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[CLJS-210] Implement Var form, var-get, and var? in CLJS Created: 27/Apr/12  Updated: 28/Oct/12

Status: Open
Project: ClojureScript
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Enhancement Priority: Minor
Reporter: Brandon Bloom Assignee: Unassigned
Resolution: Unresolved Votes: 0
Labels: patch,

Attachments: Text File vars1.patch    
Patch: Code and Test

 Description   

See discussion of Vars in CLJS here: http://dev.clojure.org/display/design/Dynamic+Binding
My goal is to eventually implement dynamic scope capture, via bound-fn and friends. The attached patch is a step in that direction.

This patch adds support for parsing the (var foo) form. The #' reader macro is provided by JVM Clojure.

#'foo emits code to construct a Var object. In this patch, each invocation of 'var will create a unique Var object. This means they are '= by fully qualified symbol, but not 'identical?. Simple memoization would fix that, but I'm not going to bother until I get to Dynamic Var objects.

The main advantage of this level of Var support is for the interactive development convenience of being able to defer dereferencing top-levels. For example, (def fog (comp f #'g)) will pick up redefinitions of 'g, but not of 'f.

 Comments   
Comment by Brandon Bloom [ 28/Apr/12 10:07 PM ]

I found two issues with this patch:

1) I never used the IVar that I declared :-P

2) (apply #'+ (range 100)) throws "Invalid arity: 101" – this is related to http://dev.clojure.org/jira/browse/CLJS-211

I'll look into fixing both. However, we should discuss supporting variable arity protocol methods...

Comment by David Nolen [ 17/Jun/12 12:17 PM ]

any support for reified vars is a low priority for the foreseeable future.

Comment by Brandon Bloom [ 01/Sep/12 7:46 PM ]

Reified vars (and binding frames!) are a requirement for a CPS transform to be correct with respect to dynamic bindings.

Comment by Tom Jack [ 10/Sep/12 5:11 PM ]

+1 towards bound-fn and friends. I had an explanation written here but now I see "async Javascript (ie. nearly all Javascript) makes the binding macro effectively useless without bound-fn". Indeed.

Comment by Herwig Hochleitner [ 28/Oct/12 7:34 PM ]

bound-fn could also be implemented by setting / resetting bound vars before / after the wrapped fn, similar to binding. We would just need to add tracking of the currently rebound dynamic vars.
That should also be CPS - friendly, no?

Comment by Tom Jack [ 28/Oct/12 9:03 PM ]

I've considered that and would love to see it happen.

But what about:

(def ^:dynamic *foo* 3)
(set! *foo* 4)
(def f (bound-fn [] *foo*))
(set! *foo* 5)
(f)
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