Golang未定义：err

I am trying out the simple following conversion program.

package main

import (
  "fmt"
  "strconv"
)

func main(){
  var numStr string="2213"
  var numVar int64
  numVar,err=strconv.ParseInt(numStr,10,64)
  fmt.Println(numVar)
}

The above throws the following compilation error.

undefined: err

Then I tried to define the err, error variable.

package main

import (
  "fmt"
  "strconv"
)

func main(){
  var numStr string="2213"
  var numVar int64
  var err error
  numVar,err=strconv.ParseInt(numStr,10,64)
  fmt.Println(numVar)
}

But then the compiler throws the following error this time.

err declared and not used

I referred to the following stackoverflow question Undefined err variable but did not get complete understanding of the behavior and hence asking the question here.

You declare err:

var err error

and then you assign something to it:

numVar,err=strconv.ParseInt(numStr,10,64)

but then you don't look at err or access it. Just assigning something a value doesn't count as using it, you have to look at err:

numVar,err=strconv.ParseInt(numStr,10,64)
if err != nil {
  // Complain or something here
}

If you really don't want to bother with the error, then say so by using the blank identifier:

numVar, _ = strconv.ParseInt(numStr, 10, 64)