[Cython] Wacky idea: proper macros

Nathaniel Smith njs at pobox.com
Sun Apr 29 09:42:14 CEST 2012

On Sat, Apr 28, 2012 at 10:25 PM, mark florisson
<markflorisson88 at gmail.com> wrote:
> On 28 April 2012 22:04, Nathaniel Smith <njs at pobox.com> wrote:
>> Was chatting with Wes today about the usual problem many of us have
>> encountered with needing to use some sort of templating system to
>> generate code handling multiple types, operations, etc., and a wacky
>> idea occurred to me. So I thought I'd through it out here.
>> What if we added a simple macro facility to Cython, that worked at the
>> AST level? (I.e. I'm talking lisp-style macros, *not* C-style macros.)
>> Basically some way to write arbitrary Python code into a .pyx file
>> that gets executed at compile time and can transform the AST, plus
>> some nice convenience APIs for simple transformations.
>> E.g., if we steal the illegal token sequence @@ as our marker, we
>> could have something like:
>> @@ # alone on a line, starts a block of Python code
>> from Cython.MacroUtil import replace_ctype
>> def expand_types(placeholder, typelist):
>>  def my_decorator(function_name, ast):
>>    functions = {}
>>    for typename in typelist:
>>      new_name = "%s_%s" % (function_name, typename)
>>      functions[name] = replace_ctype(ast, placeholder, typename)
>>    return functions
>>  return function_decorator
>> @@ # this token sequence cannot occur in Python, so it's a safe end-marker
>> # Compile-time function decorator
>> # Results in two cdef functions named sum_double and sum_int
>> @@expand_types("T", ["double", "int"])
>> cdef T sum(np.ndarray[T] arr):
>>  cdef T start = 0;
>>  for i in range(arr.size):
>>    start += arr[i]
>>  return start
>> I don't know if this is a good idea, but it seems like it'd be very
>> easy to do on the Cython side, fairly clean, and be dramatically less
>> horrible than all the ad-hoc templating stuff people do now.
>> Presumably there'd be strict limits on how much backwards
>> compatibility we'd be willing to guarantee for code that went poking
>> around in the AST by hand, but a small handful of functions like my
>> notional "replace_ctype" would go a long way, and wouldn't impose much
>> of a compatibility burden.
>> -- Nathaniel
>> _______________________________________________
>> cython-devel mailing list
>> cython-devel at python.org
>> http://mail.python.org/mailman/listinfo/cython-devel
> Have you looked at http://wiki.cython.org/enhancements/metaprogramming ?
> In general I would like better meta-programming support, maybe even
> allow defining new operators (although I'm not sure any of it is very
> pythonic), but for templates I think fused types should be used, or
> improved when they fall short. Maybe a plugin system could also help
> people.

I hadn't seen that, no -- thanks for the link.

I have to say that the examples in that link, though, give me the
impression of a cool solution looking for a problem. I've never wished
I could symbolically differentiate Python expressions at compile time,
or create a mutant Python+SQL hybrid language. Actually I guess I've
only missed define-syntax once in maybe 10 years of hacking in
Python-the-language: it's neat how if you do 'plot(x, log(y))' in R it
will peek at the caller's syntax tree to automagically label the axes
as "x" and "log(y)", and that can't be done in Python. But that's not
exactly a convincing argument for a macro system.

But generating optimized code is Cython's whole selling point, and
people really are doing klugey tricks with string-based preprocessors
just to generate multiple copies of loops in Cython and C.

Also, fused types are great, but: (1) IIUC you can't actually do
ndarray[fused_type] yet, which speaks to the feature's complexity, and
(2) to handle Wes's original example on his blog (duplicating a bunch
of code between a "sum" path and a "product" path), you'd actually
need something like "fused operators", which aren't even on the
horizon. So it seems unlikely that fused types will grow to cover all
these cases in the near future.

Of course some experimentation would be needed to find the right
syntax and convenience functions for this feature too, so maybe I'm
just being over-optimistic and it would also turn out to be very
complicated :-). But it seems like some simple AST search/replace
functions would get you a long way.

- N

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