draft proposal for Sum Types for D

[Walter Bright]

On 11/29/2022 12:37 PM, Jacob Carlborg wrote:
> ### ABI

Section added.


> ### Checking of the Tag
> 
> The DIP mention there's a runtime check of the tag, but it doesn't specify what 
> "runtime check" is.

Section added.


> This could potentially be extended to allow accessing the field without checking 
> the tag, but require checking the tag before using the value, example:

Discussed in a reply to Timon.


> ### Breaking Changes
> 
> Another breaking change, perhaps an edge case, but code that inspects all types 
> and language constructs and performs different actions, i.e. a serialization 
> library or something like `writeln`.

Added

> ### Switch Statement/Pattern Matching
> 
> The DIP mentions that pattern matching is subject of another DIP. But I think 
> it's reasonable that the switch statement should support sumtypes, in the same 
> way as the if statement.

A match statement would make this redundant. In fact, I expect the match 
statement to be lowered to a switch :-)