Length and capacity methods in slices

Reading about arrays and slices in Golang.

Why built-in functions like len and cap are used and not similar methods on slice objects?

if cap(slice) == len(slice) {
    fmt.Println("slice is full!")
}

vs

if slice.cap() == slice.len() {
    fmt.Println("slice is full!")
}

Why polluting the global namespace? Why not using object oriented style?

Also, do such methods exist and can they be used instead of built-in functions?

There is a FAQ entry on this, but here is my take:

len() and cap() are built in functions because they are understood by the compiler. You can't make your own replacements - they are very much part of the core language in a way that methods aren't, and you shouldn't confuse them with methods.

In Go a slice is made of three things, a length, a capacity and a pointer to the underlying memory. Eg the definition from reflect

type SliceHeader struct {
        Data uintptr
        Len  int
        Cap  int
}

All len(x) does is read the length from the slice structure, so it is shorthand for something like x.Len and the compiler understands that and optimises it accordingly.