Comments on: On Language Design: Magic Variables in Compojure

By: Zef Hemel

Zef Hemel — Fri, 08 Jan 2010 15:35:46 +0000

That looks a lot nicer, however the :params there should actually be just params, am I right?

By: Zef Hemel

Zef Hemel — Fri, 08 Jan 2010 14:43:41 +0000

Yes, I am aware of this, but think it's a trade off. Either keep passing loads of values around to functions (because at some point, some function may need that value), or solve it with dynamically bound variables. Neither is perfect.

I agree that limiting the availability of the request value within the handler can be good, but it should not appear out of nowhere, it should be declared as part of the syntax, like weavejester is proposing now.

By: kotarak

kotarak — Fri, 08 Jan 2010 11:51:49 +0000

OT: You can use to indent code. bleh…. Disqus should do something about that…

By: weavejester

weavejester — Fri, 08 Jan 2010 00:04:38 +0000

The "magic" local bindings in Compojure are also a feature I don't particularly like; and I wrote that piece of code. It was the least worst solution I could think of at the time.

The reason I used local bindings, rather than thread-local vars, is because I didn't want the request map to be passed implicitly to the controller functions, like so:

(defn do-something []
(str "Hello " (params :name)))

By using local bindings, I ensure that people need to explicitly choose which parts of the request they want to use. I'm of the opinion that by removing unnecessary information early, you limit the amount of things that can go wrong.

But I've never been happy with using magic variables in this way. So in the "refactor" branch of the Compojure repository I'm trying a different approach. I'm adding an extra argument to the route macros that determines the binding. Clojure's binding syntax is quite sophisticated, so I can do something like:

(GET "/book/:isbn" {{:keys [isbn]} :params}
(get-book isbn))

So now the variables are all explicitly bound. However, it's still rather verbose, especially since 99% of the time, you'll likely only want to access the parameters. So I've added some syntax sugar for handling parameters:

(GET "/book/:isbn" [isbn]
(get-book isbn))

If a vector binding is passed instead, Compojure treats it as a binding of parameters. I think this should cover the vast majority of cases. Things like sessions I'm thinking should be handled by global vars bound in middleware.

I'd be interested in your opinion on this syntax. Can you pick any holes in it?

By: Matt

Matt — Thu, 07 Jan 2010 23:53:16 +0000

While I do agree with your principles, there is more than one way to view the disappearing variables issue. Having them only available from directly within the handler forces the developer to explicitly pass only the data necessary in generating a response to further function calls. This data can include the request itself, but typically it would just be the parameters and session values of note. This way the intent of functions remains clear and there are no unintentional dependencies on variables that are context dependent. I think Rich called the danger here "action from afar" in reference to dynamic thread local bindings.
That being said, I hate magic local variables that appear from nowhere just as much as you do :)