[Please redistribute widely]
Many software packages need an extension language to make it easier
for users to add to and modify the package.
In a previous message I explained why Tcl is inadequate as an
extension language, and stated some design goals for the extension
language for the GNU system, but I did not choose a specific
At the time, I had not come to a conclusion about what to do. I knew
what sort of place I wanted to go, but not precisely where or how to
Since then, I've learned a lot more about the topic. I've read about
scsh, Rush and Python, and talked with people working on using Scheme
as an extension and scripting language. Now I have formulated a
specific plan for providing extensibility in the GNU system.
Who chooses which language?
Ousterhour, the author of Tcl, responded to my previous message by
citing a "Law" that users choose the language they prefer, and
suggested that we each implement our favorite languages, then sit back
and watch as the users make them succeed or fail.
Unfortunately, extension languages are the one case where users
*cannot* choose the language they use. They have to use the language
supported by the application or tool they want to extend. For
example, if you wanted to extend PDP-10 Emacs, you had to use TECO.
If you want to extend GNU Emacs, you have to use Lisp.
When users simply want "to write a program to do X or Y," they can use
any language the system supports. There's no reason for system
designers to try to decide which language is best. We can instead
provide as many languages as possible, to give each user the widest
possible choice. In the GNU system, I would like to support every
language people want to use--provided someone will implement them.
With the methods generally used today, we cannot easily provide many
languages for extending any particular utility or application package.
Supporting an extension language means a lot of work for the developer
of the package. Supporting two languages is twice as much work,
supposing the two fit together at all. In practice, the developer has
to choose a language--and then all users of the package are stuck with
that one. For example, when I wrote GNU Emacs, I had to decide which
language to support. I had no way to let the users decide.
When a developer chooses Tcl, that has two consequences for the
users of the package:
* They can use Tcl if they wish. That's fine with me.
* They can't use any other language. That I consider a problem.
Sometimes developers choose a language because they like it. But not
always. Sun recently announced a campaign to "make Tcl the universal
scripting language." This is a campaign to convince all the
developers who *don't* prefer Tcl that they really have no choice.
The idea is that each one of us will believe that Sun will inevitably
convince everyone else to use Tcl, and each of us will feel compelled
to follow where we believe the rest are going.
That campaign is what led me to decide that I needed to speak to the
community about the issue. By announcing on the net that GNU software
packages won't use Tcl, I hope to show programmers that not everyone
is going to jump on the Tcl bandwagon--so they don't have to feel
compelled to do so. If developers choose to support Tcl, it should be
because they want to, not because Sun convinces them they have no
Design goals for GNU
When you write a program, or when you modify a GNU program, I think
you should be the one who decides what to implement. I can't tell you
what language to support, and I wouldn't want to try.
But I am the leader of one particular project, the GNU project. So I
make the decision about which packages to include in the GNU operating
system, and which design goals to aim for in developing the GNU
These are the design goals I've decided on concerning extension
languages in the GNU system:
* As far as possible, all GNU packages should support the same
extension languages, so that a user can learn one language (any one of
those we support) and use it in any package--including Emacs.
* The languages we support should not be limited to special, weak
"scripting languages". They should be designed to be good for writing
large programs as well as small ones.
My judgement is that Tcl can't satisfy this goal. (Ousterhout seems
to agree that Tcl doesn't serve this goal. He thinks that doesn't
constitute a problem--I think it does.) That's why I've decided not
to use Tcl as the main system-wide extension language of the GNU
* It is important to support a Lisp-like language, because they
provide certain special kinds of power, such as representing programs
as data in a structured way that can be decoded without parsing.
** It is desirable to support Scheme, because it is simple and clean.
** It is desirable to support Emacs Lisp, for compatibility with Emacs
and the code already written for Emacs.
* It is important to support a more usual programming language syntax
for users who find Lisp syntax too strange.
* It would be good to support Tcl as well, if that is easy to do.
The GNU extension language plan
Here is the plan for achieving the design goals stated above.
* Step 1. The base language should be modified Scheme, with these features:
** Case-sensitive symbol names.
** No distinction between #f and (), for the sake of supporting Lisp
as well as Scheme.
** Convenient fast exception handling, and catch and throw.
** Extra slots in a symbol, to better support
translating other Lisp dialects into Scheme.
** Multiple obarrays.
** Flexible string manipulation functions.
** Access to all or most of the Unix system calls.
** Convenient facilities for forking pipelines,
making redirections, and so on.
