Pass db variable to function argument

Question

I'm trying to parse some RSS feeds using Golang. A package I found was https://github.com/jteeuwen/go-pkg-rss

My process is

Get feeds from my Postgres DB
For each feed, fetch the articles
Store each article into the Postgres DB

My code is as below

func main() {
  db := dbConnect() // returns the DB connection variable

  feeds := getRssFeeds(db) // returns feeds from my DB
  for i := 0; i < len(feeds); i++ {
    getFeedArticles(feeds[i].url, 5, db)
  }
}

func getFeedArticles(uri string, timeout int, db *sql.DB) {
  // using the package to get RSS feed contents
  feed := rss.New(timeout, true, chanHandler, itemHandler)
  if err := feed.Fetch(uri, nil); err != nil {
    fmt.Fprintf(os.Stderr, "[e] %s: %s", uri, err)
    return
  }
}

func chanHandler(feed *rss.Feed, newchannels []*rss.Channel) {
  // no need to do anything...
}

func itemHandler(feed *rss.Feed, ch *rss.Channel, newitems []*rss.Item) {
  for i := 0; i < len(newitems); i++ {
    fmt.Println(newitems[i].Title)

    // insert the article into DB here
  }
}

How do I pass the db variable to itemHandler so I can insert the article into my DB? I suppose I could call dbConnect() to create a new db variable to insert, but that would seem wasteful.

Is there a better way to do this process?

go-database-sql.org may help you.

elithrar
– elithrar

2014-01-07 07:04:11 +00:00
Commented Jan 7, 2014 at 7:04 — elithrar
– elithrar, Commented Jan 7, 2014 at 7:04

eduncan911 · Accepted Answer · 2016-05-31 21:34:52Z

3

You can use closures:

func makeHandler(db Database) rss.ItemHandler {
    return func(feed *rss.Feed, ch *rss.Channel, newitems []*rss.Item) {
         for i := 0; i < len(newitems); i++ {
             fmt.Println(newitems[i].Title)
             // Accessed via closure:
             db.InsertArticle(newitems[i])
         }
    }
}

Or some variation thereof. You can then call rss.New with makeHandler(db), or define itemHandler in getFeedArticle... In any case, itemHandler will have db being defined and accessible.

This tutorial and this part of the specs explain it in more details

edited May 31, 2016 at 21:34

eduncan911

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answered Jan 7, 2014 at 9:49

val

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5 Comments

ericbae Over a year ago

thanks for the response. I tried your suggestion of closure and called "makehandler(db)" in "rss.New" but I get "cannnot use makeHandler(db) (type handler) as type feeder.ItemHandler in function argument". Any ideas?

ericbae Over a year ago

ok, instead of calling "makeHandler(db)" in rss.New, I did rss.New(timeout, true, chanHandler, func(feed *rss.Feed, ch *rss.Channel, newitems[]*rss.Item) { ... }) and this worked. Not sure why the first approach isn't working?

val Over a year ago

The first did not work because you provided a handler where a feeder.ItemHandler was expected, although they both refer to the same function signature. You can correct it by having makeHandler return a feeder.ItemHandler instead (it has the same signature). I only defined handler for clarity, but the correct way is to return a ItemHandler directly.

val Over a year ago

And indeed: github.com/jteeuwen/go-pkg-rss/blob/master/feed.go defines type ItemHandler func(f *Feed, ch *Channel, newitems []*Item) (i.e. same as handler)

val Over a year ago

I have edited the response so that it now returns the correct func type. Thank you for your feedback !

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