]   Is Lisp inherently slower than more conventional languages such as C?
  [2-5]   Why does Common Lisp have "#'"?
  [2-6]   How do I call non-Lisp functions from Lisp?
  [2-7]   Can I call Lisp functions from other languages?
  [2-8]   I want to call a function in a package that might not exist at
          compile time. How do I do this?  
  [2-9]   What is CDR-coding?
  [2-10]  What is garbage collection?
  [2-11]  How do I save an executable image of my loaded Lisp system?
          How do I run a Unix command in my Lisp? How do I exit Lisp?
          Access environment variables?
  [2-12]  I'm porting some code from a Symbolics Lisp machine to some
          other platform, and there are strange characters in the code.
          What do they mean?  
  [2-13]  History: Where did Lisp come from?
  [2-14]  How do I find the argument list of a function?
          How do I get the function name from a function object?
  [2-15]  How can I have two Lisp processes communicate via unix sockets?
  [2-16]  How can I create a stream that acts like UNIX's /dev/null
          (i.e., gobbles any output and immediately signals EOF on
          input operations)?
  [2-17]  Read-time conditionalization of code (#+ #- and *features*)   
  [2-18]  What reader macro characters are used in major Lisp systems?
  [2-19]  How do I determine if a file is a directory or not? 
          How do I get the current directory name from within a Lisp 
          program? Is there any way to create a directory?
  [2-20]  What is a "Lisp Machine" (LISPM)?
  [2-21]  How do I tell if a symbol names a function and not a macro?

Search for \[#\] to get to question number # quickly.

Subject: [2-1] Is there a GNU-Emacs interface to Lisp?

ILISP is a powerful GNU-Emacs interface to many dialects of Lisp,
including Lucid, Allegro, {A}KCL, IBCL, and CMU.  Written by Chris
McConnell <> and now maintained by Marco Antoniotti
<> and Rick Busdiecker <>.  It is
available by anonymous ftp from []
as the file ilisp-5.6.tar.gz.  It is also available in the CMU AI
Repository in
If you want to be on the ilisp mailing list, to hear about new
releases and patches, send mail to  Please
send any comments, code, or bug reports to

Franz Inc.'s GNU-Emacs/Lisp interface includes an online Common Lisp
manual. (The manual is available by license from Franz Inc. Contact for more information.) The Emacs-Lisp interface
(without the online Common Lisp reference manual and some
Allegro-specific code) is available free from
and takes advantage of GNU-Emacs 19.X's newest features, including
support for mouse input, pulldown menus, and multifont text. The
interface also supports Epoch 3.2 and 4.2, and LEmacs 19.6 and 19.8.
For discussion of the Franz lisp-emacs interface, join the mailing list.
(See also [1-2] for a hardcopy version of the Common Lisp reference manual.)

The cl-shell package provides a major mode (cl-shell-mode) for running
Common Lisp (CL) as an Emacs subprocess.  It provides a general
mechanism for communication between CL and Emacs which does not rely
on extra processes, and should therefore be easily portable to any
version of CL.  Features include direct (i.e., not through a temp file)
evaluation and in-package compilation of forms from lisp-mode buffers,
type-ahead and a history mechanism for the cl-shell buffer, and pop-up
help facilities for the CL functions documentation, macroexpand and
describe.  Extensions for Lucid Common Lisp provide pop-up arglists
and source file editing.  Other extensions are provided to allow
editing source files of CLOS or Flavors methods.  Cl-shell is
available on the Lucid tape (in the goodies directory) or via
anonymous ftp from (

Lucid includes some other Emacs-Lisp interfaces in its goodies directory.

Harlequin's LispWorks includes an Emacs-Lisp interface.

Venue's Medley has an optional EMACS Interface.

GNU-Emacs itself is available by anonymous ftp from

Edebug, a debugger for Emacs Lisp, and some utilities for Common Lisp
debugging (Dave Gillespie's version of cl.el) are available by
anonymous ftp from
To join the Edebug mailing list send mail to For more information, write to Daniel
LaLiberte <>. 

Subject: [2-2] When should I use a hash table instead of an association list?

Both association lists (alists) and hash tables may be used to
represent tabular data. Hash tables have an O(1) running time and
alists an O(n) running time, so hash tables are ultimately more
efficient than alists. However, if the alists are small, they can be
more efficient than hash tables, which have a large initial overhead.

Alists can sometimes be more efficient if the keys are sorted
according to frequency, with the most heavily accessed keys appearing
at the front of the list. But one doesn't always know this kind of
information, and even then the frequency distribution may be flat.

In Allegro CL 4.1 [SPARC; R1], the rule of thumb is that for less than
24 elements, linear search using alists beats hashing.  In Lucid CL
4.0.1 HP 9000/700, the break-even point is at 10 elements. The
break-even points vary in other lisps from as low as 4 elements to as
high as 100 elements. So if you're using alists in your code, using 
hash tables instead may speed up your program. 

