cannot use type []rune as type rune in append

Question

package main

var lettersLower = []rune("abcdefghijklmnopqrstuvwxyz")
var lettersUpper = []rune("ABCDEFGHIJKLMNOPQRSTUVWXYZ")

func main() {
    x := append(lettersLower, lettersUpper)
}

Why does this not work? How can I append lettersLower and lettersUpper?

prog.go:7: cannot use lettersUpper (type []rune) as type rune in append

https://play.golang.org/p/ovx_o2rKPC

Answer 1

It's because append doesn't take a list to append, but rather one or more items to append. You can adapt to this with a ... on the second argument to append:

package main

import "fmt"

var lettersLower = []rune("abcdefghijklmnopqrstuvwxyz")
var lettersUpper = []rune("ABCDEFGHIJKLMNOPQRSTUVWXYZ")

func main() {
    x := append(lettersLower, lettersUpper...)
    fmt.Println(len(x))
}

Try it out on the Playground.

Note that append does not always re-allocate the underlying array (that would cause problems in terms of performance and memory usage). You're fine as far as this sample goes, but it may bite you if you ever try to use the same memory for multiple purposes. A (contrived, perhaps unclear) example:

package main

import (
    "fmt"
    "os"
)

func main() {
    foo := []byte("this is a BIG OLD TEST!!\n")
    tst := []byte("little test")
    bar := append(foo[:10], tst...)

    // now bar is right, but foo is a mix of old and new text!
    fmt.Print("without copy, foo after:  ")
    os.Stdout.Write(foo)

    // ok, now the same exercise but with an explicit copy of foo
    foo = []byte("this is a BIG OLD TEST!!\n")
    bar = append([]byte(nil), foo[:10]...) // copies foo[:10]
    bar = append(bar, tst...)

    // this time we modified a copy, and foo is its original self
    fmt.Print("with a copy, foo after:   ")
    os.Stdout.Write(foo)
}

When you try to print foo after appending to a subslice of it, you get a weird mix of old and new content.

Where the shared underlying array is a problem, you could either use strings (string bytes are immutable, a pretty effective guard against accidental overwrites) or make a copy as I did with append([]byte(nil), foo[:10]...) above.