Why people dislike global variables so much while I find them so convenient?

bauss jj_1337 at live.dk
Wed Jan 26 08:49:02 UTC 2022

On Wednesday, 26 January 2022 at 07:51:30 UTC, rempas wrote:
> On Tuesday, 25 January 2022 at 12:07:31 UTC, Ola Fosheim 
> Grøstad wrote:
>> Because the program is larger, there are more locations that 
>> could mutate the global state. ("reason" ≈ "prove correct")
> Yeah but what if it makes sense for these places to mutate it? 
> For example, I'm making a parser for my language and each 
> different statement will be parsed from a function that will be 
> in a different file. It makes sense for those files to be able 
> to mutate some global variables.
>> 1. globals are singletons, it becomes difficult to have 
>> multiple instance if you later regret having only one instance.
>> 2. because you need to use locking correctly to prevent 
>> multiple threads from mutating the state at once and then you 
>> have to prove that your locking strategy is does not lead to 
>> starvation or deadlocks.
> I don't know about most of this terms and I know only a little 
> about multi-threading programming. So thank you, I will make my 
> research and this info will come in handy!
>> I don't understand what you are trying to say here.
>> The global variable should be local to the object file. The 
>> accessor functions can be globally accessible.
> Actually the example I made with the parser will explain that 
> too. I want some specific files to be able to access the global 
> variables. If they are only local to a file then they don't 
> help me a lot and I still have to use pointers to modify that 
> specific data.
>> Passing one pointer isn't particularly expensive. What 
>> (non-TLS) globals save you on some CPUs are registers and time 
>> spent on allocation.
> Thanks! Why talking about allocations tho? We will not need to 
> allocate any memory to pass a pointer to a function and then 
> deference it. Of course if you think that you will push it to 
> the stuck in your main function and then you will call all the 
> other functions in the top.
>> For instance,  using globals can make sense in embedded 
>> programming on tiny CPUs with very little RAM.
> Well yeah! However, embedded programming is an advanced topic 
> so I suppose different rules apply there anyways...
>> https://dlang.org/spec/attribute.html#gshared
> Thanks a lot!!

For a compiler, I'd think each step shouldn't modify state but 
rather you pass the current state to the next step.

Ex. first you parse lexical information and you pass that 
information to the semantic analysis, which then passes that 
information on etc.

You should only "modify" the current localized state IMHO. Ex. 
the current statement being parsed can and should be modified, 
but you shouldn't be able to modify anything else like suddenly 
modifying the compiler configurations in the middle of semantic 
analysis etc.

I think it's especially important for compilers to not use global 
state because you want to be able to localize every single issue 
right away to a specific unit in the compiler.

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