DP Tool Club 8/CDASC08.ISO/VRAC/E17INFO.ZIP/CL

GNU Info File | 1993-06-18 | 30.8 KB | 870 lines

This is Info file ../info/cl, produced by Makeinfo-1.55 from the input
file cl.texinfo.

   Copyright (C) 1987 Cesar Quiroz


File: cl,  Node: Top,  Next: Generalities,  Up: (DIR)

Common Lisp Extensions
**********************

   The routines described in this chapter provide some of the
functionality of Common Lisp inside Emacs Lisp.

* Menu:

* gen: Generalities.           Things you need to know.
* sym: Symbols.                Gensym, gentemp, keyword-p, ...
* lis: Lists.                  List*, pairlis, acons, ...
* seq: Sequences.              Every, any, notevery, notany, ...
* con: Conditionals.           When, unless, case, ecase.
* ite: Iterations.             Do, do*, dolist, dotimes, ...
* mul: Multiple Values.        Values, values-list, ...
* ari: Integer Arithmetic.     cl-floor, cl-ceiling, cl-round, cl-truncate, ...
* stf: Generalized Variables.  Setf and friends.
* str: Structures.             Like Pascal records or C structs.
* mis: Miscellanea.            Odds and ends that didn't fit elsewhere.
* tod: To Do.                  Suggestions for future hackery.


File: cl,  Node: Generalities,  Next: Symbols,  Prev: Top,  Up: Top

Generalities
============

   This section tells you want you need to know about the routines in
the file `cl.el', so that you can use them.  The distribution also
includes this documentation and perhaps a few example files.

License, Availability, Maintenance
----------------------------------

   These files are covered by the GNU Emacs General Public License (if
you don't know its terms, try `C-h C-c') and the statement of no
warranty (again, you can type `C-h C-w' if you don't know its terms)
also applies to them.

   I, Cesar Quiroz, the original author of the software described here,
will appreciate hearing about bug reports (and fixes), suggestions and
comments, applying both to the code and to this documentation.  I don't
promise to act on those communications, but whenever they might conduce
to improvements of this software, I will make those improvements
available to the general community through the Free Software
Foundation.  You can reach me at the following net addresses:
     [e-mail] [e-mail] {allegra |
     seismo | ... } ! rochester ! quiroz CSNET: Cesar Quiroz at node
     Rochester

Purpose and Limitations
-----------------------

   These routines were written with two purposes in mind:

  1. To make programming in Emacs Lisp even more compatible with Common
     Lisp.  Indeed, my original motivation was to have a `do' macro.

  2. To facilitate to novice Lisp programmers a chance to practice with
     features commonly found only in expensive implementations of Lisp.

   In order to satisfy these purposes, the routines were written in
such a way that it is practical to use them inside Emacs: no effort was
given to implement features that could slow down the host Emacs
unnecessarily nor require recoding of the Emacs Lisp interpreter.

   For instance, no support is given to floating point arithmetic.

   So, I have tried to implement a subset of the functionality of
Common Lisp.  Whatever is implemented, has syntactic and semantic
properties like the equally named features of Common Lisp, but not all
the relevant features have been implemented (*note To Do::., for some
suggestions).  When deciding what to include, I have tried to strike a
balance between these constraints:

  1. Keep it simple, I didn't want to spend much time in this idea.

  2. Keep it compatible with Common Lisp.

  3. Keep it flexible, so my code doesn't oppose a better
     implementation (when this looked hard, I just didn't implement the
     feature).

  4. Keep an eye on the intended use of Emacs Lisp: to support an
     advanced editor.  I don't expect that more arithmetic support will
     be as conducive to this goal as having better iterations and
     conditionals will.

   For background, the reference is "Common Lisp: The Language" by Guy
Steele Jr. (Digital Press, 1984).  For all the features described here
you can assume that the intent has been to provide the syntax and
semantics of the features of like name in the book.  For the most part,
this documentation will concentrate on how my routines fail to
implement Common Lisp faithfully.

Specific Limitations
....................