A potential problem may occur, however, when the keys of an EQ or EQL
hash table are Lisp objects such as conses or arrays (or other objects
that are identified by their addresses). In most implementations, such
tables must be re-hashed after garbage collection. If your application
causes frequent GCs, this can adversely affect the performance of hash
table lookup. Since EQL-hashing and =-hashing of fixnums generally
don't require rehashing after GC, one way of avoiding this problem is
to include a unique identifier in each key object and hash on that
instead. Another solution is to use an EQUAL hash table if the keys
are conses or an EQUALP hash table if the keys are arrays or other
(non-circular!) structures.

Subject: [2-3] What is the equivalent of EXPLODE and IMPLODE in Common Lisp?

Hopefully, the only reason you need to do this is as part of trying to port
some old MacLisp code to Common Lisp.  These functions predated the
inclusion of strings as a first-class data type in Lisp; symbols were used
as strings, and they ere EXPLODEd to allow the individual characters to be
manipulated in a list.

Probably the best approximations of these are:

   (defun explode (object)
     (loop for char across (prin1-to-string object)
           collect (intern (string char))))

   (defun implode (list)
     (read-from-string (coerce (mapcar #'character list) 'string)))

An alternate definition of EXPLODE which uses MAP instead of LOOP is:

   (defun explode (object)
     (map 'list #'(lambda (char) 
                    (intern (string char)))
          (prin1-to-string object)))

The creation of N conses of garbage to process a string of N
characters is a hideously inefficient way of doing the job.  Rewrite
EXPLODE code with PRIN1-TO-STRING, or better STRING if the arguments
are symbols without funny characters.  For IMPLODE, try to make its
caller use strings and try to make the result usable as a string to
avoid having to call INTERN or READ-FROM-STRING.

Subject: [2-4] Is Lisp inherently slower than more conventional languages
               such as C?

This is a tough question to answer, as you probably expected.  In many
cases, it appears to be.  Lisp does not require the programmer to specify
the data type of variables, so generic arithmetic operators may have to
perform type checking at runtime in order to determine how to proceed.
However, Lisp code can also be denser (i.e.  there is more expressed in a
single line) than many other languages: the Lisp expression (+ A B) is more
powerful than the C expression A+B (the Lisp version supports bignums,
rationals, and complex numbers, while the C version only supports
limited-size integers and floating point); therefore, one may claim that it
is reasonable that the Lisp version take longer than the C version (but
don't expect everyone to accept this rationalization).  Solutions to this
include hardware support (e.g. processors that support type tags in data,
such as SPARC and Symbolics Lisp Machines), declarations, and specialized
variants of functions (e.g. in MacLisp, + accepts and returns only fixnums,
+$ accepts and returns only flonums, and PLUS is generic).

At one time, the MIT PDP-10 MacLisp compiler was compared to DEC's
PDP-10 Fortran compiler.  When appropriate declarations were supplied
in the Lisp code, the performance of compiled Lisp arithmetic rivaled
that of the Fortran code.  It would hardly be fair to compare Lisp
without declarations to Fortran, since the Fortran compiler would have
more information upon which it could base its optimizations. A more
recent test found that numeric code compiled with optimizations using
CMU CL is within the same ballpark as highly optimized Fortran code.
For unoptimized Fortran code, CMU CL was about 4 times faster.
Even the speed of numeric code generated by other Lisp compilers
(AKCL, Allegro, Lucid) was well within an order of magnitude of good
Fortran and C compilers (although slower than CMU CL).  Inspection of
the emitted C code from AKCL doesn't reveal many obvious sources of
inefficiency. (Since AKCL compiles Lisp into C, there are many cases
where KCL code is as fast as hand-written C code.)

See the paper
for a discussion of the speed of Lisp vis a vis Fortran or C.

Since Lisp is a good language for rapid prototyping, it is easy for a
mediocre programmer (or even a good programmer, who isn't being careful) to
generate a large amount of inefficient Lisp code. A good example is the use
of APPEND to link successive lists together, instead of keeping a pointer
to the tail of the list. Often a programmer can obtain significant
speed increases by using a time/space profiler to identify the
functions which waste time (often small functions which are called
frequently) and rewriting those functions.
Subject: [2-5] Why does Common Lisp have "#'"?

#' is a macro-character which expands #'FOO to (FUNCTION FOO).  Symbols in
Lisp have two bindings, one for values and one for functions, allowing them
to represent both variables and functions, depending on context. #'FOO
accesses FOO's lexical function binding in a context where the value
interpretation would normally occur.  #' is also used to create lexical
closures for lambda expressions. A lexical closure is a function which when
invoked executes the body of the lambda-expression in the lexical
environment within which the closure was created.  See pp. 115-117 of CLtL2
for more details.

Subject: [2-6] How do I call non-Lisp functions from Lisp?

Most Lisp implementations for systems where Lisp is not the most common
language provide a "foreign function" interface.  As of now there has been
no significant standardization effort in this area.  They tend to be
similar, but there are enough differences that it would be inappropriate to
try to describe them all here.  In general, one uses an
implementation-dependent macro that defines a Lisp function, but instead of
supplying a body for the function, one supplies the name of a function written
in another language; the argument list portion of the definition is
generally augmented with the data types the foreign function expects and
the data type of the foreign function's return value, and the Lisp
interface function arranges to do any necessary conversions.  There is also
generally a function to "load" an object file or library compiled in a
foreign language, which dynamically links the functions in the file being
loaded into the address space of the Lisp process, and connects the
interface functions to the corresponding foreign functions.

