expression templates

[enuhtac]

Hello everyone,

I'm new to D and to this list (although I've had a look onto D a few years 
ago). I hope you guys can help me with my questions.

At the moment I'm trying to implement some expression template stuff. My first 
goal is to encode an expression into a type representing that expression 
without any additional functionality (like the possibility to evaluate that 
expression). Actually this is very simple and short in D. This is my approach:

struct OpBinary( string Op, R1, R2 )
{
    alias typeof( mixin( "R1.EvalT.init" ~ Op ~ "R2.EvalT.init" ) ) EvalT;

    enum string Operator = Op;
};

struct Constant( T, T v )
{
    alias T EvalT;

    enum T value = v;
};

struct Expr( R )
{
    auto opBinary( string Op, R2 )( Expr!R2 )
    {
        return Expr!( OpBinary!( Op, R, R2 ) )();
    }

    auto opBinary( string Op, T )( T v ) if( isNumeric!T )
    {
        return Expr!( OpBinary!( Op, R, Constant!( T, v ) ) )();
    }

    auto opBinaryRight( string Op, T )( T v ) if( isNumeric!T )
    {
        return Expr!( OpBinary!( Op, Constant!( T, v ), R ) )();
    }
};

But I cannot figure out how to implement expression templates for comparison 
operators, which is crucial for my purpose. The opCmp function is great for 
implementing comparison functionality, but when building an expression template 
tree the information on the actual comparison operator is needed. opCmp just 
knows that a comparison is going on, the actual type of comparison is unknown.
What I would like to have is something like this:

    auto opCmp( string Op, R2 )( Expr!R2 )
    {
        return Expr!( OpBinary!( Op, R, R2 ) )();
    }

So opCmp knows about the actual operator and would just use my OpBinary struct 
to encode it. But this is not possible.

The only workaround for I this problem I can imagine is using my own comparison 
functions instead of the comparison operators:
op!"<"( a, b ) instead of a < b.
Another possibility would be to call opBinary explicitly:
a.opCmp!"<"( b )
In this case I would not even have to write additional code.

But these workarounds are ugly, if would greatly prefer the normal comparison 
operators.
Does anyone has an idea how to use them?

Regards,
enuhtac