If the slice syntax creates a copy, I would argue that it's simply syntactic sugar for array copying, not actually producing a slice (i.e. there is no slice type). But yes, `somefunc(ary[1:])` in Python produces a copy, not a reference to the underlying value. You could build a more "true" slice class, but the builtin stuff doesn't do that. JavaScript is similar.

Java however has `List<>.sublist` which largely behaves like Go: https://docs.oracle.com/javase/8/docs/api/java/util/List.htm... . Sometimes these kinds of things are also referred to as "views". Go calls them slices though, and this is a Go article.

edit: C# apparently has "spans": https://learn.microsoft.com/en-us/archive/msdn-magazine/2018...

It doesn’t look like any of those allow you to arbitrarily add and remove elements from the parent list in the same way Go does (if I understand Go slices correctly).

Java does let you make modifications, but only under very tight restrictions:

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

I take that to mean the parent list can’t be modified at the same time, nor can multiple views be modified.

So, I think Go’s ability to do those structural modifications on slices is rather unusual (as well as error-prone and not obviously useful).

Go slices cannot change the structure (len or cap) of their "parent" slice. (Unlike Java, the semantics of every operation are even well-defined.) In some sense this is the root of the problem - the "child" slice can start using spare capacity it 'inherited' with no way to tell the parent it has done so, and the parent may inadvertantly pass some of its length as capacity down to its child.

You can't insert or remove elements, but you can overrun the buffer??

I have a hard time thinking of a situation when that would be useful at all, let alone useful enough to be the default and widely-used behaviour! If I want to let the callee add stuff to the end of my list, can't I just pass them the whole list and let them modify that?

You cannot "overrun the buffer" nor can a callee add stuff to the end of your list; you can give them a memory buffer with unused space and they may use it, and then you can also mistakenly use it later.

Honestly, you seem to be too detached to understand this. If you really want to know how it works go through some official Go documentation. It's not fundamentally different than some feature in other languages, it's just that in Go this is the default growable list type and in other languages it's usually a non-default type.

As for whether it's ultimately useful - of course we can debate, but everyone still builds lists one element at a time and allocators still allocate by doubling, so there's at least one immediate and obvious use of capacity beyond the used length.

I coincidentally was just looking at some docs, and I think I have a handle on it (to it?:) now.

The specific strange design decision in Go is that you can create a child slice that has its own starting offset and length, but may or may not inherit its parent’s capacity. Just because it’s well-defined doesn’t mean it isn’t tricky.

Even if you kept the semantics the same but made cap=len for subslices by default, surely that would be an improvement. Rather than a “rare slice trick”, it ought to be normal. Or is there an advantage to the current default that I’m overlooking?

I mean, if you want Java, it's right there.

The advantage is that you get the fast path by default. Since Go programs are generally not awash in buffer reuse bugs despite its semantic trickiness, this seems like something different languages can reasonably prioritize differently.

cap==len is effectively the default - most slicing is done with [start:end], not [start:end:additional-cap]

No it’s not -- I just checked the language specification and it says:

After slicing the array a

a := [5]int{1, 2, 3, 4, 5}

s := a[1:4]

the slice s has type []int, length 3, capacity 4

Many of us here are arguing that the default should be cap==len==3, but it’s not, it’s 4.

Welp. TIL.

Yeah, that's much more error prone I think. It's the sort of thing that only kinda makes sense on zero-valued arrays... and even then it's dubious at best.

Bleh. I'm gonna go review some old code now D:

Another example is typed arrays in JavaScript, which have both .slice() and .subarray(). One creates a copy, and one creates a new view into the same underlying memory.

You can’t change the length of a JS ArrayBuffer, or write past the end of your view.

The difference is that those operations are on the root ArrayBuffer in JS, not the TypedArray views on that buffer.

In Go, the underlying array is fixed, and each slice independently has resize/reallocate operations.

The JS approach seems better and more comprehensible to me. (I note also that MDN lists “resizable” and “maxByteLength” as new experimental features.)