Google: "GoLang compute md5 of file"

Title of this post is my last Google query.
It got me there: https://www.socketloop.com/tutorials/how-to-generate-checksum-for-file-in-go. After reading I was surprise that Go doesn't have ReaderWriter which reads data from some object implementing io.Reader and writes data to some other object implementing io.Writer. I've grabbed the code and started cleaning it but keeping all file content in memory made me search.
I went to GoLang official doc and found: func Copy(dst Writer, src Reader) (written int64, err error)
Copy copies from src to dst until either EOF is reached on src or an error occurs.
To summarize:

• Go defines interface hash.Hash which is implemented in packages: crypto/hmac, crypto/md5, crypto/sha1, crypto/sha256, crypto/sha512.
• hash.Hash inherits io.Writer, this mean it has Write method.
• One should use io.Copy to copy data between reader and writer.

Below is the full source of main.go:

package main

import (
  "crypto/md5"
  "fmt"
  "io"
  "os"
)

func ComputeMd5(filePath string) ([]byte, error) {
  var result []byte
  file, err := os.Open(filePath)
  if err != nil {
    return result, err
  }
  defer file.Close()

  hash := md5.New()
  if _, err := io.Copy(hash, file); err != nil {
    return result, err
  }

  return hash.Sum(result), nil
}

func main() {
  if b, err := ComputeMd5("main.go"); err != nil {
    fmt.Printf("Err: %v", err)
  } else {
    fmt.Printf("main.go md5 checksum is: %x", b)
  }
}

$ go run main.go
main.go md5 checksum is: facd74ec8975d8fd84897fb352f8f87e