What is the full slice expression a[low:high:max], and when would you use it?

a[low:high:max] makes a slice with len = high-low and cap = max-low. The constraint is 0 <= low <= high <= max <= cap(a). A plain a[low:high] keeps all the capacity up to the end of the backing array. That means an append on the subslice silently overwrites the parent's elements.

s := []int{1, 2, 3, 4, 5}
s2 := s[1:3]
fmt.Println(len(s2), cap(s2)) // 2 4
s2 = append(s2, 10)
fmt.Println(s)                // [1 2 3 10 5]  -- s[3] was clobbered

s = []int{1, 2, 3, 4, 5}
s3 := s[1:3:3]                // cap limited to 2
s3 = append(s3, 10)           // forces reallocation
fmt.Println(s, s3)            // [1 2 3 4 5] [2 3 10]

Use cases:

• Returning a subslice of an internal buffer when callers may append to it.
• Splitting one allocation into several independent chunks.
• The idiom append(s[:len(s):len(s)], x) to get a guaranteed copy-on-append.

The standard library does this too, for example in bytes.Split results. Note: strings support only the two-index form.