Re: A Modest Proposal

[Noah Lavine]

These are all good points. Integrating different languages is hard.

Quite frankly, I am not yet sure that sharing a VM would be a good
idea for us, but as we talk more about it it is seeming more and more
reasonable. I sent my original message because I saw that CLisp was
becoming an embeddable lisp interpreter with an architecture similar
to Guile's, and it seemed to me that the two projects were so similar
that they might be able to collaborate productively.

I think the idea of moving to a common VM might work, because Scheme
and Common Lisp are so much closer than even Scheme and Python, or two
other similar languages. I think almost all of the semantics would map
nicely from one language to the other (although I haven't thought
about it very hard yet). It would be even easier with Guile than with
another Scheme, because Guile has already worked on integrating ELisp,
which shares some features with CL (i.e. uses nil as #f and '()).
However, I haven't thought hard about integrating Guile and CL yet.

On the surface, though, I think it could have advantages not
necessarily for us, but for other GNU projects and users, which I
outlined in a previous email. What do other people think of the idea?

Noah

On Wed, Apr 13, 2011 at 10:06 PM, William ML Leslie
<address@hidden> wrote:
> On 14 April 2011 01:23, Pascal J. Bourguignon <address@hidden> wrote:
>> In general, there may be a need for a very good lisp virtual machine to
>> run and integrate lisp code in general (CL, various schemes, and other
>> sorts of lisp-like languages, we could include perhaps implementations
>> of python,  ruby,  smalltalk,  javascript, etc).  From well afar, it
>> looks like the JVM is not good enough for lisp (ABCL, Clojure, seem to
>> have some difficulties to implement some basic lisp features on the
>> JVM).
>
> The problem of language integration has become common enough that we
> already have the opportunity to make some observations about what did
> work, and what didn't.  A concerted approach to working with distinct
> object spaces / models would be a mammoth task, it is entirely a
> different undertaking to integrating two particular languages.
>
> The particular issue I find is that the mechanisms for defining how
> two object spaces interact don't compose well.  If you work in a Java
> environment, for example, you may know how to interact with or create
> Java objects from JRuby, Kawa, Jython, E and Clojure, but that doesn't
> let you interact between any of these languages in a sane way.  Quick,
> what is the process for reifying my Jython or Kawa function as a
> callable in JRuby?
>
> This common-denominator approach applies equally well outside these
> safe bytecode VM environments - there are all sorts of mechanisms for
> generating FFI code for specific VMs given a C or C++ interface.  For
> the most part, they make you feel like you are writing C or C++.
>
> Another thing worth thinking about involves promoting idiomatic usage
> despite differences in languages.  If you've ever implemented a
> language inside a VM that was not designed for it, you will know what
> I mean when I ask questions like:
>
> 0. Objects in javascript are maps from string keys to any value.  If
> they are to be cast as hashes or dictionaries when passed to a
> language that allows non-string keys, and the language adds a
> non-string key, what happens to the javascript object?
>
> 1. How are "other" languages to make or handle multi-valued returns?
>
> 2. What happens to cons cells at the boundary between languages where
> one has immutable cons cells, the other mutable?
>
> 3. Further, what happens when a language with linked lists as the
> default sequence type and one with arrays as the default sequence type
> interact?  Consider, for example, the various options that pymacs
> provides.
>
> 4. What about operators, and the various mechanisms and naming
> conventions, single/multiple dispatch, AND the guarantees provided by
> each operator mechanism in any given language?
>
> 5. What do you want to do about subclassing and multiple inheritance?
> In a single-inheritance language like Java, the compiler is free to
> assume that in
>
>  if (x instanceof Foo) { }
>  if (x instanceof Bar) { }
>
> that if Bar and Foo are distinct concrete classes, and one is not a
> subclass of the other, the second if can be replaced with an else if.
> Method resolution order is not consistent between MI languages,
> either, and may be further complicated by how they are to interact
> with generic functions.
>
> 6. Languages may also make type-related requirements that need to be
> checked somehow; such as some languages not permitting the initialiser
> to be called more than once, final methods, &c.
>
> 7. In what way to generic functions and methods interact?  How about
> classes and typeclasses?
>
> 8. What happens when a language that is strict about arity interacts
> with a language that does not?  Does calling a javascript function
> from python with the wrong number of arguments raise a TypeError, or
> silently pass undefined?  Does the inverse pass None?
>
> 9. Do the error handling and exception mechanisms intersect - say,
> what Java exception does my Elisp args-out-of-range become?  Exception
> hierarchies can be *very* different across object spaces.
>
> 10. How do methods overloaded on static type look from a dynamic
> language? Oh, and what about virtual/non-virtual methods and extension
> methods as on ecma CLI?
>
> I'm not saying that these questions don't have answers, but I am
> saying that in order for the abstraction to compose well, there needs
> to be a way to express the solutions to problems like these, and
> preferably, a way to drop down to a lower level and specify a
> different behaviour.  This is by no means a complete list (although as
> you can probably see, it's a problem I've meditated on regularly),
> just a taste.
>
> However: Guile was *designed* to be a multi-language VM, and has a far
> nicer object model and MOP than something like the JVM or CLI.  If
> scheme were the common denominator, it might be so much nicer than
> what we've seen before that the jarring that comes from crossing
> several object space boundaries is not as severe.  I just mean to be
> careful about whether you are suggesting moving to a single VM,
> implementing an automated FFI tool with several backends, or solving
> this problem generally, because they are each projects of very
> different scale.
>
> --
> William Leslie
>
>