Comparable for equality

Orderable

Can be input on a channel

Can form sets of the type.

Is something that can be yielded on the right hand side of =>.

Something that can be extended via some means to a given type.

After type-checking, the TypeVarRef will simply be a variable that contains Nothing. This means that it can be converted into an explicit TDotable via some unknown means, or directly to the return type.

The variable argument here has a slightly special role during unification. If it contains Nothing then this is extendable via some unknown means to the specified type. If it contains a TDotable argt rt, then we know that one of the arguments is argt, and the remaining arguments are rt, which must either be a Dotable or a variable. The meaning in the former sense is recursive, the meaning in the latter case is clear. If it contains TVar tvref, then this means tvref has replaced this argument variable.

We need to do the above as we may have multiple things that are extendable in the same way, so we need to sync the arguments together.

This type is used only during type-checking, and is guaranteed to only ever appear at the top-level of the left-hand side of a TExtendable.