Re: [Caml-list] camlp4 pa_macro

[Dmitry Bely <dbely@mail.ru>]

Peter Jolly <peter@jollys.org> writes:

>> Is it possible to achieve with pa_macro something like this:
>> 
>> IFDEF NDEBUG THEN
>> DEFINE LOG(expr) = ()
>> ELSE
>> DEFINE LOG(expr) = Printf.printf expr
>> ENDIF
>> ...
>> LOG("x=%d,y=%d" x y);
>> 
>> Unfortunately the code above does not work: debug version is OK, but then
>> NDEBUG is turned on I have
>> 
>> "This expression is not a function, it cannot be applied" on LOG()
>> expression.
>
> Getting camlp4 to pretty-print the code after macro expansion is a
> useful technique for debugging this sort of problem:
>
> 	$ camlp4 pa_o.cmo pa_op.cmo pr_o.cmo pa_macro.cmo test.ml
> 	...
> 	Printf.printf ("x=%d,y=%d" x y)
>
> It should be clear why that isn't working.

I see. Thank you very much for the info.

>
>> If I use
>> 
>> LOG "x=%d,y=%d" x y;
>> 
>> then the release version surprisingly works, but the debug one gives
>> 
>> "Parse error: currified constructor"
>
> Yes, because this does not pass any arguments to the LOG macro - it
> expands it with an empty <expr>.  So this works in the latter case,
> because LOG just expands to "Printf.printf", but in the former case you
> end up with
>
> 	() "x=%d,y=%d" x y
>
> which is a syntax error as reported.
>
>> How to overcome this?
>
> 	IFDEF NDEBUG THEN
> 	DEFINE LOG = Printf.kprintf ignore
> 	ELSE
> 	DEFINE LOG = Printf.printf
> 	ENDIF

It's quite useless - 

let debug = ref true

let log fmt =
 if !debug then
   Printf.kprintf print_string fmt
 else
   Printf.kprintf ignore fmt

will in fact give the same result. It does not solve the initial problem -
completely remove the debugging code from the release binary.

> Or just replace all instances of LOG with "if debug then Printf.printf",
> on the grounds that the compiler is probably clever enough to prune
> conditions that always evaluate to false, and you probably won't notice
> any significant difference in speed even if it isn't.

But the debugging code (format strings etc.) will be there. OK, the best I
can get now is

IFDEF NDEBUG THEN
DEFINE LOG(expr) = ()
ELSE
let dprintf = Printf.printf
DEFINE LOG(expr) = expr
ENDIF
...
LOG(dprintf "x=%d,y=%d" x y);

Not very elegant, but works.

- Dmitry Bely