Go's value method receiver vs pointer method receiver

Question

I've read a Tour of Go and Effective Go, http://golang.org/doc/effective_go.html#pointers_vs_values, but still have a difficult time understanding when you would define a method on a struct using a value method receiver instead of a pointer method receiver. In other words, when would this:

type ByteSlice []byte

func (slice ByteSlice) Append(data []byte) []byte {
}

be preferable over this?

func (p *ByteSlice) Append(data []byte) {
    slice := *p
    *p = slice
}

Answer 1

Slices are one place where it's not always obvious at first. The Slice header is small, so copying it is cheap, and the underlying array is referenced via a pointer, so you can manipulate the contents of a slice with a value receiver. You can see this in the sort package, where the methods for the sortable types are defined without pointers.

The only time you need to use a pointer with a slice, is if you're going to manipulate the slice header, which means changing the length or capacity. For an Append method, you would want:

func (p *ByteSlice) Append(data []byte) {
    *p = append(*p, data...)
}