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

(Oops, let's try that again with the correct destination address this time...) 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

from people

where 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

from people

where 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