Chain two different struct specialization

[Andrea Fontana]

On Friday, 1 March 2013 at 16:03:58 UTC, bearophile wrote:
> Andrea Fontana:
>
>> BTW, compiler can't guess s1 and s2 weights, should it?
>>
>> if inside likeness() i write:
>>
>> enum test = first.weights.foo * second.weights.foo;
>>
>> it said that can't read first and second value at compile time.
>
> firstWeights and secondWeights are compile-time constants, but 
> the arguments you give to likeness are run-time values, so they 
> are unknown at compile-time inside likeness.
>
> There are ways to solve that problem, but I don't know how much 
> good this is:
>
>
> immutable struct Weights {
>     double foo, bar;
> }
>
> enum Weights firstWeights  = { foo: 0.3, bar: 0.4 },
>              secondWeights = { foo: 0.5, bar: 0.2 };
>
> struct MyStruct {
>     int prop, prop2;
>     immutable Weights weights;
>
>     this(Weights weights_, in int p, in int p2) pure nothrow {
>         this.weights = weights_;
>         this.prop = p;
>         this.prop2 = p2;
>      }
> }
>
> double likeness(alias first, alias second)() {
>     enum test = first.weights.foo * second.weights.foo;
>     double v = (first.prop - second.prop) *
>                first.weights.foo * second.weights.foo;
>     return v + (first.prop2 - second.prop2) *
>                first.weights.bar * second.weights.bar;
> }
>
> void main() {
>     enum s1 = MyStruct(firstWeights,  10,  8);
>     enum s2 = MyStruct(firstWeights,   9, 10);
>     enum s3 = MyStruct(secondWeights,  9, 10);
>
>     import std.stdio;
>     writeln(likeness!(s1, s2)());
>     writeln(likeness!(s1, s3)());
>
>     const r = [s1, s2, s3];
> }
>
> Bye,
> bearophile

but:

enum s1 = MyStruct(firstWeights,  10,  8);
enum s2 = MyStruct(firstWeights,   9, 10);

writeln(likeness(s1, s2));

still gives error: and s1 and s2 are known at compile time, 
aren't them?

Your solution will create code for each item couple, isn't it? 
Not good in my case :) Other ideas?