   Emacs Lisp and Common Lisp differ enough to make some of the
emulation difficult, expensive or nearly impractical.  Some specific
limitations are stated here:

  1. Common Lisp is lexically scoped (mostly), while Emacs Lisp is
     dynamically scoped.  Things like `block', `return', `tagbody' are
     then practically impossible to imitate correctly (in principle,
     rewriting `eval', `apply' and a couple of other functions would
     suffice, problem is that such rewriting amounts to  a new
     interpreter on top of the old.)  Things like `implicit-blocks',
     `implicit-tagbodies' and the like have not been implemented at all.
     Where they are needed, the most you can assume is that I tried to
     put `implicit-progns' around places where it made sense.

  2. Emacs Lisp's `lambda' does not support all the possible argument
     markers.  Similarly, `defmacro' doesn't support automatic
     destructuring of the calls.  An approximation to a keyword-based
     calling style was implemented, mainly for the sake of `defstruct',
     but is not general enough.

  3. Emacs Lisp supports arithmetic only on integers.

  4. Emacs Lisp doesn't support many of the basic types of Common Lisp.
     In particular, there are no arrays beyond vectors and strings
     (although these ones are compatible), characters are essentially
     small integers, etc.

  5. There are no declarations in Emacs Lisp (in the sense of Common
     Lisp's `declare', `proclaim', ...) nor there is a explicit lattice
     of types.  These limitations could be effectively overcome from
     Lisp code (to a extent), but I don't see them as a very pressing
     need, if a need at all in Emacs Lisp.  `defstruct' can be used to
     generate new types that can be recognized at runtime.

  6. The Emacs Lisp reader is not programmable.  The syntax it accepts
     is almost standard, but it preempts '?' as a dispatching macro of
     sorts.  The `format' function is incompatible with Common Lisp.
     There isn't a `quasi-constant' notation (the usual `backquote' of
     Common Lisp).  None of these differences causes any problems when
     writing Emacs Lisp (although the lack of backquoting is felt
     sorely), but they oppose a Common Lisp emulation.


Loading and Compiling
---------------------

   The file `cl.el' provides the `cl' feature, you can use this to test
whether these routines have been loaded, or to load them from your code
(by means of `(require 'cl)').  The compiled file is a little larger
than 50K bytes.

   If you need to recompile the sources, make sure you load them first
on the Emacs that will do the recompilation.  This is because many
syntactic characteristics (like the special forms) have been
implemented as macros and you need to make sure that macros are known
to the compiler before they are used.

   These extensions work correctly when interpreted in a GNU Emacs of
version 17.64 or beyond.  Compiling them requires a more recent byte
compiler, preferably one strictly younger than version 18.XX.

On Line Help
------------

   The routines in this file have documentation strings, so you can (and
should) consult them for the exact syntax allowed.  That information is
not repeated in this manual.  Some of the routines are also documented
explicitly in the Common Lisp reference, their doc-strings begin with
`[cl]' to represent this fact.

   The rest (those without the `[cl]' mark) are auxiliary functions or
macros used by the rest of the implementation.  They are not constrained
by any standard and are advertised only in as much as they can be useful
in other applications.

   Each of the following sections contains a subsection called `Features
Provided'.  It lists briefly the user-visible features of this
implementation.  In its entries, names by themselves refer to functions.
Macros and variables are identified by a `MACRO' or a `VARIABLE' ahead
of their names.


File: cl,  Node: Symbols,  Next: Lists,  Prev: Generalities,  Up: Top

Symbols
=======

   The most important omission is that of a PACKAGES mechanism.  (For a
possible implementation, *note To Do::.)  Whenever a Common Lisp
function expects a package, I have substituted an obarray.  There is a
hack to have pseudo-keywords, see below.

   There are two other notorious omissions over which I haven't lost any
sleep.  The first is the lack of a `remprop' function, which could be
easily provided if needed.  The second is the lack of ways to modify the
print name of a symbol.  This one would probably be good only to
introduce strange bugs, so I don't miss it.