If you need to do this, see the manual for your language implementation for
full details.  In particular, be on the lookout for restrictions on the
data types that may be passed.  You may also need to know details about the
linkage conventions that are used on your system; for instance, many C
implementations prepend an underscore onto the names of C functions when
generating the assembler output (this allows them to use names without
initial underscores internally as labels without worrying about conflicts),
and the foreign function interface may require you to specify this form

Franz Allegro Common Lisp's "Foreign Function Call Facility" is
described in chapter 10 of the documentation. Calling Lisp Functions
from C is treated in section 10.8.2. The foreign function interface in
Macintosh Common Lisp is similar. The foreign function interface for
KCL is described in chapter 10 of the KCL Report. The foreign function
interfaces for Lucid on the Vax and Lucid on the Sun4 are
incompatible. Lucid's interface is described in chapter 5 of the
Advanced User's Guide.

Subject: [2-7] Can I call Lisp functions from other languages?

In implementations that provide a foreign function interface as described
above, there is also usually a "callback" mechanism.  The programmer may
associate a foreign language function name with a Lisp function.  When a
foreign object file or library is loaded into the Lisp address space, it is
linked with these callback functions.  As with foreign functions, the
programmer must supply the argument and result data types so that Lisp may
perform conversions at the interface. Note that in such foreign function
interfaces Lisp is often left "in control" of things like memory
allocation, I/O channels, and startup code (this is a major nuisance
for lots of people).

Subject: [2-8]  I want to call a function in a package that might not exist at
                compile time. How do I do this?

Use (funcall (find-symbol "SYMBOL-NAME" :pkg-name) ...).

Subject: [2-9]  What is CDR-coding?

CDR-coding is a space-saving way to store lists in memory.  It is normally
only used in Lisp implementations that run on processors that are
specialized for Lisp, as it is difficult to implement efficiently
in software.  In normal list structure, each element of the
list is represented as a CONS cell, which is basically two pointers (the
CAR and CDR); the CAR points to the element of the list, while the CDR
points to the next CONS cell in the list or NIL.  CDR-coding takes
advantage of the fact that most CDR cells point to another CONS, and
further that the entire list is often allocated at once (e.g. by a call to
LIST).  Instead of using two pointers to implement each CONS cell, the CAR
cell contains a pointer and a two-bit "CDR code".  The CDR code may contain
one of three values: CDR-NORMAL, CDR-NEXT, and CDR-NIL.  If the code is
CDR-NORMAL, this cell is the first half of an ordinary CONS cell pair, and
the next cell in memory contains the CDR pointer as described above.  If
the CDR code is CDR-NEXT, the next cell in memory contains the next CAR
cell; in other words, the CDR pointer is implicitly thisaddress+1, where
thisaddress is the memory address of the CAR cell.  If the CDR code is
CDR-NIL, then this cell is the last element of the list; the CDR pointer is
implicitly a reference to the object NIL.  When a list is constructed
incrementally using CONS, a chain of ordinary pairs is created; however,
when a list is constructed in one step using LIST or MAKE-LIST, a block of
memory can be allocated for all the CAR cells, and their CDR codes all set
to CDR-NEXT (except the last, which is CDR-NIL), and the list will only
take half as much storage (because all the CDR pointers are implicit).

If this were all there were to it, it would not be difficult to implement
in software on ordinary processors; it would add a small amount of overhead
to the CDR function, but the reduction in paging might make up for it.  The
problem arises when a program uses RPLACD on a CONS cell that has a CDR
code of CDR-NEXT or CDR-NIL.  Normally RPLACD simply stores into the CDR
cell of a CONS, but in this case there is no CDR cell -- its contents are
implicitly specified by the CDR code, and the word that would normally
contain the CDR pointer contains the next CONS cell (in the CDR-NEXT case)
to which other data structures may have pointers, or the first word of some
other object (in the CDR-NIL case).  When CDR-coding is used, the
implementation must also provide automatic "forwarding pointers"; an
ordinary CONS cell is allocated, the CAR of the original cell is copied
into its CAR, the value being RPLACD'ed is stored into its CDR, and the old
CAR cell is replaced with a forwarding pointer to the new CONS cell.
Whenever CAR or CDR is performed on a CONS, it must check whether the
location contains a forwarding pointer.  This overhead on both CAR and CDR,
coupled with the overhead on CDR to check for CDR codes, is generally
enough that using CDR codes on conventional hardware is infeasible.

There is some evidence that CDR-coding doesn't really save very much
memory, because most lists aren't constructed at once, or RPLACD is done on
them enough that they don't stay contiguous.  At best this technique can
save 50% of the space occupied by CONS cells. However, the savings probably
depends to some extent upon the amount of support the implementation
provides for creating CDR-coded lists.  For instance, many system functions
on Symbolics Lisp Machines that operate on lists have a :LOCALIZE option;
when :LOCALIZE T is specified, the list is first modified and then copied
to a new, CDR-coded block, with all the old cells replaced with forwarding
pointers.  The next time the garbage collector runs, all the forwarding
pointers will be spliced out.  Thus, at a cost of a temporary increase in
memory usage, overall memory usage is generally reduced because more lists
may be CDR-coded. There may also be some benefit in improved paging
performance due to increased locality as well (putting a list into
CDR-coded form makes all the "cells" contiguous). Nevertheless, modern
Lisps tend to use lists much less frequently, with a much heavier
reliance upon code, strings, and vectors (structures).

