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Frequently Asked Questions (FAQS);faqs.354 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 standard. 1986 X3J13 forms to produce a draft for an ANSI Common Lisp standard. 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 [1-5] for information on obtaining a copy.] ---------------------------------------------------------------- [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 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 system:%function-entry-type-slot))))) 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>) Lucid: (sys::procedure-ref <fn> SYS:PROCEDURE-SYMBOL) 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>) ---------------------------------------------------------------- [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 [6-5]. See the file lisp-sockets.text in the Lisp Utilities repository described in the answer to question [6-1]. ---------------------------------------------------------------- ;;; *EOF* Xref: bloom-picayune.mit.edu comp.lang.lisp:8751 news.answers:4561 Path: bloom-picayune.mit.edu!enterpoop.mit.edu!spool.mu.edu!uunet!ogicse!das-news.harvard.edu!cantaloupe.srv.cs.cmu.edu!crabapple.srv.cs.cmu.edu!mkant From: mkant+@cs.cmu.edu (Mark Kantrowitz) Newsgroups: comp.lang.lisp,news.answers Subject: FAQ: Lisp Frequently Asked Questions 3/6 [Monthly posting] Summary: Common Pitfalls Message-ID: <lisp-faq-3.text_724237304@cs.cmu.edu> Date: 13 Dec 92 09:02:09 GMT Article-I.D.: cs.lisp-faq-3.text_724237304 Expires: Tue, 26 Jan 1993 09:01:44 GMT Sender: news@cs.cmu.edu (Usenet News System) Reply-To: lisp-faq@think.com Followup-To: poster Organization: School of Computer Science, Carnegie Mellon Lines: 707 Approved: news-answers-request@MIT.Edu Supersedes: <lisp-faq-3.text_721645325@cs.cmu.edu> Nntp-Posting-Host: a.gp.cs.cmu.edu Archive-name: lisp-faq/part3 Last-Modified: Thu Nov 5 19:30:40 1992 by Mark Kantrowitz Version: 1.27 ;;; **************************************************************** ;;; Answers to Frequently Asked Questions about Lisp *************** ;;; **************************************************************** ;;; Written by Mark Kantrowitz and Barry Margolin ;;; lisp-faq-3.text -- 32488 bytes This post contains Part 3 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 lisp-faq@think.com. This section contains a list of common pitfalls. Pitfalls are aspects of Common Lisp which are non-obvious to new programmers and often seasoned programmers as well. Common Pitfalls (Part 3): [3-0] Why does (READ-FROM-STRING "foobar" :START 3) return FOOBAR instead of BAR? [3-1] Why can't it deduce from (READ-FROM-STRING "foobar" :START 3) that the intent is to specify the START keyword parameter rather than the EOF-ERROR-P and EOF-VALUE optional parameters? [3-2] Why can't I apply #'AND and #'OR? [3-3] I used a destructive function (e.g. DELETE, SORT), but it didn't seem to work. Why? [3-4] After I NREVERSE a list, it's only one element long. After I SORT a list, it's missing things. What happened? [3-5] Why does (READ-LINE) return "" immediately instead of waiting for me to type a line? [3-6] I typed a form to the read-eval-print loop, but nothing happened. Why? [3-7] DEFMACRO doesn't seem to work. When I compile my file, LISP warns me that my macros are undefined functions, or complains "Attempt to call <function> which is defined as a macro. [3-8] Name conflict errors are driving me crazy! (EXPORT, packages) [3-9] Closures don't seem to work properly when referring to the iteration variable in DOLIST, DOTIMES and DO. [3-10] What is the difference between FUNCALL and APPLY? [3-11] Miscellaneous things to consider when debugging code. [3-12] When is it right to use EVAL? [3-13] Why does my program's behavior change each time I use it? [3-14] When producing formatted output in Lisp, where should you put the newlines (e.g., before or after the line, FRESH-LINE vs TERPRI, ~& vs ~% in FORMAT)? [3-15] I'm using DO to do some iteration, but it doesn't terminate. Search for [#] to get to question number # quickly. ---------------------------------------------------------------- [3-0] Why does (READ-FROM-STRING "foobar" :START 3) return FOOBAR instead of BAR? READ-FROM-STRING is one of the rare functions that takes both &OPTIONAL and &KEY arguments: READ-FROM-STRING string &OPTIONAL eof-error-p eof-value &KEY :start :end :preserve-whitespace When a function takes both types of arguments, all the optional arguments must be specified explicitly before any of the keyword arguments may be