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Ah, lua-users.org... That wiki which pretty much looks like the comment section in the PHP manual but with a less chance to be correct for many reasons. And I'm

Ah, lua-users.org... That wiki which pretty much looks like the comment section in the PHP manual but with a less chance to be correct for many reasons. And I'm not sure if there's other (preferably more trusted) source of snippets. Back in time when I messed with Lua [1], I needed a version of `pairs` that returns keys in the sorted order. Lua-users has a page for that [2], so I looked it up, and found that it has a subtle but important enough bug---it relies on `__orderedIndex` stored in the original table, which never gets updated nor GCed. I'm sure many enough people had been and will be tripped up by this snippet. [1] So much that I had written a type checker for Lua: https://github.com/devcat-studio/kailua/ https://github.com/devcat-studio/kailua/ [2] http://lua-users.org/wiki/SortedIteration http://lua-users.org/wiki/SortedIteration

JonChesterfield 4y ago
` t.__orderedIndex = nil` looks like it cleans up at the end of the iteration, am I missing something? Mutating a table while iterating over it is fairly hazardous anyway
- lifthrasiir 4y ago
  Ah, yeah, the problem won't be visible with a normal loop. However if you break out of the loop without hitting the very last key, or remove a key during the iteration [1], or loop against the same table inside another loop, then the snippet will fail in unexpected ways. The correct way to do this is as follows. First you collect all keys and sort them as usual, and then return an iterator function that contains those keys as an upvalue. Essentially you need to create a single-use `pairs` function that holds all necessary states. [1] You can't insert a key during the iteration, but removing an existing key is guaranteed to work correctly, no matter if you've already past the deleted key or not: https://www.lua.org/manual/5.4/manual.html#pdf-next https://www.lua.org/manual/5.4/manual.html#pdf-next

JonChesterfield 4y ago

` t.__orderedIndex = nil` looks like it cleans up at the end of the iteration, am I missing something? Mutating a table while iterating over it is fairly hazardous anyway

lifthrasiir 4y ago
Ah, yeah, the problem won't be visible with a normal loop. However if you break out of the loop without hitting the very last key, or remove a key during the iteration [1], or loop against the same table inside another loop, then the snippet will fail in unexpected ways. The correct way to do this is as follows. First you collect all keys and sort them as usual, and then return an iterator function that contains those keys as an upvalue. Essentially you need to create a single-use `pairs` function that holds all necessary states. [1] You can't insert a key during the iteration, but removing an existing key is guaranteed to work correctly, no matter if you've already past the deleted key or not: https://www.lua.org/manual/5.4/manual.html#pdf-next https://www.lua.org/manual/5.4/manual.html#pdf-next