Interesting paper on composing type definitions

[Enamex via Digitalmars-d]

On Monday, 6 March 2017 at 01:37:18 UTC, Andrei Alexandrescu 
wrote:
> On 3/4/17 10:36 PM, Andrei Alexandrescu wrote:
>> https://pdfs.semanticscholar.org/5de7/591a853ec947f8de7dc70df0b2ecc38b8774.pdf
>> -- Andrei
>
> An idea inspired by this paper would be a type Sum such that 
> Sum!(A, B) stores both states of A and B and the joint API 
> offered by the two. Would be a good exercise in applying 
> introspection. Also, when there are clashing primitives, Sum 
> might offer the option to make one of the types "dominant", 
> eliminate the function from the joint interface, or disallow 
> composition altogether. -- Andrei

I haven't read the paper yet but doesn't that sound exactly 
opposite to what 'sum types' is usually used to mean?

The value of the variable has to be either A or B. If it stores 
the status of both then it's basically a struct, right?

Probably I'm misunderstanding your point on composition and 
'joint API'.