specified. In the example above, :START becomes the value of the optional EOF-ERROR-P parameter and 3 is the value of the optional EOF-VALUE parameter. ---------------------------------------------------------------- [3-1] Why can't it deduce from (READ-FROM-STRING "foobar" :START 3) that the intent is to specify the START keyword parameter rather than the EOF-ERROR-P and EOF-VALUE optional parameters? In Common Lisp, keyword symbols are first-class data objects. Therefore, they are perfectly valid values for optional parameters to functions. There are only four functions in Common Lisp that have both optional and keyword parameters (they are PARSE-NAMESTRING, READ-FROM-STRING, WRITE-LINE, and WRITE-STRING), so it's probably not worth adding a nonorthogonal kludge to the language just to make these functions slightly less confusing; unfortunately, it's also not worth an incompatible change to the language to redefine those functions to use only keyword arguments. ---------------------------------------------------------------- [3-2] Why can't I apply #'AND and #'OR? Here's the simple, but not necessarily satisfying, answer: AND and OR are macros, not functions; APPLY and FUNCALL can only be used to invoke functions, not macros and special operators. OK, so what's the *real* reason? The reason that AND and OR are macros rather than functions is because they implement control structure in addition to computing a boolean value. They evaluate their subforms sequentially from left/top to right/bottom, and stop evaluating subforms as soon as the result can be determined (in the case of AND, as soon as a subform returns NIL; in the case of OR, as soon as one returns non-NIL); this is referred to as "short circuiting" in computer language parlance. APPLY and FUNCALL, however, are ordinary functions; therefore, their arguments are evaluated automatically, before they are called. Thus, were APPLY able to be used with #'AND, the short-circuiting would be defeated. Perhaps you don't really care about the short-circuiting, and simply want the functional, boolean interpretation. While this may be a reasonable interpretation of trying to apply AND or OR, it doesn't generalize to other macros well, so there's no obvious way to have the Lisp system "do the right thing" when trying to apply macros. The only function associated with a macro is its expander function; this function accepts and returns and form, so it cannot be used to compute the value. The Common Lisp functions EVERY and SOME can be used to get the functionality you intend when trying to apply #'AND and #'OR. For instance, the erroneous form: (apply #'and *list*) can be translated to the correct form: (every #'identity *list*) ---------------------------------------------------------------- [3-3] I used a destructive function (e.g. DELETE, SORT), but it didn't seem to work. Why? I assume you mean that it didn't seem to modify the original list. There are several possible reasons for this. First, many destructive functions are not *required* to modify their input argument, merely *allowed* to; in some cases, the implementation may determine that it is more efficient to construct a new result than to modify the original (this may happen in Lisp systems that use "CDR coding", where RPLACD may have to turn a CDR-NEXT or CDR-NIL cell into a CDR-NORMAL cell), or the implementor may simply not have gotten around to implementing the destructive version in a truly destructive manner. Another possibility is that the nature of the change that was made involves removing elements from the front of a list; in this case, the function can simply return the appropriate tail of the list, without actually modifying the list. And example of this is: (setq *a* (list 3 2 1)) (delete 3 *a*) => (2 1) *a* => (3 2 1) Similarly, when one sorts a list, SORT may destructively rearrange the pointers (cons cells) that make up the list. SORT then returns the cons cell that now heads the list; the original cons cell could be anywhere in the list. The value of any variable that contained the original head of the list hasn't changed, but the contents of that cons cell have changed because SORT is a destructive function: (setq *a* (list 2 1 3)) (sort *a* #'<) => (1 2 3) *a* => (2 3) In both cases, the remedy is the same: store the result of the function back into the place whence the original value came, e.g. (setq *a* (delete 3 *a*)) *a* => (2 1) Why don't the destructive functions do this automatically? Recall that they are just ordinary functions, and all Lisp functions