Subject: [2-10] What is garbage collection?

Garbage Collection (GC) refers to the automatic storage allocation
mechanisms present in many Lisps. There are several kinds of storage
allocation algorithms, but most fall within two main classes:

   1. Stop and Copy. Systems which copy active objects from "old"
      storage to "new" storage and then recycle the old storage.

   2. Mark and Sweep. Systems which link together storage
      used by discarded objects. 

Generational scavenging garbage collection (aka emphemeral GC) is a
variation in which memory is allocated in layers, with tenured
(long-lived) objects in the older layers. Rather than doing a full GC
of all of memory every time more room is needed, only the last few
layers are GCed during an ephemeral GC, taking much less time.
Short-lived objects are quickly recycled, and full GCs are then much
less frequent. It is most often used to improve the performance of
stop and copy garbage collectors.  It is possible to implement
ephemeral GC in mark and sweep systems, just much more difficult.

Stop and copy garbage collection provides simpler storage allocation,
avoids fragmentation of memory (intermixing of free storage with used
storage). Copying, however, consumes more of the address space, since up to
half the space must be kept available for copying all the active objects.
This makes stop and copy GC impractical for systems with a small address
space or without virtual memory.  Also, copying an object requires that you
track down all the pointers to an object and update them to reflect the new
address, while in a non-copying system you need only keep one pointer to an
object, since its location will not change. It is also more difficult to
explicitly return storage to free space in a copying system.

Garbage collection is not part of the Common Lisp standard. Most Lisps
provide a function ROOM which provides human-readable information about the
state of storage usage. In many Lisps, (gc) invokes an ephemeral garbage
collection, and (gc t) a full garbage collection.

Subject: [2-11] How do I save an executable image of my loaded Lisp system?
                How do I run a Unix command in my Lisp? How do I exit Lisp?
                Access environment variables?

There is no standard for dumping a Lisp image. Here are the
commands from some lisp implementations:
   Lucid:               DISKSAVE
   Symbolics:           Save World  [CP command]
   CMU CL:              SAVE-LISP
   Franz Allegro:       EXCL:DUMPLISP (documented) 
                        SAVE-IMAGE (undocumented)
   Medley:              IL:SYSOUT or IL:MAKESYS
   MCL:                 SAVE-APPLICATION <pathname>
                          &key :toplevel-function  :creator :excise-compiler
                          :size :resources :init-file :clear-clos-caches
   KCL:                 (si:save-system "saved_kcl")
   LispWorks:		LW:SAVE-IMAGE
Be sure to garbage collect before dumping the image. You may need to
experiment with the kind of garbage collection for large images, and
may find better results if you build the image in stages.

There is no standard for running a Unix shell command from Lisp,
especially since not all Lisps run on top of Unix. Here are the
commands from some Lisp implementations:
   Allegro:             EXCL:RUN-SHELL-COMMAND (command &key input output
                                  error-output wait if-input-does-not-exist
                                  if-output-exists if-error-output-exists)
   Lucid:               RUN-PROGRAM (name 
                                     &key input output
                                          error-output (wait t) arguments
                                          (if-input-does-not-exist :error)
                                          (if-output-exists :error)
                                          (if-error-output-exists :error))
   KCL:                 SYSTEM 
                        For example, (system "ls -l").
                        You can also try RUN-PROCESS and EXCLP, but they
                        don't work with all versions of KCL.
   CMU CL:              RUN-PROGRAM (program args
			   &key (env *environment-list*) (wait t) pty input
			   if-input-does-not-exist output
			   (if-output-exists :error) (error :output) 
			   (if-error-exists :error) status-hook before-execve)

To toggle source file recording and cross-reference annotations, use
   Allegro:             excl:*record-source-file-info*
   LispWorks:           (toggle-source-debugging nil)

Memory management:
   CMU CL:              (bytes-consed-between-gcs)  [this is setfable]
   Lucid:		(change-memory-management 
      			   &key growth-limit expand expand-reserved)
   Allegro:		*tenured-bytes-limit*
   LispWorks:           LW:GET-GC-PARAMETERS
                        (use LW:SET-GC-PARAMETERS to change them)

Environment Variable Access:
   Allegro:             (sys:getenv var)
                        (sys:setenv var value) or (setf (sys:getenv var) value)
   Lucid:               (environment-variable var)
                        (set-environment-variable var value)
   CMU CL 17:           (cdr (assoc (intern var :keyword) *environment-list*))
   {A}KCL, GCL:         (system:getenv var)
   CLISP:               (system::getenv var)

   CLISP:               EXIT
   Allegro:             EXIT (&optional excl::code &rest excl::args
                              &key excl::no-unwind excl::quiet)
   LispWorks:           BYE (&optional (arg 0))
   Lucid:               QUIT (&optional (lucid::status 0))
   CMU CL:              QUIT (&optional recklessly-p)

Subject: [2-12] I'm porting some code from a Symbolics Lisp machine to some
                other platform, and there are strange characters in the code.
                What do they mean?

