4 Jan
2017
4 Jan
'17
6:26 p.m.
On 4 Jan 2017, at 03:50, Dan Cross <crossd(a)gmail.com> wrote:
Interesting, but I'm curious how this would work in the context of C (or a C-like
variant)? The code must parse and type-check in accordance with the existing standard, no?
So if the `if(LINUX)` branch referred to, say, Linux-specific structure members, then how
would the compiler recognize that avoid spitting out a diagnostic/erroring out? The
existing C language seems defined to expressly disallow this sort of thing.
Common Lisp has a notion of 'suppressing the reader' which basically means that
the reader (which in CL is the thing which turns a stream of characters into the data
structure that is the source code of the language) will do just enough to consume a form,
but not any more than that. In particular it will ignore all sorts of things which would
make it very unhappy if it looked too closely at them. And there are then read-time
conditionals which will cause the reader to suppress the following thing, or not. It
seems to me that, even without defining how things work in the very fine-grained way that
CL does (where the data structure the reader produces is defined and you can program the
reader itself), a C-like language could define what it means to 'suppress' a
form, and support conditionals which did that. I think, reading again, that this might be
quite close to your compile-time-evaluated idea.
The thing to avoid is 'language in a string', where one language contains
another language in strings (or equivalent), because then you end up putting the inner
language together by concatenating strings, which can cross-cut constructs in the inner
language in a horrible way. C is the language in a string of the C preprocessor. Where I
work we use a tool which has a deeply horrible preprocessor which has the main syntax as
its language in a string. That syntax *in turn* has a whole other language in its
strings. Every time I look at this I want to hit someone.
--tim