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Update typecheck.md (#1)

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Arseny Kapoulkine 2020-06-18 18:43:38 -07:00 committed by GitHub
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@ -4,7 +4,7 @@ Luau supports a gradual type system through the use of type annotations and type
## Type inference modes
There are three modes currently available. Each one differ in important ways. They must be annotated on the top few lines among the comments.
There are three modes currently available. They must be annotated on the top few lines among the comments.
* `--!nocheck`,
* `--!nonstrict` (default), and
@ -44,7 +44,7 @@ local b2: B = a1 -- not ok
## Primitive types
In Lua, we have 8 primitive types: `nil`, `string`, `number`, `boolean`, `table`, `function`, `thread`, and `userdata`. Of these, only `table` and `function` are not represented by name, but have their dedicated syntax as covered in this [syntax document](syntax.md). Note that we have explicitly skipped `userdata` from the type system.
Lua VM supports 8 primitive types: `nil`, `string`, `number`, `boolean`, `table`, `function`, `thread`, and `userdata`. Of these, `table` and `function` are not represented by name, but have their dedicated syntax as covered in this [syntax document](syntax.md), and `userdata` is represented by [concrete types](#Roblox types); other types can be specified by their name.
Additionally, we also have `any` which is a special built-in type. It effectively disables all type checking, and thus should be used as last resort.
@ -107,7 +107,7 @@ f("foo") -- not ok
## Tables
From the perspective of the programmers, there are three states you should care about. They are: `unsealed table`, `sealed table`, and `generic table`. This is intended to represent the relationship of two different tables.
From the type checker perspective, each table can be in one of three states. They are: `unsealed table`, `sealed table`, and `generic table`. This is intended to represent how the table's type is allowed to change.
### Unsealed tables
@ -137,7 +137,7 @@ v2.z = 3 -- not ok
### Sealed tables
A sealed table is a table that is now locked down. This occurs when the table constructor literal had 1 or more expression.
A sealed table is a table that is now locked down. This occurs when the table constructor literal had 1 or more expression, or when the table type is spelt out explicitly via a type annotation.
```lua
local t = {x = 1} -- {x: number}
@ -261,11 +261,15 @@ local y: string = x -- ok
local z: number = x -- not ok
```
## Using Luau on Roblox
## Roblox types
All of Roblox's types are readily available for Luau's type inference engine to use, too. We can automatically deduce what your `Instance.new` is actually supposed to return.
Roblox supports a rich set of classes and data types, [documented here](https://developer.roblox.com/en-us/api-reference). All of them are readily available for the type checker to use by their name (e.g. `Part` or `RaycastResult`).
Additionally, we can automatically deduce what calls like `Instance.new` and `game:GetService` are supposed to return:
```lua
local part = Instance.new("Part")
local basePart: BasePart = part
```
Note that many of these types provide some properties and methods in both lowerCase and UpperCase; the lowerCase variants are deprecated, and the type system will ask you to use the UpperCase variants instead.