Features Provided
-----------------

`VARIABLE *gensym-index*'
`VARIABLE *gensym-prefix*'
`VARIABLE *gentemp-index*'
`VARIABLE *gentemp-prefix*'
     These variables are used to keep the state of the generator of new
     names.  Better leave them alone.

`gensym'
`gentemp'
     These do the same as the Common Lisp names of like names.

`MACRO defkeyword'
`keyword-of'
`keywordp'
     These provide the pseudo-keywords implementation.

Keywords
--------

   The lack of packages makes it difficult to implement keywords
correctly.  I have provided a macro `defkeyword' that takes a symbol
and makes sure it evaluates to itself.  (So, it is like `defconst'.)
If your programs ever need keywords, put a bunch of calls to
`defkeyword' at the beginning of your code, so when loaded they will be
in effect.

   The (standard) predicate `keywordp' tests to see if the given
symbol's name begins with a colon and then ensures that it evaluates to
itself.

   The function `keyword-of' takes a symbol and returns a keyword of
like name.

      (keyword-of 'foo)
     :foo
      (keyword-of ':bar)
     :bar

   This feature was added mainly to support `defstruct' and the tests of
the sequence functions.  It is fragile and easy to fool.

New Symbols
-----------

   A common need (especially when writing macros) is to be able to
invent new names for things.  I provide the `gensym' and `gentemp'
functions.  The global state needed is kept in the variables
`*gentemp-index*', `*gentemp-prefix*', `*gensym-index*' and
`*gensym-prefix*'.  Changing them, especially the index ones, is a very
bad idea.  I am not providing the Common Lisp default prefixes ('G' for
`gensym' and 'T' for `gentemp') because of debugging paranoia.  My
default prefixes are harder to come by when giving sane names to
things.


File: cl,  Node: Lists,  Next: Sequences,  Prev: Symbols,  Up: Top

Lists
=====

   Lists (indeed, conses) are so deeply involved in Lisp that there
seems to be little need to justify improving the list handling of a
Lisp.

   Common Lisp, however, is a rather huge Lisp.  I haven't provided all
the functions in the chapter of lists, mainly because some of them
could be implemented correctly only if keyword arguments were supported.
That explains why I haven't rushed to provide `subst', `sublis', etc.
Also, that explains the rather temporary nature of the implementation
of `member' and `adjoin'.  I will welcome any efforts to extend this
work.

Features Provided
-----------------

`endp'
`list*'
`list-length'
     Very standard utilities.  List* has proven especially useful to
     overcome the lack of a real `backquote'.  In addition, things that
     usually required the relatively clumsy
          (cons 'a (cons 'b oldlist))
          (append (list a b) oldlist)
     can now be simply put:
          (list* 'a 'b oldlist)
     See also `acons'.

`member'
     Another well known function.  Supports test with `eql' only.

`acons'
`pairlis'
     These two are part of the standards association lists
     implementation.  I am leaving `sublis' as an exercise for the
     reader.

`adjoin'
     Done mainly for the sake of `pushnew'.

`butlast'
`last'
`ldiff'
     Occasionally useful ways to access the last cons or a specified
     tail of a list.  I don't remember why there isn't a `tailp' here.

`c[ad][ad][ad][ad]r, up to four a's or d's'
     These 28 functions (and their setf inverses) have been provided
     once and for all.  Many packages contributed to Emacs Lisp contain
     macros that implement some of these, I think this code will make
     most of them unnecessary.

`first'
`rest'
`second'
`third'
`fourth'
`fifth'
`sixth'
`seventh'
`eighth'
`ninth'
`tenth'
     More standard accessors (and their setf inverses).  Not
     particularly useful but easy to provide.

`setnth'
`setnthcdr'
     These functions are non-standard.  They are here for `defsetf'
     purposes only, but they might be useful on their own.


File: cl,  Node: Sequences,  Next: Conditionals,  Prev: Lists,  Up: Top

Sequences
=========

   Sequences are partly supported in Emacs Lisp (see, for instance, the
`elt' function).  This limited support is compatible with Common Lisp,
so it should be easy to extend.  However, the lack of keyword arguments
makes many of the functions impossible so far (but, as mentioned below,
a basic framework for that extension is provided here).

