：=运算符和Golang中的if语句

Question

The following works as a function to open a file

func openFile(filename string) {
  var file *os.File
  var err error
  if file, err = os.Open(filename); err != nil {
    log.Printf("Failed to open the file: %s.", filename)
    return
  }
  defer file.Close()
  // blahblahblah
}

however, this does not work, when I try to use := to declare the variable file

func updateFrequencies(filename string, frequencyForWord map[string]int) {
  if file, err := os.Open(filename); err != nil {
     ....
  }
}

error: ./word_frequencies_2.go:30: undefined: file

But if I changed this slightly, this works

file, err := os.Open(filename)
if err != nil {
   log.Printf("Failed to open the file: %s.", filename)
   return
}

why can't I use := as part of the if statement?

Answer 1

Why can't I use := as part of the if statement?

You can, but then the variables are defined within the scope of the if block. So, file is not defined outside of your if block.

The same rule applies to definitions in for, switch, and similar blocks.