Steven Watanabe wrote:
Eric Niebler wrote:
Consider:
and_< when<G1, T1> ,when<G2, T2>
T1 requires that any expression passed to it matches G1. Likewise for T2 and G2. Now, imagine that the result of applying T1 is some type that *doesn't* match G2 ... it might not even be an expression! If you tried to pass it to T2, you would be violating T2's preconditions. T2's transform will likely fail.
How about passing it the _state parameter?
So that and_'s transform works a bit like fold? That's an interesting suggestion. Can you give me a use case? The times I've needed and_ in my grammar, it's looked like: and_< terminal<_> ,if_<is_base_of<MyBase,_arg>() >
or something. In this case, passing the _state parameter to terminal<>'s transform would violate its preconditions (it expects a terminal expression), so this might not be such a great idea. Today, this transform isn't terribly interesting ... it just returns the terminal expression unmodified. -- Eric Niebler Boost Consulting www.boost-consulting.com