are called by value. Therefore, these functions do not know where the lists they are given came from; they are simply passed the cons cell that represents the head of the list. Their only obligation is to return the new cons cell that represents the head of the list. One thing to be careful about when doing this is that the original list might be referenced from multiple places, and all of these places may need to be updated. For instance: (setq *a* (list 3 2 1)) (setq *b* *a*) (setq *a* (delete 3 *a*)) *a* => (2 1) *b* => (3 2 1) ; *B* doesn't "see" the change (setq *a* (delete 1 *a*)) *a* => (2) *b* => (3 2) ; *B* sees the change this time, though One may argue that destructive functions could do what you expect by rearranging the CARs of the list, shifting things up if the first element is being deleted, as they are likely to do if the argument is a vector rather than a list. In many cases they could do this, although it would clearly be slower. However, there is one case where this is not possible: when the argument or value is NIL, and the value or argument, respectively, is not. It's not possible to transform the object referenced from the original cell from one data type to another, so the result must be stored back. Here are some examples: (setq *a* (list 3 2 1)) (delete-if #'numberp *a) => NIL *a* => (3 2 1) (setq *a* nil *b* '(1 2 3)) (nconc *a* *b*) => (1 2 3) *a* => NIL The names of most destructure functions (except for sort, delete, rplaca, rplacd, and setf of accessor functions) have the prefix N. In summary, the two common problems to watch out for when using destructive functions are: 1. Forgetting to store the result back. Even though the list is modified in place, it is still necessary to store the result of the function back into the original location, e.g., (setq foo (delete 'x foo)) If the original list was stored in multiple places, you may need to store it back in all of them, e.g. (setq bar foo) ... (setq foo (delete 'x foo)) (setq bar foo) 2. Sharing structure that gets modified. If it is important to preserve the shared structure, then you should either use a nondestructive operation or copy the structure first using COPY-LIST or COPY-TREE. (setq bar (cdr foo)) ... (setq foo (sort foo #'<)) ;;; now it's not safe to use BAR Note that even nondestructive functions, such as REMOVE, and UNION, can return a result which shares structure with an argument. Nondestructive functions don't necessarily copy their arguments; they just don't modify them. ---------------------------------------------------------------- [3-4] After I NREVERSE a list, it's only one element long. After I SORT a list, it's missing things. What happened? These are particular cases of the previous question. Many NREVERSE and SORT implementations operate by rechaining all the CDR links in the list's backbone, rather than by replacing the CARs. In the case of NREVERSE, this means that the cons cell that was originally first in the list becomes the last one. As in the last question, the solution is to store the result back into the original location. ---------------------------------------------------------------- [3-5] Why does (READ-LINE) return "" immediately instead of waiting for me to type a line? Many Lisp implementations on line-buffered systems do not discard the newline that the user must type after the last right parenthesis in order for the line to be transmitted from the OS to Lisp. Lisp's READ function returns immediately after seeing the matching ")" in the stream. When READLINE is called, it sees the next character in the stream, which is a newline, so it returns an empty line. If you were to type "(read-line)This is a test" the result would be "This is a test". The simplest solution is to use (PROGN (CLEAR-INPUT) (READ-LINE)). This discards the buffered newline before reading the input. However, it would also discard any other buffered input, as in the "This is a test" example above; some implementation also flush the OS's input buffers, so typeahead might be thrown away. ---------------------------------------------------------------- [3-6] I typed a form to the read-eval-print loop, but nothing happened. Why? There's not much to go on here, but a common reason is that you haven't actually typed a complete form. You may have typed a doublequote, vertical bar, "#|" comment beginning, or left parenthesis that you never matched with another doublequote, vertical bar, "|#", or right parenthesis, respectively. Try typing a few right parentheses followed by