I didn't see a discussion of equality checking here, so I'll mention it. It's really useful if sum types (aka variants or algebraic data types) do not have iden

I didn't see a discussion of equality checking here, so I'll mention it. It's really useful if sum types (aka variants or algebraic data types) do not have identity, i.e. that creating two identical values with the same tag (or case) with the same values compare equal. This allows the compiler to represent sums efficiently, e.g. by packing two 32-bit values into a single 64-bit word. With reference identity (two constructed values are only equivalent if they refer to the same on-heap representation), then this optimization becomes harder (basically only works with escape analysis). Virgil has variants and also enums. They are slightly different things, though treated much the same under the hood. Neither have identity. (How they are different is that enums can have arguments in Virgil, but there are still is a fixed list of named values, whereas variants have named cases and are potentially recursive).