** Two interfaces for call-outs to C code.
One allows the C code to work on arbitrary Scheme data.
The other passes strings only, and is compatible with Tcl
C callouts provided the C function does not try to call
the Tcl interpreter.
** Cheap built-in dynamic variables (as well as Scheme's lexical variables).
** Support for forwarding a dynamic variable's value
into a C variable.
** A way for applications to define additional Scheme data types
for application-specific purposes.
** A place in a function to record an interactive argument reading spec.
** An optional reader feature to convert nil to #f and t to #t,
for the sake of supporting Lisp as well as Scheme.
** An interface to the library version of expect.
** Backtrace and debugging facilities.
All of these things are either straightforward or have already been
done in Scheme systems; the task is to put them together. We are
going to start with SCM, add some of these features to it, and write
the rest in Scheme, using existing implementations where possible.
* Step 2. Other languages should be implemented on top of Scheme.
** Rush is a cleaned-up version of the Tcl language, which runs far
faster than Tcl itself, by means of translation into Scheme. Some
kludgy but necessary Tcl constructs don't work in Rush, and Tcl
aficionadoes may be unhappy about this; but Rush provides cleaner ways
to get the same results, so users who write extensions should like it
better. Developers looking for an extension language are likely to
prefer Rush to Tcl if they are not already attached to Tcl.
Here are a couple of examples supplied by Adam Sah:
*** To pass an array argument without copying it, in Tcl you must use
upvar or make the array a global variable. In Rush, you can simply
declare the argument "pass by reference".
*** To extract values from a list and pass them as separate arguments
to a function, in Tcl you must construct a function call expression
using that list, and then evaluate it. This can cause trouble if the
other arguments contain text that includes any special Tcl syntax. In
Rush, the apply function handles this simply and reliably.
*** Rush eliminates the need for the "expr" command by allowing infix
mathematical expressions and statements. For example, the Tcl
computation `"set a [expr $b*$c]' can be written as `a = b*c' in
Rush. (The Tcl syntax works also.)
[SBD94] Adam Sah, Jon Blow and Brian Dennis. "An Introduction to the Rush
Language." Proc. Tcl'94 Workshop. June, 1994.
[SB94] Adam Sah and Jon Blow. "A New Architecture for the Implementation of
Scripting Languages." Proc. USENIX Symp. on Very High Level Languages.
October, 1994. to appear.
** It appears that Emacs Lisp can be implemented efficiently by
translation into modified Scheme (the modifications are important).
** Python appears suitable for such an implementation, as far as I can
tell from a quick look. By "suitable" I mean that mostly the same
language could be implemented--minor changes in semantics would be ok.
(It would be useful for someone to check this carefully.)
** A C-like language syntax can certainly be implemented this way.
* Distribution conditions.
We will permit use of the modified Scheme interpreter in proprietary
programs, so as to compete effectively with alternative extensibility
Translators from other languages to modified Scheme will not be part
of any application; each individual user will decide when to use one
of these. Therefore, there is no special reason not to use the GPL as
the distribution terms for translators. So we will encourage
developers of translators to use the GPL as distribution terms.
Until today, users have not been able to choose which extension
language to use. They have always been compelled to use whichever
language is supposed by the tool they wish to extend. And that has
meant many different languages for different tools.
Adopting Tcl as the universal scripting language offers the
possibility of eliminating the incompatibility--users would be able to
extend everything with just one language. But they wouldn't be able
to choose which language. They would be compelled to use Tcl and
By making modified Scheme the universal extension language, we can
give users a choice of which language to write extensions in. We can
implement other languages, including modified Tcl (Rush), a Python
variant, and a C-like language, through translation into Scheme, so
that each user can choose the language to use. Even users who choose
modified Tcl will benefit from this decision--they will be happy with
the speedup they get from an implementation that translates into
Only Scheme, or something close to Scheme, can serve this purpose.
Tcl won't do the job. You can't implement Scheme or Python or Emacs
Lisp with reasonable performance on top of Tcl. But modified Scheme
can support them all, and many others.
The universal extension language should be modified Scheme.
Request for Volunteers
If you understand Scheme implementation well, and you want to
contribute a substantial amount of time to this project, please send
mail to Tom Lord, email@example.com.
If you expect to have time later but don't have time now, please send
mail when you do have time to work. Participation in a small way is
probably not useful until after the package is released.