Login light
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

// See page 145.

// Title2 prints the title of an HTML document specified by a URL.
// It uses defer to simplify closing the response body stream.
package main

import (
    "fmt"
    "net/http"
    "os"
    "strings"

    "golang.org/x/net/html"
)

// Copied from gopl.io/ch5/outline2.
func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
    if pre != nil {
        pre(n)
    }
    for c := n.FirstChild; c != nil; c = c.NextSibling {
        forEachNode(c, pre, post)
    }
    if post != nil {
        post(n)
    }
}

//!+
func title(url string) error {
    resp, err := http.Get(url)
    if err != nil {
        return err
    }
    defer resp.Body.Close()

    ct := resp.Header.Get("Content-Type")
    if ct != "text/html" && !strings.HasPrefix(ct, "text/html;") {
        return fmt.Errorf("%s has type %s, not text/html", url, ct)
    }

    doc, err := html.Parse(resp.Body)
    if err != nil {
        return fmt.Errorf("parsing %s as HTML: %v", url, err)
    }

    // ...print doc's title element...
    //!-
    visitNode := func(n *html.Node) {
        if n.Type == html.ElementNode && n.Data == "title" &&
            n.FirstChild != nil {
            fmt.Println(n.FirstChild.Data)
        }
    }
    forEachNode(doc, visitNode, nil)
    //!+

    return nil
}

//!-

func main() {
    for _, arg := range os.Args[1:] {
        if err := title(arg); err != nil {
            fmt.Fprintf(os.Stderr, "title: %v\n", err)
        }
    }
}