i think the most interesting design detail of lua tables is just glossed over as “nil-holes” in this article. namely, that nil values do not exist. there is no table.delete(key) method, you just zero out the value and the key stops existing. the same thing is true for any variable, if you set it to nil it ceases to be. i find that implementation fascinating.
I assume they did that for performance reasons, because removing entries is slow probably? From user perspective, it would have made it more sense to remove the key instead defining it as nil and then expecting the user to handle the nil. The key does not stop existing, right? I don’t program in Lua.
Except it’s ass if you want to do non-destructive data processing of arbitrary structures and your input and output might have null as a value. You can’t just know about fields a, b, and c of the table and leave everything else as it is, you need to know the whole structure and make sure you write null in the output for fields that have nil in them.
Or, more realistically, use libraries that implement null as custom user data.
Iirc Roberto Ierusalimschy even considered introducing a null value in one of the recent versions, of course confusingly named ‘undefined’ — but changed his mind. Perhaps it’s for the better than to have such a backwards name for it.
To my knowledge, Lisps like Emacs Lisp implement this better: trying to get a value for a nonexistent key will get you nil, but you can still retrieve the list of all keys, including ones that are set to nil.
i think the most interesting design detail of lua tables is just glossed over as “nil-holes” in this article. namely, that nil values do not exist. there is no
table.delete(key)method, you just zero out the value and the key stops existing. the same thing is true for any variable, if you set it to nil it ceases to be. i find that implementation fascinating.I assume they did that for performance reasons, because removing entries is slow probably? From user perspective, it would have made it more sense to remove the key instead defining it as nil and then expecting the user to handle the nil. The key does not stop existing, right? I don’t program in Lua.
Except it’s ass if you want to do non-destructive data processing of arbitrary structures and your input and output might have null as a value. You can’t just know about fields a, b, and c of the table and leave everything else as it is, you need to know the whole structure and make sure you write null in the output for fields that have nil in them.
Or, more realistically, use libraries that implement null as custom user data.
Iirc Roberto Ierusalimschy even considered introducing a null value in one of the recent versions, of course confusingly named ‘undefined’ — but changed his mind. Perhaps it’s for the better than to have such a backwards name for it.
To my knowledge, Lisps like Emacs Lisp implement this better: trying to get a value for a nonexistent key will get you nil, but you can still retrieve the list of all keys, including ones that are set to nil.
yeah that’s probably when you should drop down to C.