[boost] [proto][design]why unary_expr,binary_expr; but no nullary_expr?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-04-27 07:01:43

Although there's unary_expr<Tag,*>, binary_expr<tag.*,*>
in traits.hpp there's no nullary_expr<Tag>.  I realize
this would be a replacement to the current terminal template
but with a user specified tag.  After all,
if a user specified tag for n-ary (where n>0) expressions
*is* desirable, why is a user specified tag for 0-ary
expressions *not* desirable?  On case where this would be
useful is for algebraic morphisms where n-ary ops in a
source algebra are mapped to n-ary ops in a target algebra.
This *includes* 0-ary ops.  An op would be the equivalent
to proto tags.  I think this could be done by another
transform similar to the pass_through transform but
allowing transformation of the tag.  Currently
pass_through just uses the source tag in the target
expression.  It seems like it should be pretty easy
to allow transformation on the tag also.