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You can just create a sentinel value and use that. eg. `local mySentinel = {}`. Then insert that value when that's the behavior you want. `nil` on its own rarel

You can just create a sentinel value and use that. eg. `local mySentinel = {}`. Then insert that value when that's the behavior you want. `nil` on its own rarely makes for a good semantic understanding of what value is actually in the table anyways. Also, in Lua 5.4, the behavior is different -- you need to do `t[key] = undef` (which is the only context you can use `undef` in). And `nil` can actually be a value in a table. Not saying what's better or worse, but that the behavior in 5.4 is different.

pansa2 6y ago
Did `undef` actually make it into 5.4? I thought it was removed during the beta period.
- samatman 6y ago
  It was.
aasasd 6y ago
Do libraries like Penlight know about your sentinels or ‘undef’? Maybe in ten years they will use undef, or maybe you whipped up your own functional library and keep it optimized and full-featured. But now we have what we have. Besides, please tell: if you receive an empty object as a value in JSON, how do you preserve it in your ‘sentinel’ approach? Gotta need another special value for that.
- nikki93 6y ago
  For the JSON case, it doesn't make sense to use an empty object as a sentinel, for sure. JSON parsers usually have their own sentinel for `null`. Lua has a lot of issues, but `nil` being not-present hasn't been an issue that has come up a lot in practice. If you have any specific cases where that has been an issue, would be interesting to hear about them.
  - aasasd 6y ago
    As I wrote in the first comment, the use-case is e.g. API integration, which is what ‘scripting’ often is, these days. More widely, any transformations or filtering or helper code where you accept structures and pass them on after mild fiddling—again, quite typical use for Python or JS if you ever pipe data between scripts in the terminal or over the network. You don't put your data schema in each of those scripts and maintain them on every change at the source. Imagine implementing `jq` in Lua with all those ‘sentinels’ throughout the whole function library. The issue is that interoperability is crippled. The fact that you haven't so far run into these use-cases doesn't help with the problem that an entire, very common, class of functionality can't be done without tracking those nulls at each and every step with some crutches that bring their own troubles.
    - nikki93 6y ago
      Yeah I actually don't think Lua is great for those things, tbh. Lua is not a good tool for manipulating JSON--JavaScript is probably sensible for that (it is JavaScript's object format, after all). And yeah I don't think it's good to use a bunch of external libraries in Lua, since as you say there's not a common ground for how data is organized. The libraries I rely on are are more native integration things like lpeg, luasocket, luasec, love2d, ... They've all been quite good. And yeah you keep saying 'sentinels' in quotes but honestly it's worked fine when using lua-cjson. There's always an impedance mismatch between JavaScript's object format and Lua's though. Lua also allows non-string keys which are great in Lua but not a thing in JavaScript, and in practice I've found the array/object-table mismatch more problematic than `nil`/`null` and so on. I've mostly applied Lua for embedding in game engines, and to that end there's nothing that comes close, short of rolling your own VM that's exactly built the way you want. The main way to use it I think is to embed it somewhere and build up the set of utilities / libraries that you need to script over. But yeah it's not great for API glue code bc. APIs tend to bias toward JavaScript / JSON and other ways of doing things.
      - aasasd 6y ago
        Well, my lament in the first place is that Lua could be a good and fast general-purpose scripting language. Instead, I can have ‘good’ but then I'll have to sit around twiddling thumbs while Python or JS start up. Especially Fennel runs circles around ClojureScript in terms of toolchain and build-cycle lightness. JSON is pretty non-specific to JavaScript, which is probably a part of its popularity. And it's not the only serialization format. Null works about the same way in the majority of popular languages—so you're bound to need it for interoperability if you go outside C. By libraries, I mean Lua libs like Penlight. Once you touch functional programming and function composition, you want structures to be preserved as they are, unless explicitly mangled. > And yeah you keep saying 'sentinels' in quotes It's because I haven't seen that notion before, at least under this name.

pansa2 6y ago

Did `undef` actually make it into 5.4? I thought it was removed during the beta period.

samatman 6y ago
It was.

aasasd 6y ago

Do libraries like Penlight know about your sentinels or ‘undef’? Maybe in ten years they will use undef, or maybe you whipped up your own functional library and keep it optimized and full-featured. But now we have what we have. Besides, please tell: if you receive an empty object as a value in JSON, how do you preserve it in your ‘sentinel’ approach? Gotta need another special value for that.

nikki93 6y ago
For the JSON case, it doesn't make sense to use an empty object as a sentinel, for sure. JSON parsers usually have their own sentinel for `null`. Lua has a lot of issues, but `nil` being not-present hasn't been an issue that has come up a lot in practice. If you have any specific cases where that has been an issue, would be interesting to hear about them.
- aasasd 6y ago
  As I wrote in the first comment, the use-case is e.g. API integration, which is what ‘scripting’ often is, these days. More widely, any transformations or filtering or helper code where you accept structures and pass them on after mild fiddling—again, quite typical use for Python or JS if you ever pipe data between scripts in the terminal or over the network. You don't put your data schema in each of those scripts and maintain them on every change at the source. Imagine implementing `jq` in Lua with all those ‘sentinels’ throughout the whole function library. The issue is that interoperability is crippled. The fact that you haven't so far run into these use-cases doesn't help with the problem that an entire, very common, class of functionality can't be done without tracking those nulls at each and every step with some crutches that bring their own troubles.
  - nikki93 6y ago
    Yeah I actually don't think Lua is great for those things, tbh. Lua is not a good tool for manipulating JSON--JavaScript is probably sensible for that (it is JavaScript's object format, after all). And yeah I don't think it's good to use a bunch of external libraries in Lua, since as you say there's not a common ground for how data is organized. The libraries I rely on are are more native integration things like lpeg, luasocket, luasec, love2d, ... They've all been quite good. And yeah you keep saying 'sentinels' in quotes but honestly it's worked fine when using lua-cjson. There's always an impedance mismatch between JavaScript's object format and Lua's though. Lua also allows non-string keys which are great in Lua but not a thing in JavaScript, and in practice I've found the array/object-table mismatch more problematic than `nil`/`null` and so on. I've mostly applied Lua for embedding in game engines, and to that end there's nothing that comes close, short of rolling your own VM that's exactly built the way you want. The main way to use it I think is to embed it somewhere and build up the set of utilities / libraries that you need to script over. But yeah it's not great for API glue code bc. APIs tend to bias toward JavaScript / JSON and other ways of doing things.
    - aasasd 6y ago
      Well, my lament in the first place is that Lua could be a good and fast general-purpose scripting language. Instead, I can have ‘good’ but then I'll have to sit around twiddling thumbs while Python or JS start up. Especially Fennel runs circles around ClojureScript in terms of toolchain and build-cycle lightness. JSON is pretty non-specific to JavaScript, which is probably a part of its popularity. And it's not the only serialization format. Null works about the same way in the majority of popular languages—so you're bound to need it for interoperability if you go outside C. By libraries, I mean Lua libs like Penlight. Once you touch functional programming and function composition, you want structures to be preserved as they are, unless explicitly mangled. > And yeah you keep saying 'sentinels' in quotes It's because I haven't seen that notion before, at least under this name.