How to get undeclared global variable warning on interface?

[Wuzzy]

Hi!
In a previous version of Minetest, Minetest showed a warning directly on the interface when an undeclared global variable has been accessed.
But now, this feature got removed again and it is only written in debug.txt.

Personally, I find this warning one of the most useful warnings of Minetest, because accessing undeclared global variables is not only bad practice, but a likely cause of bugs. Just today I have squashed another bug with the help of these warnings.
IMO a good Lua script should avoid this completely.

First question: How do I get the undeclared global variable warning on interface?

Second question: Why is it not shown in the interface by default anymore? I know, many mods use this kind of bad practise, so you'll get many such warnings, but it is unintentionally most of the time and it is very easy to forget a “local” and (seemingly) get away with it.
Screaming it right into the interface would force mod makers to fix this kind of bad practice. Also, because mod users would quickly start to complain. But maybe there is a better rationale to remove it from the interface anyways?

But I certainly want Minetest to scream loudly and proudly this warning to the interface, at least in my Minetest installation.

Here are some examples of the warning messages I am talking about:

16:29:15: WARNING: Undeclared global variable "pos" accessed at ...test/games/hungry_games/mods/hungry_games/engine.lua:27
16:30:57: WARNING: Assignment to undeclared global "obj" inside a function at ...test/games/hungry_games/mods/hungry_games/engine.lua:36.
16:30:57: WARNING: Assignment to undeclared global "winnerPos" inside a function at ...test/games/hungry_games/mods/hungry_games/engine.lua:107.
16:30:57: WARNING: Assignment to undeclared global "winnerName" inside a function at ...test/games/hungry_games/mods/hungry_games/engine.lua:108.

Most of these bugs are fixed by simply writing “local” in front of the name. Or you make an explicit global assignment to the variable beforehand, if it was that you wanted to do.

[rubenwardy]

It's confusing for new players.

https://github.com/minetest/minetest/bl ... ua#L51-L52

Change

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warn(("Undeclared global variable %q accessed at %s:%s")
            :format(name, info.short_src, info.currentline))

to

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minetest.log("error", ("Undeclared global variable %q accessed at %s:%s")
            :format(name, info.short_src, info.currentline))

[Wuzzy]

It's confusing for new players.

True, but the warning really should not be there in the first place. The modder is to blame. ;-)
Anyways, it's not that important.

But thanks for you help. :-)

[prestidigitator]

There's one case where I don't like the warning. Sometimes when creating a namespace you don't want to depend on the order of execution. So you might write this in two or more different places:

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namespace = namespace or {};

namespace.var = ...;
function namespace.func() ... end;


I'd be nice to differentiate a simple check for nil like this and true dependence on value, but there's really no such (simple, built-in) mechanism in Lua. You'd have to iterate through all the keys of the table or create a secondary table with a dedicated API for presence testing or something like that. Maybe:

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namespace = existing_or_empty_global_table("namespace");


...which is quite ugly (though admittedly clear in its meaning).

[TeTpaAka]

There's one case where I don't like the warning. Sometimes when creating a namespace you don't want to depend on the order of execution. So you might write this in two or more different places:

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Code: Select all

namespace = namespace or {};

namespace.var = ...;
function namespace.func() ... end;


I'd be nice to differentiate a simple check for nil like this and true dependence on value, but there's really no such (simple, built-in) mechanism in Lua. You'd have to iterate through all the keys of the table or create a secondary table with a dedicated API for presence testing or something like that. Maybe:

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Code: Select all

namespace = existing_or_empty_global_table("namespace");


...which is quite ugly (though admittedly clear in its meaning).

There seems to be a way, I think. Intllib used to do it this way:
https://github.com/kaeza/minetest-intllib/commit/2ef1abad55f0e1b8b76169cdb1da3bc3f8749c67#diff-d7d74284e2e39f6c06d199c8e2bedbd0R2

[prestidigitator]

There seems to be a way, I think. Intllib used to do it this way:
https://github.com/kaeza/minetest-intllib/commit/2ef1abad55f0e1b8b76169cdb1da3bc3f8749c67#diff-d7d74284e2e39f6c06d199c8e2bedbd0R2

Hmm. It's a way of getting around the warning, true, but it's also perhaps a little subversive to use rawget() or rawset(). For example, if metatables were used for additional functionality using an always empty global table (e.g. to detect when certain variables are changed as well as initially set), using rawget() might always return nil. To be fair I suppose such a mechanism could replace the rawget() and/or rawset() functions too, but outside a security mechanism there's often a limit to the amount of paranoia you want to apply.

[rubenwardy]

See minetest.global_exists()

It's confusing for new players.

True, but the warning really should not be there in the first place. The modder is to blame. ;-)
Anyways, it's not that important.

But thanks for you help. :-)

You can't rely on maintainers to go through all their mods and correct this. It would be nice to have a mod debug mode. The craft recipe checker could be included too. Although mod developers could just not know about such a mode.

If there were no existing mods, it would be fine to put it on the interface. Except maybe that libraries such as Lua socket trigger the warnings.



As an additional note, I much prefer if foobar then to if minetest.global_exists("foobar") then

[prestidigitator]

See minetest.global_exists()

Cool. Added to dev wiki.

[christoferlevich]

Most of these bugs are fixed by simply writing “local” in front of the name.

Please forgive me if this question sounds ignorant, I am very new to the game, but are you saying if I simply add the word 'local' to the file it solves the error? I would like to minimize errors regardless of their impact on the game. I am using it in a school district, and I want to get the students involved in the game and the coding (as we go).

[Wuzzy]

No, the word “local” must be written in front of the variable name, not in front of the file name.

I think it would be best if you beef up your Lua knowledge first and learn about scope, local variables and global variables. :P

The general Minetest recommendation is to only use variables by default and only use them if you REALLY need them. And if you use them, use them with care. Because all mods have to share the same namespace which means that the more global variables exist, the larger the risk of a conflict.

(I am looking at you, countless forks of PilzAdam's “Simple Mobs” mod which all overwrite the same global variable called “mobs”. >:O )

[rubenwardy]

I suggest having a look at these two resources:

http://rubenwardy.com/minetest_modding_ ... s/lua.html
https://www.lua.org/pil/

[Byakuren]

(I am looking at you, countless forks of PilzAdam's “Simple Mobs” mod which all overwrite the same global variable called “mobs”. >:O )

Not to mention all of the global variables not called "mobs" that get overwritten by those mods.

[christoferlevich]

No, the word “local” must be written in front of the variable name, not in front of the file name.

I think it would be best if you beef up your Lua knowledge first and learn about scope, local variables and global variables. :P

I'm working on it. I tend to always jump way ahead of where I should be when learning. Thanks for the response. In time, I won't be so 'green'