Surprising behavior when passing arrays and slices to functions in Go

ex_passing_slices_and_arrays_by_val.go

package main

import (
	"fmt"
)

var a1 = [3]int{1, 2, 3}
var s1 = a1[0:3]

func main() {
	fmt.Println("array at beginning", a1)
	fmt.Println("slice at beginning", s1)
	passingByVal()
	fmt.Println("array at end", a1)
	fmt.Println("slice at end", s1)
}

func passingByVal() {
	fmt.Println("  array before outer calls", a1)
	fmt.Println("  slice before outer calls", s1)
	aByVal(a1)
	fmt.Println("  array between outer calls", a1)
	fmt.Println("  slice between outer calls", s1)
	sByVal(s1)
	fmt.Println("  array after outer calls", a1)
	fmt.Println("  slice after outer calls", s1)
}

func aByVal(a2 [3]int) {
	fmt.Println("    array before inner call", a2)
	a2[0] = 4
	fmt.Println("    array after inner call", a2)
}

func sByVal(s2 []int) {
	fmt.Println("    slice before inner call", s2)
	s2[1] = 5
	s2 = append(s2, 6)
	fmt.Println("    slice after inner call", s2)
}

output.txt

array at beginning [1 2 3]
slice at beginning [1 2 3]
  array before outer call [1 2 3]
  slice before outer call [1 2 3]
    array before inner call [1 2 3]
    array after inner call [4 2 3]
  array between outer calls [1 2 3]
  slice between outer calls [1 2 3]
    slice before inner call [1 2 3]
    slice after inner call [1 5 3 6]
  array after outer call [1 5 3]
  slice after outer call [1 5 3]
array at end [1 5 3]
slice at end [1 5 3]