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Getting Started with Gambit and Snow (or any other Scheme)

From: "andrew cooke" <andrew@...>

Date: Sun, 12 Aug 2007 20:04:09 -0400 (CLT)

This largely states the obvious, but boy, did I need this earlier today. 
So perhaps this will help someone else.

OK, so the context is - you want to use a Scheme.  You also want to be
able to use and share libraries.  And you want unit tests.  And you'd
rather not write code specific to one particular Scheme.  But you want
namespaces/packages...

So the solution, as far as I can see, is to use Snow (aka Scheme Now) -
http://snow.iro.umontreal.ca/ - plus whatever Scheme takes your fancy (I
am using Gambit because the idea of Erlang with a better syntax seems
pretty cool - if that makes no sense, check
http://lambda-the-ultimate.org/node/841).

But how does all this work?  That is the obvious thing that wasn't...

First, you install whatever Scheme you want (if you're running Linux you
may already have Guile installed).  I installed Gambit.  It's the usual
configure and make; pretty painless.

Next, you install Snow.  That means downloading the "generic Snow" and
then doing the config and make install process.

One you've done that (and set your paths), you end up with a couple of
utilities.  First, snowman, which is like yast or apt, or rpm or cspan,
whatever - a package manager that lets you install packages.  It's
documented 2/3 of the way down this page:
http://snow.iro.umontreal.ca/?tab=Documentation

Second, you have a program called "snow" which is what you use to invoke
your scheme code.  I wasn't expecting this, but it makes sense - it hides
away whatever magic is needed to convert your particular Scheme to support
the cross-platform snow API.

The simplest way to use snow is as an interpreter (you can also compile
code using "snow --compile" but I've not explored that yet).  The Snow
docs give the following example.  Note that the first line is some kind of
bash magic that invokes the same file using "snow" (I think it is written
like that so that the syntax is acceptable to bath bash and scheme).

":";exec snow -- "$0" "$@"

(package* go/v1.0.0
 (require: hostos/v1)
 (require: fixnum/v1))

(define (double x)
  (snow-fxarithmetic-shift-left x 1))

(test* (expect* (= 8 (double 4))))

(write (double (string->number (cadr (snow-command-line)))))
(newline)

To see that working, put it in a file called "go.scm", then chmod +x so
that it's executable, and invoke it with an argument:

> ./go.scm 500
1000

That's it, really.  That is running (in my case) Gambit, with all the
relevant Snow bits added.  So you get the module/namespace functionality
(the package* bit) and the unit test functionality (test*, expect*).

One final cute trick.  I said earlier that Guile is probably already
installed on Linux.  That's true in my case, and Snow can invoke scripts
on "all schemes", which makes testing compatibility easy.  Here's how:

> SNOW_HOST=all ./go.scm 500
------------------------------------------------------------ gambit
1000
------------------------------------------------------------ guile
1000

Once you grok the above, the Snow site documentation makes a lot more
sense.  All this is there anyway (what got me started was someone sending
me a cut+paste of the relevant sections!).  But somehow, for me at least,
the "big picture" took time to form.

Andrew

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