Return. ---------------------------------------------------------------- [3-7] DEFMACRO doesn't seem to work. When I compile my file, LISP warns me that my macros are undefined functions, or complains "Attempt to call <function> which is defined as a macro. When you evaluate a DEFMACRO form or proclaim a function INLINE, it doesn't go back and update code that was compiled under the old definition. When redefining a macro, be sure to recompile any functions that use the macro. Also be sure that the macros used in a file are defined before any forms in the same file that use them. Certain forms, including LOAD, SET-MACRO-CHARACTER, and REQUIRE, are not normally evaluated at compile time. Common Lisp requires that macros defined in a file be used when compiling later forms in the file. If a Lisp doesn't follow the standard, it may be necessary to wrap an EVAL-WHEN form around the macro definition. Most often the "macro was previously called as a function" problem occurs when files were compiled/loaded in the wrong order. For example, developers may add the definition to one file, but use it in a file which is compiled/loaded before the definition. To work around this problem, one can either fix the modularization of the system, or manually recompile the files containing the forward references to macros. Also, if your macro calls functions at macroexpand time, those functions may need to be in an EVAL-WHEN. For example, (defun some-function (x) x) (defmacro some-macro (y) (let ((z (some-function y))) `(print ',z))) If the macros are defined in a file you require, make sure your require or load statement is in an appropriate EVAL-WHEN. Many people avoid all this nonsense by making sure to load all their files before compiling them, or use a system facility (or just a script file) that loads each file before compiling the next file in the system. ---------------------------------------------------------------- [3-8] Name conflict errors are driving me crazy! (EXPORT, packages) If a package tries to export a symbol that's already defined, it will report an error. You probably tried to use a function only to discover that you'd forgotten to load its file. The failed attempt at using the function caused its symbol to be interned. So now, when you try to load the file, you get a conflict. Unfortunately, understanding and correcting the code which caused the export problem doesn't make those nasty error messages go away. That symbol is still interned where it shouldn't be. Use unintern to remove the symbol from a package before reloading the file. Also, when giving arguments to REQUIRE or package functions, use strings or keywords, not symbols: (find-package "FOO"), (find-package :foo). A sometimes useful technique is to rename (or delete) a package that is "too messed up". Then you can reload the relevant files into a "clean" package. ---------------------------------------------------------------- [3-9] Closures don't seem to work properly when referring to the iteration variable in DOLIST, DOTIMES and DO. DOTIMES, DOLIST, and DO all use assignment instead of binding to update the value of the iteration variables. So something like (let ((l nil)) (dotimes (n 10) (push #'(lambda () n) l))) will produce 10 closures over the same value of the variable N. ---------------------------------------------------------------- [3-10] What is the difference between FUNCALL and APPLY? FUNCALL is useful when the programmer knows the length of the argument list, but the function to call is either computed or provided as a parameter. For instance, a simple implementation of MEMBER-IF (with none of the fancy options) could be written as: (defun member-if (predicate list) (do ((tail list (cdr tail))) ((null tail)) (when (funcall predicate (car tail)) (return-from member-if tail)))) The programmer is invoking a caller-supplied function with a known argument list. APPLY is needed when the argument list itself is supplied or computed. Its last argument must be a list, and the elements of this list become individual arguments to the function. This frequently occurs when a function takes keyword options that will be passed on to some other function, perhaps with application-specific defaults inserted. For instance: (defun open-for-output (pathname &rest open-options) (apply #'open pathname :direction :output open-options)) FUNCALL could actually have been defined using APPLY: (defun funcall (function &rest arguments) (apply function arguments)) ---------------------------------------------------------------- [3-11] Miscellaneous things to consider