The CAPI Manifesto

[Gor Gyolchanyan]

Sure. You're right. Weird macros are often used for handy shortcuts
(like the aforementioned for loop header), which are not handy enough
to invent a just-as-weird way of translating them. Everything else is
either for compiler extensions (which either have built-in support in
D or are simply nit translatable) or hacks for compile-time activity
(can be replaced by more intuitive counterparts in D).
Translating _any_ C code is impossible. The C preprocessor cannot be
fully simulated in D. But translating _this_ C code is not as hard,
because it's a bad practice to abuse macros in C and no good C library
would do that.
The translator should not be a "_any_ C code translator", it should be
an "arbitrary _this_ C code translator", which makes a few assumptions
and does not guarantee 100% convertibility of the code.
The most important part of the translator should be the problem
reporter, which should point you to all the suspicious and
untranslated parts of the C code, so you can deal with them manually.

On Tue, Oct 25, 2011 at 6:36 AM, Marco Leise <Marco.Leise at gmx.de> wrote:
> Am 21.10.2011, 10:32 Uhr, schrieb Gor Gyolchanyan
> <gor.f.gyolchanyan at gmail.com>:
>
>> That's ALL you can do in C. fill structs and call functions
>> (fundamental type manipulation doesn't count).
>> My personal research shows the following use cases of C macros (sorted
>> by popularity in descending order):
>> 1. enum
>> 2. alias (most notably, conditionally compiled ones)
>> 3. CTFE function
>> 4. mixin template
>> 5. syntactic alias
>> 6. syntactic mixin template
>>
>> only the last 2 out of 6 cannot be translated to D.
>> An example of a syntactic alias is this very common piece of C code:
>> #ifdef __VERY_VERY_OLD_C_COMPILER__
>>    #define CONST
>> #else
>>    #define CONST const
>> #endif
>
> Maybe these cases can often be solved when, as you say, they are for
> compatibility with other compilers. The header converter would simply assume
> to be the latest and greatest of the known compilers and evaluate the code
> like this:
> - there is an #ifdef
> - if it a define from the list of "known C compilers with quirks"
>  - jump right to the else block
> The #define obviously can become more than one thing in the .d file. But
> that is determined by the instantiation site. The above CONST would probably
> be ignored when it is used in a const parameter declaration, because of D's
> transitivity and become 'immutable __gshared' when used on a global
> variable. Do you get the idea?
>