   The functionality really provided here is given by the functions
(essentially, predicates) `every', `some', `notevery', `notany'.  I
have found them useful countless times, so I thought to provide them
before anything else.

   That still leaves many omissions, though.

Features Provided
-----------------

`every'
`notany'
`notevery'
`some'
     Extremely useful functions.  If your favorite Lisp doesn't have
     them, you are missing a lot.

`setelt'
     A setf-inverse to `elt'.

`add-to-klist'
`build-klist'
`extract-from-klist'
     A "klist" is just an alist whose keys are keywords.  I based the
     pseudo-keyword argument support of `defstruct' on this idea, but
     their best fit is here, as they could help to write the remaining
     sequence-handling functions (`find', `substitute', ...) that I
     didn't provide for the lack of a good keyword arguments mechanism.

`elt-satisfies-if-not-p'
`elt-satisfies-if-p'
`elt-satisfies-test-p'
`elts-match-under-klist-p'
     The Common Lisp book defines some of the semantics of sequence
     functions in terms of satisfaction of certain tests.  These
     predicates provide that functionality, but I haven't integrated
     them with the rest of the extensions.  However, I thought it was
     better to include them anyway, as they can serve somebody else as
     a starting point.


File: cl,  Node: Conditionals,  Next: Iterations,  Prev: Sequences,  Up: Top

Conditionals
============

   An elementary incompatibility prevents us from producing true Common
Lisp here.  The `if' forms are different.  In Emacs Lisp, `if' can take
any number of subforms, there being a CONDITION form, a THEN form, and
after them any number of ELSE subforms, which are executed in an
implicit `progn'.  Moreover, that style is widely used in the Emacs
sources, so I thought most impractical to break with it to support
Common Lisp's `if' (where only one ELSE form is tolerated).  For the
most part, I use `cond' almost always, so it doesn't bother me much.
If you use single-branch `if's often, consider `when' or `unless' as
alternatives.

   `case' and `ecase' are a convenient way to write things that usually
end up in a very baroque `cond'.

Features Provided
-----------------

`MACRO case'
`MACRO ecase'
`MACRO unless'
`MACRO when'
     The standard stuff, completely implemented.


File: cl,  Node: Iterations,  Next: Multiple Values,  Prev: Conditionals,  Up: Top

Iterations
==========

   Having a `do' macro was my original motivation.  The alternatives in
standard Emacs Lisp are either expensive (recursion) or correspond
directly to the expansion of my macros:
      (macroexpand '
       (do ((i 0) (j 1 (+ 1 j)))
           ((> j (foo i)) (cons i bar))
         (setq i (baz i j))))

     (let ((i 0) (j 1))
       (while (not (> j (foo i)))
         (setq i (baz i j))
         (psetq j (+ 1 j)))
       (cons i bar))
   So I prefer to leave to the macros the problem of remembering the
details right.

   The incompatibilities are due to the problems already discussed
(*note Generalities::., for more details).

   If you write Emacs Lisp code often, you will find enough uses for
these.  Examples are cooking up a translation table to move C-s out of
the way of multiplexers, switches, concentrators and similar fauna, or
building keymaps.

Features Provided
-----------------

`MACRO do'
`MACRO do*'
`MACRO dolist'
`MACRO dotimes'
     The standard, but for the lack of implicit blocks.

`MACRO loop'
     The basic standard one, not the fancy one.  As per the book, warns
     you about atomic entries at the surface of the macro (to guarantee
     that the fancy `loop' macros can be made standard later).

`MACRO do-all-symbols'
`MACRO do-symbols'
     These operate on obarrays, the default is the current one.


File: cl,  Node: Multiple Values,  Next: Integer Arithmetic,  Prev: Iterations,  Up: Top

Multiple Values
===============

   The multiple values mechanism covers (simply and elegantly, in my
opinion) various common needs:

  1. The case where a function returns a composite value, that has to be
     assembled in the callee and disassembled in the caller.  An
     example is `pair-with-newsyms'.

