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78 lines
2.1 KiB
Markdown
78 lines
2.1 KiB
Markdown
# Unsealed table literals
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**Status**: Implemented
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## Summary
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Currently the only way to create an unsealed table is as an empty table literal `{}`.
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This RFC proposes making all table literals unsealed.
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## Motivation
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Table types can be *sealed* or *unsealed*. These are different in that:
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* Unsealed table types are *precise*: if a table has unsealed type `{ p: number, q: string }`
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then it is guaranteed to have only properties `p` and `q`.
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* Sealed tables support *width subtyping*: if a table has sealed type `{ p: number }`
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then it is guaranteed to have at least property `p`, so we allow `{ p: number, q: string }`
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to be treated as a subtype of `{ p: number }`
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* Unsealed tables can have properties added to them: if `t` has unsealed type
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`{ p: number }` then after the assignment `t.q = "hi"`, `t`'s type is updated to be
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`{ p: number, q: string }`.
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* Unsealed tables are subtypes of sealed tables.
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Currently the only way to create an unsealed table is using an empty table literal, so
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```lua
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local t = {}
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t.p = 5
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t.q = "hi"
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```
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typechecks, but
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```lua
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local t = { p = 5 }
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t.q = "hi"
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```
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does not.
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This causes problems in examples, in particular developers
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may initialize properties but not methods:
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```lua
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local t = { p = 5 }
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function t.f() return t.p end
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```
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## Design
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The proposed change is straightforward: make all table literals unsealed.
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## Drawbacks
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Making all table literals unsealed is a conservative change, it only removes type errors.
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It does encourage developers to add new properties to tables during initialization, which
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may be considered poor style.
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It does mean that some spelling mistakes will not be caught, for example
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```lua
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local t = {x = 1, y = 2}
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if foo then
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t.z = 3 -- is z a typo or intentional 2-vs-3 choice?
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end
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```
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In particular, we no longer warn about adding properties to array-like tables.
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```lua
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local a = {1,2,3}
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a.p = 5
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```
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## Alternatives
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We could introduce a new table state for unsealed-but-precise
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tables. The trade-off is that that would be more precise, at the cost
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of adding user-visible complexity to the type system.
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We could continue to treat array-like tables as sealed.
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