On a macOS machine that uses Homebrew, you can find the Go code of the net/url package at /usr/local/Cellar/go/1.9.2/libexec/src/net/url/url.go. If for some reason you cannot locate the source code of the net/url package on your Unix machine, you can execute the following command and wait for its output:

$ find / -name url.go 2>/dev/null

The output of the preceding command on a Debian Linux machine is as follows:

/usr/share/go/src/pkg/net/url/url.go
/usr/share/go/src/pkg/html/template/url.go

Apart from the net/url package, there is also an url.go file that belongs to the html/template standard Go package. However, what interests us is the /usr/share/go/src/pkg/net/url/url.go file. Now that you have located the url.go file, we can start playing with it.

The preamble of the net/url package is as follows:

// Copyright 2009 The Go Authors. All rights reserved. 
// Use of this source code is governed by a BSD-style 
// license that can be found in the LICENSE file. 
 
// Package url parses URLs and implements query escaping. 
package url 

More or less, all packages in the standard Go library have a similar preamble. Now, imagine that you want to get a list of all the functions in the net/url package. Standard Unix command line utilities such as grep(1) are here to help you:

$ grep '^func' /usr/local/Cellar/go/1.9.2/libexec/src/net/url/url.go
func (e *Error) Error() string { return e.Op + " " + e.URL + ": " + e.Err.Error() }
func (e *Error) Timeout() bool {
func (e *Error) Temporary() bool {
func ishex(c byte) bool {
func unhex(c byte) byte {
func (e EscapeError) Error() string {
...
func (u *URL) MarshalBinary() (text []byte, err error) {
func (u *URL) UnmarshalBinary(text []byte) error { 

Now let's say that you want to look at the implementation of one of the functions found in net/url, and the function that interests you is, say, User(). Its implementation is as follows:

// User returns a Userinfo containing the provided username 
// and no password set. 
func User(username string) *Userinfo { 
        return &Userinfo{username, "", false} 
} 

The documentation of the User() function states that it returns a Userinfo, which is a structure defined in net/url:

// The Userinfo type is an immutable encapsulation of username and 
// password details for a URL. An existing Userinfo value is 
// guaranteed to have a username set (potentially empty, 
// as allowed by RFC 2396), and optionally a password. 
type Userinfo struct { 
        username    string 
        password    string 
        passwordSet bool 
} 

Now things are becoming clearer. If you continue delving into the code of net/url, you will find more in-depth information about it.

The next subsection will look into the Go code of a more interesting package.