Re: non-scheme scripts: proposed solutions and their pros/cons

[Noah Lavine <noah.b.lavine@gmail.com>]

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As you say, the only real solution is to do more than one of these things.

For instance, I think it's really important to be able to load modules
written in other languages. However, this may be language-dependent to a
certain extent, because some languages (Python) already have ways to define
modules. In those cases we should stick with their conventions, and use our
other methods for figuring out what language the file is in.

However, if we're using Guile in one language and loading an executable in
a different language, then we can't use a command-line argument or a
different executable to signal it. The only choices left are heuristics and
explicit markers. I think the only reasonable choice is both - use
heuristics, and let the user supply a marker if the heuristics are wrong.
(In the module case, one can imagine a heuristic based on having a
language-specific load path for each language, which might be very
effective.)

But for using Guile as an interpreter for different languages, a
command-line argument or argv[0] switch make a lot of sense.

What do you think?
Noah



On Tue, Nov 20, 2012 at 7:15 AM, Ian Price <ianprice90@googlemail.com>wrote:

>
> As promised in the other thread, here is my list. This was really a
> response to the even the earlier thread I started, which I
> (unfortunately) didn't reply to at the time.
>
> First off, they important question "why do we need this?". Well, guile
> is a multi-language vm in principle, even if Scheme is it's first and
> foremost citizen. If guile is to be a full-fledged vm for these other
> languages, they need all (or at least most of) the rights and privileges
> scheme does. This includes the ability to be run as scripts.
>
> So, what are the solutions? Well, a few have cropped up, but I'm not
> sure that individually any of them constitute a complete
> solution. They are: a command-line argument, some marker in the
> module, a different executable, and heuristics based on file
> extension.
>
> I shall treat these in order, though I can't pretend I will be
> comprehensive. Clarifications and additions welcome.
>
>
> * a command line argument
> This was the position I initially proposed, and so had some bias
> towards. The idea is very simple, we add a --language extension, the
> argument to which would be the language of the file(s) we are trying
> to execute.
>
> ** Pros
> - Existing source files in the language do not need to be modified to
>   use this.
> - Requires (in theory) modifying only the command-line parsing
>
> ** Cons
> - Has nothing to say about how e.g. scheme modules interact with elisp
> modules.
> - #! only gives you one argument, and not all languages are going to
>   support a \ type solution
>
> * a file marker
> The idea here it to have some way of marking in a file which language
> it is written in, with some token like #!javascript, or perhaps
> something like #!language javascript
>
> ** Pros
> - works well with current module system
>   e.g. say we have a file foo/bar.js and that had a #!javascript
>   marker, then I could (use-modules (foo bar)) and guile would notice
>   the marker, switch to javascript mode for it.
> - we already do this for switching to curly-infix and r6rs reader
>   modes
>
> ** Cons
> - requires modifying existing source code, I couldn't just import an
>   existing elisp (or whatever) file and use as is.
> - the existing mechanism allows switching reader mode at an arbitrary
>   part of the file. Not a big con, for #!language, I would simply say
>   we disallow it in arbitrary places. However, people that extend
>   guile as a language (i.e. lilypond), might disagree and would want
>   to take advantage of this.
>
> * different executable
> This was a position posed in response to mine, that I was initially
> against, but am somewhat more open to now. The idea is that for every
> language $foo, we have a script guile-$foo which invokes that
> language.
> Technically it need not be a different executable, but one whose
> action depends on argv[0] and just performs the appropriate action,
> but in many respects these can be treated the same.
>
> ** Pros
> - Existing source files in the language do not need to be modified to
>   use this, they can change the existing languages symlink.
> - We can handle common switches used by the language
> - If you go for argv[0], may only require modifying the command-line
>   parsing.
>
> ** Cons
> - proliferation of names, even if just symlinks
> - has nothing to say about cross-language interoperability
>
>
> * heuristics
> This always gets proposed, and I never like it, but hey ho. The idea
> is to have some simple way to guess what language a file is written
> in, for example, with a mapping of file extensions to languages.
>
> ** Pros
> Best case scenario, you do nothing. You type guile foo.js, and it just
> works.
> - same excutable name
> - don't need to modify existing code
>
> ** Cons
> *** guessing on file extension
> - guile allows user defined extensions
>   Need to have a way of associating modes with new extensions
> - some language share extensions
>   I'm thinking of .pl for perl and prolog, but I'm sure there are
>   other conflicts. Might not be a big con in practice
> - Many files don't have an extension, think running with ./foo
> *** guessing on content type
> - Seems complicated to implement, especially for syntactically close
>   languages. Probably undecidable in general.
>
>
> My thanks to William Leslie[0], Neil Jerram[1], and quotemstr on #emacs for
> pointing out various pros/cons.
>
>
> So, the million dollar question: what do we do?
>
> Well, I don't know. :)
>
> Maybe the argv[0] solution, with some extension for requiring modules
> in other languages built into use-modules (similarly for other
> languages). But that's less nice since now the calling module needs to
> know the language of the module it's requiring.
>
> 0. https://lists.gnu.org/archive/html/guile-devel/2012-07/msg00067.html
> 1. https://lists.gnu.org/archive/html/guile-devel/2012-07/msg00068.html
>
> --
> Ian Price -- shift-reset.com
>
> "Programming is like pinball. The reward for doing it well is
> the opportunity to do it again" - from "The Wizardy Compiled"
>
>

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