  2. The case where a function might cheaply compute redundant
     information that is useful to the caller only eventually.  For
     instance, routines that compute quotients and remainders together,
     whose callers might be more often interested in just receiving the
     quotient.

  3. The case of functions that usually return a useful value, but
     might need to elaborate on occasion (say, returning a reason code
     too).

   The general idea is that one such function always returns the extra
values, but only callers that are aware of this ability receive them.
Unaware callers just receive the first value.

   I think my implementation is pretty much complete.  I am imposing no
limits on the number of multiple values a function may return, so I am
not providing the constant `multiple-values-limit'.  You can assume
multiple values are bound by the memory size only.

Features Provided
-----------------

`values'
`values-list'
     These are the forms that produce multiple values.

`MACRO multiple-value-bind'
`MACRO multiple-value-call'
`MACRO multiple-value-list'
`MACRO multiple-value-prog1'
`MACRO multiple-value-setq'
     These are the forms that receive multiple values.

`VARIABLE *mvalues-count*'
`VARIABLE *mvalues-values*'
     Used by the implementation.  Don't touch them!


File: cl,  Node: Integer Arithmetic,  Next: Generalized Variables,  Prev: Multiple Values,  Up: Top

Integer Arithmetic
==================

   I have provided most of the functions that are supposed to act on
integers.  Of those that take arbitrary numbers, I have implemented
those that have a reasonable implementation if restricted to integers
only, although some more could be added (like a restricted form of
`expt').

   Being a little worried about the bad fame that affects some
implementations of the '%' C operator, I have taken perhaps unnecessary
precautions whenever integer division is concerned (see the function
`safe-idiv').  This should be of interest only when dividing numbers
that might be negative, but I have preferred here to be safe rather
than fast.

Features Provided
-----------------

`abs'
`signum'
     The usual.

`gcd'
`lcm'
     The usual.

`isqrt'
     A rather annoying function.  Only use I can think of: to cut short
     a prime number sieve.

`evenp'
`oddp'
`plusp'
`minusp'
     A few predicates that use to come handy.

`cl-ceiling'
`cl-floor'
`cl-round'
`cl-truncate'
`mod'
`rem'
     The intention is to give everybody his preferred way to divide
     integers.  I have tried not to depend on the unreliable semantics
     of C's integer division, I hope I got it right.  Read the code
     when in doubt.


File: cl,  Node: Generalized Variables,  Next: Structures,  Prev: Integer Arithmetic,  Up: Top

Generalized Variables
=====================

   This implementation has many limitations.  Take a look to see if you
want to overcome them, the fixes might prove unnecessarily expensive for
Emacs purposes.  The ones I am clearly aware of:

  1. Common Lisp suggests an underlying mechanism (the setf-methods) to
     implement generalized variables.  I have used ad-hoc ideas that
     gave me a rather trivial implementation that suffers from some
     inflexibility.  As a result, `defsetf' only admits the simplest
     form and there is no `define-modify-macro' nor there are functions
     to handle the (nonexistent) setf-methods.

  2. I haven't implemented (I was uninterested) `getf' and friends.
     This shouldn't be hard.

   In addition to providing this mechanism, I have written `defsetf's
for almost every accessor I thought of.  There is room for improvement
here, as Emacs Lisp provides many types of its own (buffers, windows,
keymaps, syntax tables, ...) for which pairs of accessors and mutators
could be defined.

   If you want to check whether a function has a setf-inversor, look at
the property `:setf-update-fn' of its name.  This is a characteristic
of my implementation, not mandated by Common Lisp, so you shouldn't use
it in code, but only to determine interactively what can be setf'd.

Features Provided
-----------------

`MACRO setf'
`MACRO psetf'
     Almost complete implementation.  `Setf' should handle `apply'
     inside itself and not in a `defsetf', but the difference is so
     minute I feel lazy about fixing this. `Psetf' is the version where
     the assignments occur in parallel.

`MACRO defsetf'
     Very sketchy implementation.  I will appreciate if somebody puts
     some time in implementing the whole works of setf-methods and such.

`MACRO incf'
`MACRO decf'
     The usual and standard.

