Comment by lifthrasiir
2 years ago
I'm not sure how it can be possible. In my experience the notion of three-part slices does exist in C but only implicitly. For example,
size_t trim_end(const char *p, size_t len) {
while (len > 0) {
if (p[len - 1] != ' ') break;
--len;
}
return len;
}
Conceptually this function accepts a slice `(p, len, cap)` and returns a slice `(p, len2, cap)` where `len2 <= len` and the capacity never changes. But the actual argument doesn't have `cap`, and the return argument doesn't have `p`. Everything is implicit and it's typical for C programmers to fully document and follow such implicits. Go's slice operator can't come out of such implicit practices in my opinion.
In comparison, your claim only makes sense when the following was somehow normal:
struct slice { const char *p; size_t len, cap; };
struct slice trim_end(const struct slice *s) {
struct slice out = *s;
while (out.len > 0) {
if (out.p[out.len - 1] != ' ') break;
out = subslice(out, 0, out.len - 1);
}
return out;
}
Note that a hypothetical `subslice` function call maps perfectly to a Go code `out[0:len(out)-1]`, and my complaint will equally apply: there should be two clearly named variants of `subslice` that may or may not keep the capacity. But I hardly saw such construction in C.
I feel like were talking past each other. I'm not saying that C has a slicing operator, or that it's typical to define one as a function, or that it's typical to define a slice-like struct in C.
I'm saying that if you look at how arrays get used in C, you'll see that you're usually passing around extra numbers with them. So Go added syntax than encapsulates this. And it encapsulates the most general case (hence both a length and a capacity, even though most cases in C only use one number).
Instead of passing (char *, int) in C, you just pass (slice) in Go. And the Go slicing operator gives a nice syntax for selecting ranges of buffers.
But a Go slice is just a pointer to an array underneath, and I think it's best to always think about them that way. And then it's natural that mySlice[:2] would keep the same underlying capacity, and that future appends would mutate that capacity. Defaulting mySlice[:2] to mean mySlice[:2:2] seems less convenient overall to me (though prevents mistakes like in your original hi/lo example, but those come from not thinking of slices in terms of the underlying array).
> Instead of passing (char *, int) in C, you just pass (slice) in Go.
Maybe that's a point of disagreement here. I don't exactly see (char *, int) as a single entity, so it cannot be replaced with a slice (a definitely single entity) in my view.
They do appear together in arguments, but they have to be separate variables when you manipulate them and no C syntax will suggest that they are indeed related. So you have to rely on conventions (say, `name`, `name_len` and `name_cap`) which tend to be fragile. You can't expect such annoyance from Go slices, so I argue they are necessarily different from each other.
It works fine if you replace that by the same thing, which is what most langages (with slices) do.
The problem is that Go does not do that, because its designers could not be arsed to provide a separate vector type it’s (ptr, int, int) and now you can start stomping on out-of-slice memory unless the creator of the slice has used the extended slicing form.