The Symbolics Zetalisp character set includes the following
characters not present in other Lisps (^ means control):
   ^]      >=      greater than or equal to
   ^\      <=      less than or equal to
   ^Z      !=      not equal to
   ^^      ==      equivalent to 
   ^E      not
   ^G      pi
   ^L      +/-     plus/minus
   ^H      lambda
   ^F      epsilon
   ^W      <-->    left/right arrow
   ^X      <--     left arrow
   ^Y      -->     right arrow
   ^A              down arrow
   ^K              up arrow
   ^D              up caret
   ^_              down caret
   ^T              forall
   ^U              there exists
   ^B              alpha
   ^C              beta
   ^I              gamma
   ^J              delta
   ^O              partial delta  
   ^N              infinity
   ^M              circle +
   ^V              circle x

Other special characters to look out for are the font-change characters,
which are represented as a ^F followed by a digit or asterisk. A digit
means to push font #N onto the stack; an asterisk means to pop the most
recent font from the stack. You can clean up the code by replacing "\^F."
with "". In format statements, ^P and ^Q are used to delimit text to
be printed in a particular character style.
Subject: [2-13] History: Where did Lisp come from?

John McCarthy developed the basics behind Lisp during the 1956 Dartmouth
Summer Research Project on Artificial Intelligence.  He intended it as an
algebraic LISt Processing (hence the name) language for artificial
intelligence work. Early implementations included the IBM 704, the IBM
7090, the DEC PDP-1, the DEC PDP-6 and the DEC PDP-10. The PDP-6 and
PDP-10 had 18-bit addresses and 36-bit words, allowing a CONS cell to
be stored in one word, with single instructions to extract the CAR and
CDR parts. The early PDP machines had a small address space, which
limited the size of Lisp programs. 

Milestones in the development of Lisp:

   1956            Dartmouth Summer Research Project on AI.

   1960-65         Lisp1.5 is the primary dialect of Lisp.

   1964-           Development of BBNLisp at BBN.

   late 60s        Lisp1.5 diverges into two main dialects:
                   Interlisp (originally BBNLisp) and MacLisp.

   early 70s       Development of special-purpose computers known as Lisp
                   Machines, designed specificly to run Lisp programs. 
                   Xerox D-series Lisp Machines run Interlisp-D. 
                   Early MIT Lisp Machines run Lisp Machine Lisp 
                   (an extension of MacLisp).

   1969            Anthony Hearn and Martin Griss define Standard Lisp to
                   port REDUCE, a symbolic algebra system, to a variety
                   of architectures.  

   late 70s        Macsyma group at MIT developed NIL (New Implementation
                   of Lisp), a Lisp for the VAX.

                   Stanford and Lawrence Livermore National Laboratory
                   develop S-1 Lisp for the Mark IIA supercomputer.

                   Franz Lisp (dialect of MacLisp) runs on stock-hardware
                   Unix machines.

                   Gerald J. Sussman and Guy L. Steele developed Scheme,
                   a simple dialect of Lisp with lexical scoping and
                   lexical closures, continuations as first-class objects,
                   and a simplified syntax (i.e., only one binding per symbol).

                   Advent of object-oriented programming concepts in Lisp.
                   Flavors was developed at MIT for the Lisp machine,
                   and LOOPS (Lisp Object Oriented Programming System) was
                   developed at Xerox. 

   early 80s       Development of SPICE-Lisp at CMU, a dialect of MacLisp
                   designed to run on the Scientific Personal Integrated
                   Computing Environment (SPICE) workstation.

   1980            First biannual ACM Lisp and Functional Programming Conf.

   1981            PSL (Portable Standard Lisp) runs on a variety of platforms.

   1981+           Lisp Machines from Xerox, LMI (Lisp Machines Inc) 
                   and Symbolics available commercially.

   April 1981      Grass roots definition of Common Lisp as a description
                   of the common aspects of the family of languages (Lisp
                   Machine Lisp, MacLisp, NIL, S-1 Lisp, Spice Lisp, Scheme). 

   1984            Publication of CLtL1. Common Lisp becomes a de facto 

   1986            X3J13 forms to produce a draft for an ANSI Common Lisp

   1987            Lisp Pointers commences publication.

   1990            Steele publishes CLtL2 which offers a snapshot of
                   work in progress by X3J13.  (Unlike CLtL1, CLtL2
                   was NOT an output of the standards process and was
                   not intended to become a de facto standard.  Read
                   the Second Edition Preface for further explanation
                   of this important issue.) Includes CLOS,
                   conditions, pretty printing and iteration facilities. 

   1992            X3J13 creates a draft proposed American National
                   Standard for Common Lisp. This document is the
                   first official successor to CLtL1. 

[Note: This summary is based primarily upon the History section of the
draft ANSI specification. More detail and references can be obtained from
that document. See [4-12] for information on obtaining a copy.]

