Larry Evans wrote:
I think the nullary expression concept is good. I've been trying to see how proto relates to algebra's and their transforms (or homo morphisms), and algebra's have nullary expressions (true and false are nullaries in bool algebra) and the transform from one algebra to another map the nullaries of the source algebra to nullaries of target algebra. Now one particular algebra relevant to proto and spirit and deterministic parsing is what I'll call the "derives_empty" algebra. Expressions in this target algebra are created by a transform from a source algebra which is the algebra of grammar expressions. In that transform (call it gram_to_empty), the transforms are:
gram_to_empty(terminal) :-> empty_not gram_to_empty(epsilon) :-> empty_yes gram_to_empty(non_terminal) :-> empty_unknown gram_to_empty(E0 | E1 | ... | En) :-> gram_to_empty(E0) + gram_to_empty(E1) + ... + gram_to_empty(En) gram_to_empty(E0 >> E1 >> ... >> En) :-> gram_to_empty(E0) * gram_to_empty(E1) * ... * gram_to_empty(En)
<snip> The presence or absence of terminals as nullary expressions in Proto doesn't affect your ability to implement this transform in Proto. The transform would be pretty straightforward, I think. I think it would make a terrific example, actually. I've been working on the docs for Proto's transforms. You can find the latest here: http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/user_s_... Check out the examples for fold<>, fold_tree<> and applyN<>. I think they would be the most useful ones for this problem. -- Eric Niebler Boost Consulting www.boost-consulting.com