On 03/20/08 16:53, Eric Niebler wrote:
Larry Evans wrote:
What about recording the arity *with* the tag instead of in the expression associated with the tag? IOW, instead of:
expr<Tag,Args,Arity>
have:
expr<tag_arity<Tag,Arity>,Args>
Why? Well, it directly associates the arity with the Tag instead of with the expression. After all, sin(X) is not expressed as sin(x,1). [or in lisp terms, (sin x) is not expressed as (sin x 1) ].
What problem does that solve?
I don't think it solves any problem that can't already be solved, but it makes the solution clearer. It's clearer because the arity is associated with the tag *not* the expression. The arity of a tag does not depend on the expression; so, it should not be in the expression. Instead the arity of the tag determines the expression validity. OK, but you may say "the arity *still* is in the expression: expr<tag_arity<Tag,Arity>,Args> "; however, it's "more closely" associated with the tag than the expression, so it's really more a matter of style, I guess. OTOH, I would guess it would make specifying morphisms between algebra's easier since any morphism would map equal arity's to equal arities; and if the arity were directly associate with the tag, as in tag_arity<Tag,Arity>, then the mapping would be easier to understand and verify (with maybe a boost concepts class which assured the pairs in the mpl map had equal arities on both sides).