Gabriel and Steele's "The Evolution of Lisp", which appeared in the
1993 ACM History of Programming Languages conference, is available by
anonymous ftp from	[]

Brad Miller maintains a Lisp History web page at

Subject: [2-14]  How do I find the argument list of a function?
                 How do I get the function name from a function object?

There is no standard way to find the argument list of a function,
since implementations are not required to save this information.
However, many implementations do remember argument information, and
usually have a function that returns the lambda list. Here are the
commands from some Lisp implementations:

   Lucid:                               arglist
   Allegro:                             excl::arglist
   Symbolics:                           arglist
   LispWorks:                           lw:function-lambda-list

CMU Common Lisp, new compiler:
   #+(and :CMU :new-compiler)
   (defun arglist (name)
     (let* ((function (symbol-function name))
            (stype (system:%primitive get-vector-subtype function)))
       (when (eql stype system:%function-entry-subtype)
         (cadr (system:%primitive header-ref function

The draft ANSI standard does include FUNCTION-LAMBDA-EXPRESSION and
FUNCTION-KEYWORDS, which can be used to create an ARGLIST function.

If you're interested in the number of required arguments you could use

   (defun required-arguments (name)
     (or (position-if #'(lambda (x) (member x lambda-list-keywords))
                      (arglist name))
         (length (arglist name))))

To extract the function name from the function object, as in
        (function-name #'car) ==> 'car
use the following vendor-dependent functions:

   Symbolics: (si::compiled-function-name <fn>)
	(unless (si:lexical-closure-p <fn>) ...)
   Lucid:     (sys::procedure-ref <fn> SYS:PROCEDURE-SYMBOL)
	(when (sys:procedurep <fn>) ..)
   Allegro:   (xref::object-to-function-name <fn>)
   CMU CL:    (kernel:%function-header-name <fn>)
   AKCL:      (system::compiled-function-name <fn>)
   MCL:       (ccl::function-name <fn>)
   LispWorks: (system::function-name <fn>)

If a vendor-dependent function does not exist, the following
(inefficient) code maps over all symbols looking for one whose
function-cell matches the function object.

(defun function-name (fobject)
   (do-all-symbols (fsymbol)
      (when (and (fboundp fsymbol)
                 (eq (symbol-function fsymbol) fobject))
        (return fsymbol))))

If a vendor supports FUNCTION-LAMBDA-EXPRESSION, the third value is
the name of the function, if available. 

Subject: [2-15] How can I have two Lisp processes communicate via unix sockets?

CLX uses Unix sockets to communicate with the X window server. Look at
the following files from the CLX distribution for a good example of
using Unix sockets from Lisp:
        defsystem.lisp          Lucid, AKCL, IBCL, CMU.
        socket.c, sockcl.lisp   AKCL, IBCL
        excldep.lisp            Franz Allegro CL
You will need the "socket.o" files which come with Lucid and Allegro.
To obtain CLX, see the entry for CLX in the answer to question [7-1].

See the file sockets.tar.gz in the Lisp Utilities repository
described in the answer to question [6-1].

Subject: [2-16]  How can I create a stream that acts like UNIX's /dev/null
                 (i.e., gobbles any output and immediately signals EOF on
                 input operations)?

(defparameter *dev-null*
  (make-two-way-stream (make-concatenated-stream) (make-broadcast-stream))
  ;; Since Lisp Machines have a built-in /dev/null which handles
  ;; additional, non-standard operations, we'll use that instead.
  #+lispm #'system:null-stream)

Subject: [2-17] Read-time conditionalization of code (#+ #- and *features*)

The #+ and #- syntax provides for the read-time conditionalization of
lisp code, depending on the presence or absence of keywords on the
*features* list. The nascent Common Lisp standard does not specify
what keywords an implementation must have on its features list.
Nevertheless, most implementations have features that allow one to
distinguish the implementation from other implementations. This allows
one to write implementation-dependent code that is run only in the
relevant implementations.

Here is a list of the features to use to specify a particular Common
Lisp implementation. Unfortunately, not every vendor has a
unique keyword that distinguishes their family of implementations from
those of other vendors, nor major and minor versions of the implementation.

