[Python-Dev] AST Transformation Hooks for Domain Specific Languages

[Jon Riehl]

Hi Nick, all,

Just for the record, I would point to Mython (mython.org) as an
existing provider of this capability.  I've already added an AST node
called "Quote" that functions like your DSL node, along with well
defined lexical, concrete syntax, and compile-time properties.

I have a mostly functioning front end for 2.X that does these
expansions (MyFront), and I'm waiting for a stable Mercurial migration
(I've been only lightly lurking on python-dev, so if this already
exists, someone should ping me) so I can publish a 3.X branch that
will get rid of a lot of the code I have to maintain by just building
on top of CPython (CMython? *smirk*).

It looks like you have some ideas about import semantics and managing
compile-time dependencies.  I would invite further elaboration on the
mython-dev Google group.  I currently have two different mechanisms
implemented, one via import hooks and the other by forced global
recompilation, but neither of these satisfies because you are imposing
a compile-time concept into a thoroughly dynamic language.

...and yes, compile-time metaprogramming is insane.

Regards,
-Jon

http://mython.org/ - Make Python yours.

On Fri, Apr 8, 2011 at 6:29 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> A few odds and ends from recent discussions finally clicked into
> something potentially interesting earlier this evening. Or possibly
> just something insane. I'm not quite decided on that point as yet (but
> leaning towards the latter).
>
> Anyway, without further ado, I present:
>
> AST Transformation Hooks for Domain Specific Languages
> ======================================================
>
> Consider:
>
> # In some other module
> ast.register_dsl("dsl.sql", dsl.sql.TransformAST)
>
> # In a module using that DSL
> import dsl.sql
> def lookup_address(name : dsl.sql.char, dob : dsl.sql.date) from dsl.sql:
>    select address
>    from people
>    where name = {name} and dob = {dob}
>
>
> Suppose that the standard AST for the latter looked something like:
>
>    DSL(syntax="dsl.sql",
>        name='lookup_address',
>        args=arguments(
>            args=[arg(arg='name',
>                      annotation=<Normal AST for "dsl.sql.char">),
>                  arg(arg='dob',
>                      annotation=<Normal AST for "dsl.sql.date">)],
>            vararg=None, varargannotation=None,
>            kwonlyargs=[], kwarg=None, kwargannotation=None,
>            defaults=[], kw_defaults=[]),
>        body=[Expr(value=Str(s='select address\nfrom people\nwhere
> name = {name} and dob = {dob}'))],
>        decorator_list=[],
>        returns=None)
>
> (For those not familiar with the AST, the above is actually just the
> existing Function node with a "syntax" attribute added)
>
> At *compile* time (note, *not* function definition time), the
> registered AST transformation hook would be invoked and would replace
> that DSL node with "standard" AST nodes.
>
> For example, depending on the design of the DSL and its support code,
> the above example might be equivalent to:
>
>    @dsl.sql.escape_and_validate_args
>    def lookup_address(name: dsl.sql.char, dob: dsl.sql.date):
>       args = dict(name=name, dob=dob)
>       query = "select address\nfrom people\nwhere name = {name} and
> dob = {dob}"
>       return dsl.sql.cursor(query, args)
>
>
> As a simpler example, consider something like:
>
>    def f() from all_nonlocal:
>        x += 1
>        y -= 2
>
> That was translated at compile time into:
>
>    def f():
>        nonlocal x, y
>        x += 1
>        y -= 2
>
> My first pass at a rough protocol for the AST transformers suggests
> they would only need two methods:
>
>  get_cookie() - Magic cookie to add to PYC files containing instances
> of the DSL (allows recompilation to be forced if the DSL is updated)
>  transform_AST(node) - a DSL() node is passed in, expected to return
> an AST containing no DSL nodes (SyntaxError if one is found)
>
> Attempts to use an unregistered DSL would trigger SyntaxError
>
> So there you are, that's the crazy idea. The stoning of the heretic
> may now commence :)
>
> Where this idea came from was the various discussions about "make
> statement" style constructs and a conversation I had with Eric Snow at
> Pycon about function definition time really being *too late* to do
> anything particularly interesting that couldn't already be handled
> better in other ways. Some tricks Dave Malcolm had done to support
> Python level manipulation of the AST during compilation also played a
> big part, as did Eugene Toder's efforts to add an AST optimisation
> step to the compilation process.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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