[boost] Operation representation in boost::etl

From: Markus Werle (numerical.simulation_at_[hidden])
Date: 2003-11-25 06:02:10

Hi!

This is a question to those of you who have written and used
Expression Templates within their own libs.
I need some premature feedback during the redesign phase of Daixtrose. 
I consider some early design decisions debatable.  

Daixtrose can represent two Operations, a binary operation
(binop) and an unary operation (unop), which both 
have compile-time parameters describing their 
detailed nature, like
binop<BinaryPlus, LHS, RHS>

This means, any expression within Daixtrose
has a tree structure with a maximum of 2 branches.
E.g. the information is somewhat redundant: binop<BinaryPlus, ...
states the nature as binary operator twice.
Also isn't an unop a binop with a dead branch?

Actually I was thinking about a generalized 
representation, which allows for an arbitrary number
of arguments (though more than 2 are out of my current scope),
something like

template <typename Op, typename Args> struct op;

where Args is a mpl::typelist, tuple or similar structure. 
Drawback: 
This introduces one extra nesting level per node for the
template representing a normal expressions like a + b + c + d. 
This may be a serious disadvantage, but OTOH a canonical
representation of any operation would allow easy introduction
for optimized extra features like reordering 
op<SumOverAll, cons<Matrix, Matrix, Matrix, Matrix> >
instead of 
op<BinaryPlus, cons<Matrix, op<BinaryPlus, Matrix, etc...

you get the point? 

I think that this new representation may also allow
an easier reuse of boost::mpl features (like
mpl::transform<List, Simplify>), which is my
primary goal during the redesign phase.

Please comment and tell me what you think,

best regards,

Markus

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