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Update compatibility.md

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Call out the upcoming change to `print` and slightly reword a couple of rows. Also add a list of differences from 5.x that was previously requested (the list is likely incomplete)
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Arseny Kapoulkine 2021-04-21 09:25:04 -07:00 • committed by GitHub
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@ -47,10 +47,11 @@ Sandboxing challenges are [covered in the dedicated section](sandbox).
| feature | status | notes |
|---------|--------|------|
| yieldable pcall and metamethods | ✔️/❌ | pcall/xpcall supports yielding but metamethods don't |
| yieldable pcall/xpcall | ✔️ | |
| yieldable metamethods | ❌ | significant performance implications |
| ephemeron tables | ❌ | this complicates the garbage collector esp. for large weak tables |
| emergency garbage collector | ❌ | Luau runs in environments where handling memory exhaustion in emergency situations is not tenable |
| goto statement | ❌ | this complicates the compiler due to handling of locals and doesn't address a significant need |
| goto statement | ❌ | this complicates the compiler, makes control flow unstructured and doesn't address a significant need |
| finalizers for tables | ❌ | no `__gc` support due to sandboxing and performance/complexity |
| no more fenv for threads or functions | 😞 | we love this, but it breaks compatibility |
| tables honor the `__len` metamethod | ❌ | performance implications, no strong use cases
@ -110,9 +111,16 @@ Floor division is less harmful, but it's used rarely enough that `math.floor(a/b
| `utf8` library accepts codepoints up to 2^31 | 🤷‍♀️ | no strong use cases |
| The use of the `__lt` metamethod to emulate `__le` has been removed | 😞 | breaks compatibility and doesn't seem very interesting otherwise |
| When finalizing objects, Lua will call `__gc` metamethods that are not functions | ❌ | no `__gc` support due to sandboxing and performance/complexity |
| The function print calls `__tostring` instead of tostring to format its arguments. | 🔜 | |
| The function print calls `__tostring` instead of tostring to format its arguments. | ✔️ | |
| By default, the decoding functions in the utf8 library do not accept surrogates. | 😞 | breaks compatibility and doesn't seem very interesting otherwise |
Lua has a beautiful syntax and frankly we're disappointed in the `<const>`/`<toclose>` which takes away from that beauty. Taking syntax aside, `<toclose>` isn't very useful in Luau - its dominant use case is for code that works with external resources like files or sockets, but we don't provide such APIs - and has a very large complexity cost, evidences by a lot of bug fixes since the initial implementation in 5.4 work versions. `<const>` in Luau doesn't matter for performance - our multi-pass compiler is already able to analyze the usage of the variable to know if it's modified or not and extract all performance gains from it - so the only use here is for code readability, where the `<const>` syntax is... suboptimal.
If we do end up introducing const variables, it would be through a `const var = value` syntax, which is backwards compatible through a context-sensitive keyword similar to `type`.
## Lua 5.x differences
We have a few behavior deviations from Lua 5.x that come from either a different implementation, or our desire to clean up small inconsistencies in the language/libraries:
* Order of table assignment in table literals follows program order in mixed tables (Lua 5.x assigns array elements first in some cases)
* Equality comparisons call `__eq` metamethod even when objects are rawequal (which matches other metamethods like `<=` and facilitates NaN checking)