In my opinion this is actually kinda awesome, because it’s as if every element in the domain maps to an element in the range, so there are no disjoint or undefined mappings in a table. It’s not how we’re used to thinking but when the logic is designed correctly it’s pretty useful and easy.
Should accessing an absent record really be an _error_? Shouldn’t it just be a “None” or “Missing” option or something like that instead? It doesn’t seem to me like indexing a set with an absent key is really an error, it’s just not a value.
There are definitely different approaches with different tradeoffs. Lua tends to prioritize implementation simplicity so I would guess that's a factor here.
Ideally yeah you'd return the Nothing side of an Option type but that's not representable in lua's type system. Returning an {:ok, data} tuple erlang-style is a pretty solid middle ground, and that is a common convention in lua with multiple return of ok, result.
But throwing an error has advantages too. There is a potentially important semantic difference between an unused key and a null data. The difference becomes especially important when you go mixing data types IMO. I'm begrudgingly fine with a hashmap returning null for unset keys, but not with an array returning it for an out of bounds index. With lua's approach you can't easily differentiate these things and it does cause serious problems. Everyone hated this in php, I don't see why it's such a popular choice when lua does it.
The problem with bounds errors is that it brings little value for a correct program and there are better ways to help with correctness than a runtime error (but that transcends Lua-like langs).
Looking at python, it’s more often and irritating to convert to .get() after an error than receiving an error and realizing it was helpful. Code that optimistically assumes non-null results rarely survives few next lines after getting nulls anyway.
Should accessing an absent record really be an _error_? Shouldn’t it just be a “None” or “Missing” option or something like that instead? It doesn’t seem to me like indexing a set with an absent key is really an error, it’s just not a value.