Am Dienstag, 6. April 2004 23:13 schrieb Carl Witty:
On Tue, 2004-04-06 at 12:05, Wolfgang Jeltsch wrote:
Look at XML. You are able to include XML code from other files but the code you import must be a complete XML element. Similarily, the brances of a Haskell #ifdef should be producable out of a non-terminal, and, more specific, they should be producable out of the same non-terminal. With these restrictions, one would be able to guarantee syntactical correctness without evaluating the #ifdef conditions, for example.
This doesn't work for one possible (and, I expect, common) use of a preprocessor -- if one of the branches is syntactically incorrect for some compilers. For instance, perhaps one branch uses the GHC unboxed values extensions for performance, and the other branch uses H98 for portability.
Well, the preprocessor would have to be able to parse all extensions. Maybe this is too difficult.
[...]
Wolfgang