create a web crawler in go

To create a web crawler in Go, we first need to understand the basic concept behind it. A web crawler is an automated program that searches the internet and retrieves content. The content can be anything like images, videos, or text.

Here is a basic skeleton of the web crawler in Go:

main.go
package main

import(
    "fmt"
    "net/http"
)

func main(){
    resp, err := http.Get("https://www.example.com/")
    if err!=nil{
        fmt.Println(err)
    }
    defer resp.Body.Close()
    fmt.Println(resp.Status)
}
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In the above program, we are fetching the content of the website "https://www.example.com". Here, the http.Get() function is used to fetch the content of the website. The variable ‘resp’ stores the response received from the server. The ‘defer’ keyword is used to ensure that the response body is closed at the end of the function.

We can also use a package like goquery to parse the HTML document once we have fetched it. Here is an example of how to use goquery to extract all links from a webpage:

main.go
package main

import (
    "log"
    "fmt"
    "strings"
    "net/http"
    "github.com/PuerkitoBio/goquery"
)

func main() {
    site := "https://www.example.com"
    resp, err := http.Get(site)
    if err != nil {
        log.Fatal(err)
    }
    defer resp.Body.Close()

    doc, err := goquery.NewDocumentFromReader(resp.Body)
    if err != nil {
        log.Fatal(err)
    }

    doc.Find("a").Each(func(index int, item *goquery.Selection) {
        href, _ := item.Attr("href")
        if strings.HasPrefix(href, "/") {
            href = fmt.Sprintf("%s%s", site, href)
        }
        fmt.Printf("Link #%d: %s\n", index, href)
    })
}
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This program uses a package called goquery which makes it very easy to parse the HTML document. We first fetch the HTML document using the http.Get function the same as before. Then we use NewDocumentFromReader function of Goquery to create an object to manipulate. We then find all anchor tags(links) in the webpage and extract their href attribute using Attr method. Finally, we print all the links found on the webpage.

Using Go's concurrency with channels and goroutines can also make web crawling much faster.

Overall, building a web crawler in Go is not too difficult, and it can be a great exercise to improve your programming skills.