Re: Chain two different struct specialization

[bearophile]

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