   :lucid                       Lucid Common Lisp
   :lcl3.0                      Lucid Common Lisp v3.0 and above
   :lcl4.0                      Lucid Common Lisp v4.0 and above
   (and :allegro :franz-inc)    Franz Allegro Common Lisp
   :excl                        Franz Allegro Common Lisp 
   :aclpc                       Franz Allegro Common Lisp\PC.
   :allegro-v3.0                Franz Allegro Common Lisp v3.0 
   :allegro-v3.1                Franz Allegro Common Lisp v3.1 
   :allegro-v4.0                Franz Allegro Common Lisp v4.0 
   :allegro-v4.1                Franz Allegro Common Lisp v4.1 
   :cmu                         CMU Common Lisp
   (and :cmu :new-compiler)     CMU Common Lisp w/Python compiler
   (and :cmu :python)           CMU Common Lisp w/Python compiler
   :cmu17                       CMU Common Lisp v17 and above
   kcl                          Kyoto Common Lisp
   akcl                         Austin KCL
   :ibcl                        Ibuki Common Lisp 
   :mcl                         Macintosh Common Lisp
   :coral                       Coral Lisp; bought by Apple to become
				MACL, then MCL
   :ccl                         Coral Common Lisp
	[Note: Harlequin LispWorks also uses :ccl]
   :ccl-1                       Coral Common Lisp v1
   :ccl-1.3                     Coral Common Lisp v1.3 and higher
   :ccl-2                       present in Macintosh Common Lisp 2.0 and higher
   :harlequin-common-lisp       Harlequin Common Lisp
   :harlequin-unix-lisp         Harlequin on Unix platforms
   :harlequin-PC-lisp           Harlequin on PC platforms
   :lispworks                   Harlequin LispWorks development environment
   :lispworks3                  major release of Harlequin LispWorks
   :lispworks3.1                major and minor release of Harlequin LispWorks
   :harlequin                   All Harlequin products. not always present?
   :clisp                       CLISP Common Lisp
   :symbolics                   Symbolics Genera
   :imach                       Symbolics Genera for Ivory architecture
   :cloe-runtime                Symbolics CLOE
   :cloe                        CLOE 3.1
   :procyon                     Procyon Common Lisp
   (and :procyon :macintosh)    Procyon Common Lisp, Macintosh version
   (and :procyon :os2)          Procyon Common Lisp, OS2 version
   :gclisp                      Golden Common Lisp
   (and dec vax common)         DEC VAXlisp
   :explorer                    TI Explorer Lisp Machine  | used
   :TI                          TI Explorer Lisp Machine  | interchangeably
   :elroy                       TI Explorer release 3 and successors
   :Xerox                       Medley (Venue's CL/InterLisp combo) to rel2.01
   :medley                      Medley releases 3.0 and up
      Use  (IL:UNIX-GETPARM "mach") and (IL:UNIX-GETPARM "arch") to
      distinguish platforms under Medley.
   :ecl                         ECoLisp
   :lispm                       Symbolics, TI, and LMI Lisp machines

   In the cases where a feature is not a keyword, it is almost always
   in the LISP package.

The draft ANSI standard defines some other useful features:

  :cltl1                Compatible with the 1st edition of Steele
  :cltl2                Compatible with the 2nd edition of Steele
  :IEEE-Floating-Point  IEEE floating point support
  :X3J13                conforms to some particular draft of the ANSI
                        CL specification 
  :draft-ANSI-CL        conforms to first full public review draft
  :ANSI-CL              conforms to ANSI CL after its adoption
  :common-lisp          language family "Common Lisp"

Other features used by some Lisps include:

  :clos                 Contains a native CLOS implementation.
  :pcl                  Contains the PCL implementation of CLOS.
  :flavors              Has an implementation of Symbolics Flavors
  :loop                 Contains the :cltl1 version of the Loop macro
  :ansi-loop            Contains the ANSI Loop macro
  :clx or :xlib         Contains CLX
  :clxr4 or :CLX-MIT-R4 Contains CLX for X11R4
  :clxr5 or :CLX-MIT-R5 Contains CLX for X11R5
  :compiler             Contains a compiler  
  :windows              MS Windows version
  :color                Color display
  :monochrome           Monochrome display
  :multiprocessing      Has multiprocessing capabilities.
  :profiler             Has a PC-monitoring based profiler.

Platform-specific features, CPU-dependent features, and
operating-system specific features are also important because they can
indicate changes between different implementations of the same lisp,
such as compiled file extensions (e.g., .sbin, .hbin, etc.).
Unfortunately, not every vendor includes such features, and the naming
conventions are inconsistent. Where there are several names for the
same feature, we've put the preferred name first. Hopefully the
vendors will begin to standardize their use of these features.
CPU-dependent features include :sparc (used in CMU CL, Lucid CL,
Harlequin, and Allegro CL), :mips (used in Allegro CL), :r2000 (used
in Allegro CL even on r4000 machines), :mc68000, and :pa (HP's
9000/800 RISC cpu).  Platform-specific features include :sun (used in
Allegro CL and Lucid), :sun4 (used in CMU CL and Allegro CL), :sgi
(used in Allegro CL), :hp300, :hp400, :hp500, :sun3, :vax, :prime,
:dec, :dec3100, :macintosh (used in Procyon but not MCL), :ibm-pc,
:ibm-rt-pc.  OS-specific features include :unix (used in CMU CL, IBCL,
and Lucid CL), :vms, :sunos (used in CMU CL), :sun-os (used in Lucid),
:sunos4.0 and :sunos4 (used in various Allegro versions independent of
the actual version of SunOS), :mach (used in CMU CL), :hpux, :ultrix,
:os2, and :svr4.


   :allegro alone doesn't suffice to distinguish Franz Allegro Common
   Lisp from Macintosh Allegro Common Lisp (an early version of
   Macintosh Common Lisp). :excl specifies that the EXCL package (a
   set of Allegro extensions to Common Lisp) is present, but this has
   since become synonymous with Franz Allegro Common Lisp.

   Thanks to Vincent Keunen for gathering the information in this list.

