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Added a comparison with Flow, and did some wordmithing

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ajeffrey@roblox.com 2022-06-30 16:04:21 -07:00
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22
rfcs/shape-types.md
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@ -19,6 +19,8 @@ but sealed and unsealed tables cannot.
## Design
### Shape types
Rename table types to shape types.
Allow a class instance to be a member of a shape type if has all the named properties, and they have a matching type.
@ -30,10 +32,14 @@ Allow a class to be a subtype of a table type if has all the named properties, a
With these changes, `Part & { p : number }` is now an inhabited type, which it currently is not.
Note that we can recover the current semantics if we also adopt the [Function and table types RFC](function-and-table-types.md),
since the current semantics of `{ p : number }` is `table & { p : number }`.
since the current semantics of `{ p : number }` is `table & { p : number }`. We may want dedicated syntax for this.
### Built-in APIs
We should audit the built-in APIs to check which ones require tables and which ones are also prepared to accept class instances.
### Bounded generics
This design will future-proof us for bounded generics, for example the inferred type of
```lua
@ -46,6 +52,8 @@ will, with bounded generics and this RFC, have type:
getP : <a, t <: {p:a}> (t) -> a
```
### Related work
This design is essentially the same as
[WebIDL](https://webidl.spec.whatwg.org/#idl-objects), where what
we're calling shape types they call "callback interfaces".
@ -55,7 +63,17 @@ bindings](https://github.com/microsoft/TypeScript/blob/main/lib/lib.dom.d.ts)
use interface types, so presumably TS interfaces are inhabited by both
platform objects and ECMAScript objects.
We should bikeshed the concept name: table types? Tably types? Shape types? Interface types?
Flow distinguishes between class types and object types.
```js
class C { p : number }
var c : C = new C()
var t : { p : number } = c
```
gives error
```
var t : { p : number } = c
^ Cannot assign `c` to `t` because `C` [1] is not a subtype of object type [2].
```
## Drawbacks