On Sun, 28 Nov 2010 21:09:34 -0800, Jeffrey Lee Hellrung, Jr. wrote:
I believe the now-impossible-to-find "Avalanche Preprocessor library" that that a user posted a link to some time back used a somewhat different syntax for lambdas. Recalling strictly from memory, it would look something like
CHAOS_PP_EXPR( CHAOS_PP_ENUM( 5, CHAOS_PP_BIND( CHAOS_PP_PRIMITIVE_CAT_, ( class T ), 1 ) CHAOS_PP_BIND( CHAOS_PP_WHEN_, 1, CHAOS_PP_BIND( CHAOS_PP_CAT_, ( = T ), CHAOS_PP_BIND( CHAOS_PP_DEC_, 1 ) ) )
In other words, there was some very heavily-used CHAOS_PP_BIND-like macro that essentially initiated a lambda sequence, which accepted the macro to bind followed by the arguments to bind. Arguments appearing as numerals (1, 2, etc.) were placeholders, while arguments appearing in parantheses were literals. I don't remember much of the syntax, semantics, or mechanics past that. What I wrote above is probably not considered preferable to using the existing Chaos lambda syntax, but maybe some combination of these ideas could be massaged into a better syntax with faster compile times, given willing investigative souls.
The lambda expressions in Chaos are made up of a relatively few primitives. All (or, at least, almost all) library primitives define lambda bindings such as the following: #define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__) #define CAT_ID() CAT #define CAT_ LAMBDA(CAT_ID)() The LAMBDA macro (as well as the other lambda operators such as ARG(1), ARG(2), etc.) expand to expressions such as the following for LAMBDA (CAT_ID)() (,,,,, (etc) ,,,, 0xLAMBDA, (CAT_ID))() The parser than has a "horizontal" limit on how far over it can parse an expression (the current limit is 50, so there is about 50 commas in the above). Otherwise, the parser has no (practical) limit on depth. It parses and substitutes general expressions. The main problem is a general problem with preprocessor metaprogramming. The parser cannot "get past" some input. You cannot safely use token- pasting against arbitrary input, as that leads to undefined behavior. Even if it didn't, you still couldn't get past other stuff like operators. E.g. CHAOS_PP_EXPR(CHAOS_PP_SEQ_FOR_EACH( (CHAOS_PP_ARG(1) + CHAOS_PP_ARG(2)), (a, b)(x, y)(p, q) )) This yields garbage output because their is no way for the parser to see past the '+' opreator to find the ARG(2). -Paul