Names and design of partial sumtypes

[monkyyy]

I think full sumtypes are probably usually overkill and I rarely 
use it; but I have some lib ideas that need... something

```d
struct shape{
   int w,h;
   bool isCircle()=>h==-1;
   bool isSquare()=>w==h;
   bool isRect()=>h!=-1 && w!=h;
}
void drawCircle(shape s){
   assert(s.isCircle)
...
```

Classification could be pulled out from tag and it being a full 
union in the above example.

Radix sorts classify using a user defined function to then index 
an array of pointers to make a copy

fizzbuzz using ranges is kinda tricky, despite being 
computationally trivial, maintaining the control flow from 
classification while having access to the data is hard

etc.

----

so api wise...

classify: returns an int from the value

classmax: defines that max value classify may return(enum)

match: calls classify, swizzles the value to a collection of 
lamdas

etc.

what preexisting code plays with such ideas and how do they name 
their things?