`MACRO pop'
`MACRO push'
`MACRO pushnew'
     Should be the usual, but I haven't had the time to test them
     properly.

`MACRO rotatef'
`MACRO shiftf'
     Very fancy.  Good for implementing history rings and such.  To
     swap two values, the following forms are equivalent:
          (rotatef a b)
          (psetf a b b a)
          (psetq a b b a)  ;not good for anything but variables


File: cl,  Node: Structures,  Next: Miscellanea,  Prev: Generalized Variables,  Up: Top

Structures
==========

   I haven't had the time to construct a complete implementation of
structures, but the part provided should stand on its own for many
purposes.  I am not supporting `BOA constructors', nor typed slots (the
`:type', `:named' and `:initial-offset' options), nor explicit
representational types.  The rest should be pretty much complete.  See
the example file `fractions.el' for an idea of how complete the
implementation is, and for exercises.

   When writing these functions, I noticed I was incurring in lots of
auxiliaries.  I used dollar signs in their names, in the hope that this
could prevent clashes with user functions.  In retrospect, I should have
done it in the other sections, too.

Features Provided
-----------------

`MACRO defstruct'
     Create records (a la C structs) and use them as types in your
     programs.  Almost completely standard.

`make$structure$instance'
     This non-standard function implements most of the `guts' of the
     `make-' constructors.  It can be used as an example of the pseudo
     keyword-arguments.


File: cl,  Node: Miscellanea,  Next: To Do,  Prev: Structures,  Up: Top

Miscellanea
===========

Features Provided
-----------------

`MACRO psetq'
     A parallel-assignments version of `setq', makes the expansions of
     `do' and `do*' be very similar, as they should.  Otherwise used to
     swap two values, now superseded by `rotatef'.

`duplicate-symbols-p'
`pair-with-newsyms'
`reassemble-argslists'
`unzip-list'
`zip-lists'
     These are utilities I find useful when parsing a call or
     generating code inside a macro.  Non standard.


File: cl,  Node: To Do,  Prev: Miscellanea,  Up: Top

To Do
=====

   No doubt many people will like to extend the functionality of these
routines.  When considering doing so, please try and do it in such a way
that your implementation of a subset of the functionality of Common Lisp
is not inimical with a more extensive or more correct one.  For
definiteness, ask yourself the questions:

   * Will my code run under a correct implementation of Common Lisp?

   * Will a correct implementation of Common Lisp run if my code is
     loaded?

The first question tests the pertinence of your extensions.  The second
tries to discover "extensions" that prevent correct implementations of
other features.  Please tell me if you notice a case in which my code
fails to pass any of those tests.

   The next subsections propose some more extensions.  I hope that they
are attempted by people learning Lisp, as a way to enhance their
understanding.  Of course, experts are also admitted.

Keyword Arguments
-----------------

   Some effort has been done to handle keywords almost right.  For
instance, a structure constructor (*note Structures::.) can be invoked
with keyword arguments.

   Look for the functions whose names have a `klist' in them.  They
were written to facilitate parsing calls with keyword arguments, but I
haven't done a complete implementation yet.  (Note that `member',
`assoc' and perhaps some other function, have to be implemented
independently of the general framework.  More details by Email if you
want to try your hand at this.)

Mapping Functions
-----------------

   There is enough support to write `maplist', `mapl', etc.  Emacs Lisp
already provides some of the mapping functions, the trick now is to
code the rest in a very efficient manner, so there will be an incentive
to use `maplist' over an explicit iteration.  I have a draft
implementation, but I don't have the time to test it now.

Complete the current implementation
-----------------------------------

   Some of the features described above are only a partial
implementation of the Common Lisp features.  Things that cry for a more
complete form:

`defsetf'
     Only the simplest format is supported.  The most general one is
     needed too.  Also, try to get `define-setf-method' and
     `get-setf-method' to work.

`define-modify-macro'
     Same as above.  The modify-macros provided are all ad hoc.

`defstruct'
     I think my version recognizes all the options and then proceeds to
     ignore most of them.  Making sure that at least good error
     messages are produced would be nice.  Also, what about BOA
     constructors?

