Pattern matching in D?

[Nick Sabalausky via Digitalmars-d]

On 10/23/2016 11:55 PM, Nick Sabalausky wrote:
>
> --------------------------------
> // An equivalent std.variant.Algebraic would be clunky by comparison:
>   variant RgbColor {
>     | Red
>     | Yellow
>     | Green
>     | Different {
>         red : float;
>         green : float;
>         blue : float;
>       }
>   }
> --------------------------------

Just to compare to equivalent D:

--------------------------------
struct RgbColor_ // Don't clutter the namepsace
{
     struct Red {}
     struct Yellow {}
     struct Green {}
     struct Different {
         float red;
         float green;
         float blue;
     }
}
alias RgbColor = Algenraic!(
     RgbColor_.Red,
     RgbColor_.Yellow,
     RgbColor_.Green,
     RgbColor_.Different,
}
--------------------------------

It's just...I mean, yea, it works, and you could probably DRY it up a 
little with a type contructing template ("alias RgbColor = 
DoMagic!RgbColor_"), but...meh...

And then the pattern matching end would be similarly "ehh...meh...":

--------------------------------
RgbColor color = ...;
auto x = color.visit(
     (RgbColor_.Red a) => "red",
     (RgbColor_.Yellow a) => "yellow",
     (RgbColor_.Green a) => "green",
     (RgbColor_.Red a) =>
         mixin(interpolateStr!`rgb(${a.red}, ${a.green}, ${a.blue})`),
);
--------------------------------

Again, technically works, but...ehh, it's like doing slices or 
high-order funcs in C.