[LLVMdev] [PATCH] - Union types, attempt 2

[Joachim Durchholz]

Talin schrieb:
> Well, the fact that union members have to be indexed by number means 
> that the ordering has to be part of the type - so even though 
> type-theoretically union { i32, float } is the same as union { float, 
> i32 }, in my implementation they are distinct types. However, from the 
> standpoint of a frontend, this is not a great concern, because the 
> frontend will most likely sort the list of types before constructing the 
> IR type.

Hm... it's placing a burden on the frontend developer.

More importantly, it's something that the fronend developer must not 
forget to do, so you better make sure this is documented in capital 
letters in a place where the frontend developer is likely to look when 
preparing code generation.

Most importantly, however, this will create a lot of hassles when making 
code interoperable between compilers: Compiler writers need to agree on 
a language-independent canonical ordering.
That said, if the ordering is canonical, it could be established at the 
IR level. E.g. by ordering alphabetically.

When coding, please consider that many languages establish assignment 
compatibility between union types. E.g. a union {i32, float} value could 
be assigned to a name that's typed as a union {i32, i64, float}.
This probably means the need for conversion operators, and it definitely 
means that indexes aren't meaningful by themselves, only in conjunction 
with their union type.

 > By always putting the types in a canonical order, regardless of
> the order that they appear in the source code, you can ensure that 
> unions of equal types are always compatible. In other words, you can 
> treat the members like an ordered set rather than like a list.  

Yes, that's closer to the frontend semantics: the variants of a union 
type don't have any natural ordering, so list semantics could cause 
problems.

Regards,
Jo