Subject: [2-18]  What reader macro characters are used in major Lisp systems?

The draft ANSI standard for Common Lisp leaves many dispatching macro
characters unassigned. Of these, the following are explicitly reserved
for the user and hence will never be defined by Common Lisp:
   #!, #?, #[, #], #{, and #}. 
All other unassigned macro characters are not reserved for the user,
and hence the user has no guarantee that they won't be used by some
Lisp implementation. 

As a result, there is the potential of portability clashes between
systems that use the same macro characters. This question lists the
non-standard macro character usage of major Lisp systems, in an effort
to avoid such conflicts.

   #"		AKCL; pathnames
   #$		Macintosh Common Lisp; traps
   #%		Cyc; references to constants in the representation language
   #%		Harlequin LispWorks; ?
   #@		Macintosh Common Lisp; Points notation
   #@ 		Defsystem	
   #I		Portable Infix Package
   #L		Allegro Common Lisp; logical pathnames
   #M		Series
   #T 		Allegro Common Lisp; ?
   #Y		CLISP; ?
   #Z		Series
   #_		Macintosh Common Lisp; traps
   #`		Harlequin LispWorks; ?

There is a proposal in the ANSI draft to have COMPILE-FILE and LOAD
bind *READTABLE*, which would allow one to locally redefine syntax
through private readtables. Unfortunately, this doesn't help with the
Infix Package, where one wants to globally extend syntax.

Subject: [2-19] How do I determine if a file is a directory or not? 
                How do I get the current directory name from within a Lisp 
                program? Is there any way to create a directory?

There is no portable way in Common Lisp of determining whether a file
is a directory or not. Calling DIRECTORY on the pathname will not
always work, since the directory could be empty. For UNIX systems
   (defun DIRECTORY-P (pathname)
      (probe-file (concatenate 'string pathname "/.")))
seems to work fairly reliably. (If "foo" is a directory, then "foo/."
will be a valid filename; if not, it will return NIL.) This won't, of
course, work on the Macintosh, or on other operating systems (e.g.,
MVS, CMS, ITS). On the Macintosh, use DIRECTORYP.

Moreover, some operating systems may not support the concept of
directories, or even of a file system. For example, recent work on
object-oriented technology considers files to be collections of
objects. Each type of collection defines a set of methods for reading
and writing the objects "stored" in the collection. 

There's no standard function for finding the current directory from
within a Lisp program, since not all Lisp environments have the
concept of a current directory. Here are the commands from some Lisp
   Lucid:               WORKING-DIRECTORY (which is also SETFable)
                        PWD and CD also work
   Allegro:             CURRENT-DIRECTORY (use excl:chdir to change it)
   LispWorks:           LW:*CURRENT-WORKING-DIRECTORY* 
                        (use LW:CHANGE-DIRECTORY to change it)

Allegro also uses the variable *default-pathname-defaults* to resolve
relative pathnames, maintaining it as the current working directory.
So evaluating (truename "./") in Allegro (and on certain other
systems) will return a pathname for the current directory. Likewise,
in some VMS systems evaluating (truename "[]") will return a pathname
for the current directory.

There is no portable way of creating a new directory from within a
Lisp program. 

Subject: [2-20] What is a "Lisp Machine" (LISPM)?

A Lisp machine (or LISPM) is a computer which has been optimized to run lisp
efficiently and provide a good environment for programming in it. The
original Lisp machines were implemented at MIT, with spinoffs as LMI (defunct)
and Symbolics (bankrupt). Xerox also had a series of Lisp machines
(Dandylion, Dandytiger), as did Texas Instruments (TI Explorer). The
TI and Symbolics Lisp machines are currently available as cards that
fit into Macintosh computers (the so-called "Lisp on a chip").

Optimizations typical of Lisp machines include:

   - Hardware Type Checking. Special type bits let the type be checked
     efficiently at run-time.

   - Hardware Garbage Collection. 

   - Fast Function Calls. 

   - Efficient Representation of Lists.

   - System Software and Integrated Programming Environments.

For further information, see:

   Paul Graham, "Anatomy of a Lisp Machine", AI Expert, December 1988.

   Pleszkun and Thazhuthaveetil, "The Architecture of Lisp Machines",
   IEEE Computer, March 1987.

   Ditzel, Schuler and Thomas, "A Lisp Machine Profile: Symbolics 3650",
   AI Expert, January 1987.

   Peter M. Kogge, "The Architecture of Symbolic Computers",
   McGraw-Hill 1991. ISBN 0-07-035596-7.

[Derived from a post by Arthur Pendragon <>.]

Subject: [2-21] How do I tell if a symbol names a function and not a macro?

FBOUNDP tests whether the symbol is globally bound to an operator
(e.g., a function, macro, or special form). SYMBOL-FUNCTION returns
the contents of a symbol's "function slot" if the symbol names a
function. But if the symbol names a macro or special form, it is
completely unspecified what a call to SYMBOL-FUNCTION will return.
Instead, use code like the following to test whether a symbol names a

   (defun fbound-to-function-p (symbol)
     (and (fboundp symbol)
          (not (macro-function symbol))
          (not (special-operator-p symbol))))

;;; *EOF*

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