Comment by gwd 2 days ago
So sometimes you want it lexical scope, and sometimes function scope; For example, maybe you open a bunch of files in a loop and need them all open for the rest of the function.
Right now it's function scope; if you need it lexical scope, you can wrap it in a function.
Suppose it were lexical scope and you needed it function scope. Then what do you do?
What's an example of where you'd need to do that?
I can't recall ever needing that (but that might just be because I'm used to lexical scoping for defer-type constructs / RAII).
Someone already replied, but in general when you conditionally acquire a resource, but continue on failing. E.g., if you manage to acquire it, defer the Close() call, otherwise try to get another resource.
Another example I found in my code is a conditional lock. The code runs through a list of objects it might have to update (note: it is only called in one thread). As an optimization, it doesn't acquire a lock on the list until it finds an object that has to be changed. That allows other threads to use/lock that list in the meantime instead of waiting until the list scan has finished.
I now realize I could have used an RWLock...
I frequently use that pattern. For example, something like this:
a := Start()
if thingEnabled {
thing := connectToThing()
defer thing.Close()
a.SetThing(thing)
}
a.Run(ctx)Really? I find the opposite is true. If I need lexical scope then I’d just write, for example
f.Close() // without defer
With lexical scope, there’s only three ways to safely jump the scope:
1. reaching the end of the procedure, in which case you don’t need a defer)
2. A ‘return’, in which case you’re also exiting the function scope
3. a ‘break’ or ‘continue’, which admittedly could see the benefit of a lexical scope defer but they’re also generally trivial to break into their own functions; and arguably should be if your code is getting complex enough that you’ve got enough branches to want a defer.
If Go had other control flows like try/catch, and so on and so forth, then there would be a stronger case for lexical defer. But it’s not really a problem for anyone aside those who are also looking for other features that Go also doesn’t support.
I don't write Go, but reading this, it feels like you don't actually need functional scoping at all. In Java, I would simply write:
try (SomeResource foo = SomeResource.open()) { method(foo); }
or
public void method() { try(...) { // all business logic wrapped in try-with-resources } }
To me it seems like lexical scoping can accomplish nearly everything functional scoping can, just without any surprising behavior.
Minor nit but the second example is ostensibly still just functional scoping (as in not literally but more pragmatically) because you don’t contain any branching outside of the try block within your method.
But that’s a moot point because I appreciate it’s just an example. And, more importantly, Go doesn’t support the kind of control flows you’re describing anyway (as I said in my previous post).
A lot of the comments here about ‘defer’ make sense in other languages that have different idioms and features to Go. But they don’t apply directly to Go because you’d have to make other changes to the language first (eg implementing try blocks).
That's a theoretical problem that almost never surfaces in practice.
Using `defer...recover` is computationally expensive within hot paths. And since Go encourages errors to be surfaced via the `error` type, when writing idomatic Go you don't actually need to raise exceptions all that often.
So panics are reserved for instances where your code reaches a point that it cannot reasonably continue.
This means you want to catch panics at boundary points in your code.
Given that global state is an anti-pattern in any language, you'd want to wrap your mutex, file, whatever operations in a `struct` or its own package and instantiate it there. So you can have a destructor on that which is still caught by panic and not overly reliant on `defer` to achieve it.
This actually leads to my biggest pet peeve in Go. It's not `x, err := someFunction()` and nor is it `defer/panic`, these are all just ugly syntax that doesn't actually slow you down. My biggest complaint is the lack of creator and destructor methods for structs.
the `NewClass`-style way of initialising types is an ugly workaround and it constantly requires checking if libraries require manual initilisation before use. Not a massive time sink but it's not something the IDE can easily hint to you so you do get pulled from your flow to then either Google that library or check what `New...` functions are defined in the IDEs syntax completion. Either way, it's a distraction.
The lack of a destructor, though, does really show up all the other weaknesses in Go. It then makes `defer` so much more important than it otherwise should be. It means the language then needs runtime hacks for you to add your own dereference hooks[0][1] (this is a problem I run into often with CGO where I do need to deallocate, for example, texture data from the GPU). And it means you need to check each struct to see if it includes a `Close()` method.
I've heard the argument against destructors is that they don't catch errors. But the counterargument to that is the `defer x.Close()` idiom, where errors are ignored anyway.
I think that change, and tuples too so `err` doesn't always have to be its own variable, would transform Go significantly into something that feels just as ergonomic as it is simple to learn, and without harming the readability of Go code.
[0] https://medium.com/@ksandeeptech07/improved-finalizers-in-go...
[1] eg https://github.com/lmorg/ttyphoon/blob/main/window/backend/r...
You put the files in the collection as you open them, and you register the defer before opening any of them. It works fine. Defer should be lexically scoped.
Making it lexical scope would make both of these solvable, and would be clear for anyone reading it.
You can just introduce a new scope wherever you want with {} in sane languages, to control the required behavior as you wish.