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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 ## Design
### Shape types
Rename table types to 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. 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. 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), 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. 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 This design will future-proof us for bounded generics, for example the inferred type of
```lua ```lua
@ -46,6 +52,8 @@ will, with bounded generics and this RFC, have type:
getP : <a, t <: {p:a}> (t) -> a getP : <a, t <: {p:a}> (t) -> a
``` ```
### Related work
This design is essentially the same as This design is essentially the same as
[WebIDL](https://webidl.spec.whatwg.org/#idl-objects), where what [WebIDL](https://webidl.spec.whatwg.org/#idl-objects), where what
we're calling shape types they call "callback interfaces". 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 use interface types, so presumably TS interfaces are inhabited by both
platform objects and ECMAScript objects. 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 ## Drawbacks