   There are other places where your programming ingenuity would help us
all.  For instance, `subst', `sublis' and the like could be easily
provided in the LISTS section.  (I haven't done it because I wanted to
have the keyword arguments stuff first.)

Hash Tables
-----------

   A very simple implementation of hash tables would admit only strings
as keys.  For each string and a given number of buckets (a prime is
desirable here), add the numeric values of all (or of a reasonable
subset) of the characters and assign the bucket whose index is the
remainder of the sum modulo the (prime) number of buckets.

   A more convenient implementation can then be based on using
`prin1-to-string' on an arbitrary Lisp object and using the output
string as a key. This should make it easy to write `sxhash'.  Remember
that one needs to ensure that `(equal x y)' should imply that
`(= (sxhash x) (sxhash y))'; and also that the keys are state-less (so
you can write them to a file and read them back later).

   Don't forget to provide a `defsetf' for `gethash'.

Packages
--------

   Packages should be easy to implement on top of a good hash table
implementation, either by using it directly or by reusing some shared
code.  Don't worry too much about efficiency: package qualification has
no run-time cost, only read- and print-time costs.

   The difficult thing is to integrate it correctly.  You have to
replace the built-in functions `read' and `write'.  This is not as bad
as writing a programmable reader, but still a pain.  For starters, your
routines could remember the default definitions of the above mentioned
functions:

     (setf def-reader  (symbol-function 'read))
     (setf def-printer (symbol-function 'print))
     ...

   And then your specialized functions could just use `apply' to
exercise the default ones, intercepting their activity in time to do the
package qualification.  You might have to do this to `prin1',
`prin1-to-string' and friends.

Streams and Files
-----------------

   This is the first "To Do" that might require doing some C
programming.  The purpose is to construct an efficient byte stream
abstraction that will allow Streams and Files to be handled.  Think of
stdio, not Unix I/O, because Emacs already runs under other operating
systems.  Also, think of doing it in a way that can be generalized
easily (for instance, streams kept in memory without a file behind,
streams as an interface to a windowing system, etc.)  Of course, the
intended syntax is that of Common Lisp.

Reader and Printer
------------------

   The Emacs Lisp reader (the C function `Fread') is not reentrant nor
programmable.  It could be fixed as Lisp Code, but that is probably
uglily expensive (as bad as redoing `eval' and `apply' to support
lexical scoping).  Doing this  extension is probably a bad idea: a
Common Lisp reader is incompatible with Emacs Lisp code (because of the
`?\' constructions) and the most important rule to keep in mind is that
this code is running under Emacs, so the host shouldn't be burdened too
much with these emulations.  Same goes for a more complete printer (a
Common Lisp `format' would be incompatible with the Emacs Lisp one).

Backquote
---------

   Even if the reader is not made programmable nor reentrant, a
backquoting mechanism could come handy.  You need to study the way the
current reader does `quote' and hack from there.  This might be a more
worthwhile extension than the complete rewrite of the reader.

Wild Ideas
----------

   Perhaps there is a way to implement `block', `tagbody', `return' and
friends in spite of the dynamic scoping of Emacs Lisp.  By this, I mean
an adequate definition that preserves remotely the original intent and
still provides a sensible set of constructs.  Other dynamically scoped
Lisps have these features, so implementing them is not necessarily
impossible.

   In the same spirit of these extensions would be to provide a port of
something like Flavors (was there a PD version from Maryland, for Franz
perhaps?) and then rephrase the language of major and minor modes in an
Object Oriented paradigm.

   Also, the rather gross `loop' macros that are out there in many Lisp
systems could be helpful to some people (but then think of a
`lisp-indent-hook' that handles them properly).



Tag Table:
Node: Top125
Node: Generalities1096
Node: Symbols8275
Node: Lists10739
Node: Sequences12894
Node: Conditionals14689
Node: Iterations15681
Node: Multiple Values17136
Node: Integer Arithmetic18851
Node: Generalized Variables20204
Node: Structures22545
Node: Miscellanea23713
Node: To Do24264

End Tag Table