...

package main

import (
  "fmt"
  "error"
  "math"
)

func main() {
  result, err := sqrt(16)
  
  if err != nil {
    fmt.Println(err)
  } else {
    fmt.Println(result)
  }
}

func sqrt(x float64) (float64, error ) {
  if x < 0 {
    return 0, errors.New("Undefined for negative numbers")
  }
  return math.Sqrt(x), nil
} 

Method Set

func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    ...
}

func (s *GracefulServer) BlockingClose() bool {
    ...
}

This is called the 'receiver'. In the first case (h handler) it is a value type, in the second (s *GracefulServer) it is a pointer. The way this works in Go may vary a bit from some other languages. The receiving type however, works more or less like a class in most object-oriented programming. It is the thing you call the method from, much like if I put some method A in side some class Person then I would need an instance of type Person in order to call A (assuming it's an instance method and not static!).

One gotcha here is that the receiver gets pushed onto the call stack like other arguments so if the receiver is a value type, like in the case of handler then you will be working on a copy of the thing you called the method from meaning something like h.Name = "Evan" would not persist after you return to the calling scope. For this reason anything that expects to change the state of the receiver, needs to use a pointer or return the modified value (gives more of an immutable type paradigm if you're looking for that).

Here's the relevant section from the spec; https://golang.org/ref/spec#Method_sets

Structs/Types

Example of Person Type

...