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Update generalized-iteration.md (#640)

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Use a more precise pseudocode snippet for __iter handling which matches runtime semantics
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Arseny Kapoulkine 2022-08-17 10:02:08 -07:00 committed by GitHub
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@ -51,8 +51,10 @@ end
To support self-iterating objects, we modify the iteration protocol as follows: instead of simply expanding the result of expression `iter` into three variables (`gen`, `state` and `index`), we check if the first result has an `__iter` metamethod (which can be the case if it's a table, userdata or another composite object (e.g. a record in the future). If it does, the metamethod is called with `gen` as the first argument, and the returned three values replace `gen`/`state`/`index`. This happens *before* the loop: To support self-iterating objects, we modify the iteration protocol as follows: instead of simply expanding the result of expression `iter` into three variables (`gen`, `state` and `index`), we check if the first result has an `__iter` metamethod (which can be the case if it's a table, userdata or another composite object (e.g. a record in the future). If it does, the metamethod is called with `gen` as the first argument, and the returned three values replace `gen`/`state`/`index`. This happens *before* the loop:
```lua ```lua
if getmetatable(gen) and getmetatable(gen).__iter then local genmt = rawgetmetatable(gen) -- pseudo code for getmetatable that bypasses __metatable
gen, state, index = getmetatable(gen).__iter(gen) local iterf = genmt and rawget(genmt, "__iter")
if iterf then
gen, state, index = iterf(gen)
end end
``` ```