expression templates

[Don]

Mr enuhtac wrote:
> Hello everyone,
> 
> I'm new to D and 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

In the present language I don't think it's possible to do expression 
templates involving opCmp or opEquals, since you have no control over 
the return type.

One other solution I did: I had some code where the result of the 
expression template was used in a mixin, so I moved the mixin outside 
the expression. Then I changed all the >, <, == into something else 
before converting it into an expression template.
But this is ugly as well.