when debugging code. This question lists a variety of problems to watch out for when debugging code. This is sort of a catch-all question for problems too small to merit a question of their own. See also question [1-2] for some other common problems. Functions: * (flet ((f ...)) (eq #'f #'f)) can return false. * The function LIST-LENGTH is not a faster, list-specific version of the sequence function LENGTH. It is list-specific, but it's slower than LENGTH because it can handle circular lists. * Don't confuse the use of LISTP and CONSP. CONSP tests for the presence of a cons cell, but will return NIL when called on NIL. LISTP could be defined as (defun listp (x) (or (null x) (consp x))). * Use the right test for equality: EQ tests if the objects are identical -- numbers with the same value need not be EQ, nor are two similar lists necessarily EQ. Similarly for characters and strings. For instance, (let ((x 1)) (eq x x)) is not guaranteed to return T. EQL Like EQ, but is also true if the arguments are numbers of the same type with the same value or character objects representing the same character. (eql -0.0 0.0) is not guaranteed to return T. EQUAL Tests if the arguments are structurally isomorphic, using EQUAL to compare components that conses, arrays, strings or pathnames, and EQ for all other data objects (except for numbers and characters, which are compared using EQL). EQUALP Like EQUAL, but ignores type differences when comparing numbers and case differences when comparing characters. = Compares the values of two numbers even if they are of different types. CHAR= Case-sensitive comparison of characters. CHAR-EQUAL Case-insensitive comparison of characters. STRING= Compares two strings, checking if they are identical. It is case sensitive. STRING-EQUAL Like STRING=, but case-insensitive. * Some destructive functions that you think would modify CDRs might modify CARs instead. (E.g., NREVERSE.) * READ-FROM-STRING has some optional arguments before the keyword parameters. If you want to supply some keyword arguments, you have to give all of the optional ones too. * If you use the function READ-FROM-STRING, you should probably bind *READ-EVAL* to NIL. Otherwise an unscrupulous user could cause a lot of damage by entering #.(shell "cd; rm -R *") at a prompt. * Only functional objects can be funcalled in CLtL2, so a lambda expression '(lambda (..) ..) is no longer suitable. Use #'(lambda (..) ..) instead. If you must use '(lambda (..) ..), coerce it to type FUNCTION first using COERCE. Methods: * PRINT-OBJECT methods can make good code look buggy. If there is a problem with the PRINT-OBJECT methods for one of your classes, it could make it seem as though there were a problem with the object. It can be very annoying to go chasing through your code looking for the cause of the wrong value, when the culprit is just a bad PRINT-OBJECT method. Initialization: * Don't count on array elements being initialized to NIL, if you don't specify an :initial-element argument to MAKE-ARRAY. For example, (make-array 10) => #(0 0 0 0 0 0 0 0 0 0) Iteration vs closures: * DO and DO* update the iteration variables by assignment; DOLIST and DOTIMES are allowed to use assignment (rather than a new binding). (All CLtL1 says of DOLIST and DOTIMES is that the variable "is bound" which has been taken as _not_ implying that there will be separate bindings for each iteration.) Consequently, if you make closures over an iteration variable in separate iterations they may nonetheless be closures over the same variable and hence will all refer to the same value -- whatever value the variable was given last. For example, (let ((fns '())) (do ((x '(1 2) (cdr x))) ((null x)) (push #'(lambda () x) fns)) (mapcar #'funcall (reverse fns))) returns (nil nil), not (1 2), not even (2 2). Thus (let ((l nil)) (dolist (a '(1 2 3) l) (push #'(lambda () a) l))) returns a list of three closures closed over the same bindings, whereas (mapcar #'(lambda (a) #'(lambda () a)) '(1 2 3)) returns a list of closures over distinct bindings. Defining Variables and Constants: * (defvar var init) assigns to the variable only if it does not already have a value. So if you edit a DEFVAR in a file and reload the file only to find that the value has not changed, this is the reason. (Use DEFPARAMETER if you want the value to change upon reloading.) DEFVAR is used to declare a variable that is changed by the program; DEFPARAMETER is used to declare a variable that is normally constant, but which can be changed to change the functioning of a program.