How to Correctly Append Byte Slices and Arrays in Go?

Appending Byte Slices and Arrays in Go: Troubleshooting and Solutions

In Go, appending two byte array slices can lead to unexpected errors. Consider the following code:

one := make([]byte, 2)
two := make([]byte, 2)
one[0] = 0x00
one[1] = 0x01
two[0] = 0x02
two[1] = 0x03

log.Printf("%X", append(one[:], two[:]))

three := []byte{0, 1}
four := []byte{2, 3}

five := append(three, four)

This code produces the following errors:

cannot use four (type []uint8) as type uint8 in append
cannot use two[:] (type []uint8) as type uint8 in append

Contrary to Go's reputation for robust slices, these errors seem inexplicable. To understand and resolve this issue, we need to delve into the Go Programming Language Specification.

Understanding the Specification

According to the Go Specification, the append function takes a slice s of type S and zero or more values x of type T, where T is the element type of S. The result is a new slice of type S with the elements of s followed by the elements of x.

Crucially, for the final argument, if it is assignable to a slice type []T, it can be passed unchanged as the value for a ...T parameter if followed by ....

Addressing the Errors

In the provided code, the final argument to append is not followed by .... To resolve this, we need to explicitly specify []T... for the final argument. In our case, T is byte, so the code should be:

fmt.Println(append(one[:], two[:]...))

For the second example, we should make a similar modification:

five := append(three, four...)

Additional Notes

• The ... operator in Go allows us to pass a slice as a variadic argument.
• When appending slices of different lengths, the resulting slice will have a length equal to the sum of the lengths of the input slices.
• The []T... syntax is also used when we want to initialize a slice with a variadic list of elements.

Conclusion

By understanding the nuances of the append function and the Go Specification's guidelines for variadic arguments, we can effectively append byte arrays in Go and avoid the errors encountered in the original code.

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