Assignments and Usage of global Variables in Clojure

Question

I've got the following problem:

I do understand that the concept of functional programming languages do not allow immutable variables, so classic variable assignment and usage like

var foo = bar();

after that in another function...

var baz = funnything(foo);

is not possible (correct me if it is, I'm still trying to get the whole idea and concepts behind functional programming).

I'm trying to play around a bit with Clojure and come to a point where I stuck:

I want to get a webpage and save the DOM into a variable for later usage. The program starts, a prompt is spawning and the user can enter a URL which he want to fetch.

After the DOM of the website is fetched he can navigate through the DOM by for example getting the title, or all links or whatever.

The Problem: Because I can't use a construct like

; More or less pseudo-code
(def page nil)

(get-url [url]
  (page (browser/get url)))

(get-title
    (println (html/select page [:title])))

I have to use the following construct:

(get-title [url]
    (println (html/select (browser/get url) [:title])))

Due to this construct everytime I want to access an element of the site via get-title, get-links or whatever I have to download the whole webpage whenever the user is entering one of those methods on the command prompt.

Do I miss something here? Are there classical assigments in Clojure? Am I doing the complete thing wrong and should study a whole lot more functional programming concepts before getting hands-on and learning-by-doing?

Oin · Accepted Answer · 2013-04-11 08:54:08Z

2

You are misunderstanding assignment in clojure.

(page (browser/get url))

is not equivalent to: page = browser/get(url), it is equivalent to page(browser/get(url)).

I think you don't really need a global variable, just composable functions. Your example could look something like this:

(defn get-title [url]
  (let [page (browser/get url)]
    (println (html/select page [:title]))))

That and using memoize, like Joost mentioned, if you want to cache the return values of broser/get.

answered Apr 11, 2013 at 8:54

Oin

7,5993 gold badges38 silver badges58 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Joost Diepenmaat · Accepted Answer · 2013-04-11 09:08:47Z

2

You can easily cache the value of (browser/get url) so you don't have to refetch the page every time you want to examine a property of it. See clojure.core/memoize. But that will only solve part of the problem.

In your particular case, your program requires some sort of global (or session scoped), mutable, concept of "the current page" or at least, "the current url", since that is what you're expecting your user to enter and modify. Clojure provides multiple constructs for managing mutable state.

Since you're only dealing with one page at a time, this is probably best modeled by using atoms. See http://clojure.org/atoms and http://clojuredocs.org/clojure_core/clojure.core/swap!

edited Apr 11, 2013 at 9:08

answered Apr 11, 2013 at 8:46

Joost Diepenmaat

17.8k3 gold badges48 silver badges55 bronze badges

1 Comment

akluth Over a year ago

Great, atoms were exactly that what I was looking for, Thanks!

Collectives™ on Stack Overflow

Assignments and Usage of global Variables in Clojure

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related