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57 lines
2 KiB
Text
57 lines
2 KiB
Text
# Type Metamethod Support
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## Summary
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Add support for the `__type` metamethod on Luau defined tables.
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## Motivation
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The motivation of allowing developers to use the `__type` metamethod is to provide a better runtime mechanism for comparing between table types.
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Right now, the type checker can compare table types, however, using typeof on these tables will still return `table`, as to the runtime, these are the same type. Support for the `__type` metamethod is already supported for host-defined userdatas, so it would be trivial to extend this to tables.
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## Design
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The design would simply involve adding a new metamethod `__type`, that can be set in the metatable of another table:
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```lua
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local Cat_MT = {
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__index = Cat,
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__type = "Cat"
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}
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```
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When calling the `Cat` object with `typeof`, it should return the value of the defined `__type` metamethod if it exists:
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```lua
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local cat = Cat.new()
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print(typeof(cat)) --> "Cat"
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```
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## Drawbacks
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The main drawback is that this allows spoofing of host-defined userdatas, lets take the Roblox datum `Color3`. This object contains a `__type` flag of Color3. If we allow __type on userdatas, there's no easy way to check between a host-defined userdata, and a spoofed Color3 Luau userdata/table at the Luau level.
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The easiest fix is to include a hardcoded prefix on the `__type` field to indicate its a Lua-defined table.
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## Alternatives
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This can already be implemented using a field exposed on the table:
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```lua
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local t1 = {__type = "Table1"}
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local t2 = {__type = "Table2"}
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local function useTable(t)
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if t.__type == "Table1" then
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print("table is of type 1")
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elseif t.__type = "Table2" then
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print("table is of type 2")
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end
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end
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```
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The issue here is that it requires each table passed to the function to implement a `__type` field, which has its own problems
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* If the table is not frozen (or cant be frozen), someone can easily change the `__type` field and make the code take a path other than expected.
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* It creates a contract that every object table has a key of the same name to check types.
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