Interfaces

- structural interfaces are a key part of typescript
- type vs interface
- how dynamic do we want to go
- shape mutation
- excess properties
- declaration exprsesions
- dynamic prototypes
- all sorts of JS hacks that everyone hates anyway

- funky signatures:
- call signatures (just a Function fat pointer)
- construct signatures (also a Function fat pointer)
- index signatures
    - can I read through index signatures? can I call through them?
    - (index signatures use dynamic.find at runtime, which is a linear scan over string equality!)

- in general interfaces should "just work" by default, like everything in TS++
- when we want / need to be explicit, Dynamic<T>
- like explicit dyn T (but fixed size fat pointer)
- unknown is just Dynamic<unknown>

interface Named {
    name: string;
}

function label(value: Named): string {
    value.name
}
Representation

- closed: aliases, newtypes, and object shapes have one concrete representation
- open: bare structural interfaces and indexed shapes are open and store through Dynamic<T>
- indexed structural fields are readonly and return T | undefined; represented collections such as Map implement IndexSet for writes