Mock functions in Go

Personally, I don't use gomock (or any mocking framework for that matter; mocking in Go is very easy without it). I would either pass a dependency to the downloader() function as a parameter, or I would make downloader() a method on a type, and the type can hold the get_page dependency:

Method 1: Pass get_page() as a parameter of downloader()

type PageGetter func(url string) string

func downloader(pageGetterFunc PageGetter) {
    // ...
    content := pageGetterFunc(BASE_URL)
    // ...
}

Main:

func get_page(url string) string { /* ... */ }

func main() {
    downloader(get_page)
}

Test:

func mock_get_page(url string) string {
    // mock your 'get_page()' function here
}

func TestDownloader(t *testing.T) {
    downloader(mock_get_page)
}

Method2: Make download() a method of a type Downloader:

If you don't want to pass the dependency as a parameter, you could also make get_page() a member of a type, and make download() a method of that type, which can then use get_page:

type PageGetter func(url string) string

type Downloader struct {
    get_page PageGetter
}

func NewDownloader(pg PageGetter) *Downloader {
    return &Downloader{get_page: pg}
}

func (d *Downloader) download() {
    //...
    content := d.get_page(BASE_URL)
    //...
}

Main:

func get_page(url string) string { /* ... */ }

func main() {
    d := NewDownloader(get_page)
    d.download()
}

Test:

func mock_get_page(url string) string {
    // mock your 'get_page()' function here
}

func TestDownloader() {
    d := NewDownloader(mock_get_page)
    d.download()
}