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Update generalized-iteration.md

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Arseny Kapoulkine 2022-02-01 20:35:06 -08:00 • committed by GitHub
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rfcs/generalized-iteration.md
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@ -36,7 +36,13 @@ This is a general mechanism that can support iteration through many containers,
Thus, today the loop `for k, v in tab do` effectively executes `k, v = tab()` on the first iteration, which is why it yields `attempt to call a table value`. If the object defines `__call` metamethod then it can act as a self-iterating method, but this is not idiomatic, not efficient and not pure/clean.
This proposal comes in two pars: general support for `__iter` metamethod and default implementation for tables without one.
This proposal comes in two pars: general support for `__iter` metamethod and default implementation for tables without one. After this proposal, there's going to be a single, idiomatic, general and performant way to iterate through the object of any type:
```lua
for k, v in obj do
...
end
```
### __iter
@ -71,7 +77,7 @@ Luau compiler already emits a bytecode instruction, FORGPREP*, to perform initia
If the argument is a table and it does not implement `__iter` metamethod, we treat this as an attempt to iterate through the table using the builtin iteration order.
> Note: we also check if the table implements `__call`; if it does, we fall back to the default handling. We may be able to remove this check in the future, but we will need this initially to preserve backwards compatibility with custom table-driven iterator objects that implement `__call`.
> Note: we also check if the table implements `__call`; if it does, we fall back to the default handling. We may be able to remove this check in the future, but we will need this initially to preserve backwards compatibility with custom table-driven iterator objects that implement `__call`. In either case, we will be able to collect detailed analytics about the use of `__call` in iteration, and if neither is present we can emit a specialized error message such as `object X is not iteratable`.
To have a single, unified, iteration scheme over tables regardless of whether they are arrays or dictionaries, we establish the following semantics: