What does "^:static" do in Clojure?

Question

I've seen the ^:static metadata on quite a few function in the Clojure core.clj source code, e.g. in the definition of seq?:

(def
 ^{:arglists '([x])
   :doc "Return true if x implements ISeq"
   :added "1.0"
   :static true}
 seq? (fn ^:static seq? [x] (instance? clojure.lang.ISeq x)))

What precisely does this metadata do, and why it it used so frequently throughout core.clj?

score 41 · Accepted Answer · edited May 07 '16 at 20:41

41

In the development of Clojure 1.3 Rich wanted to add the ability for functions to return types other than Object. This would allow native math operators to be used without having to cram everything into one function.

The original implementation required functions that supported this to be marked :static. this meta data caused the compiler to produce two versions to the function, one that returned Object and one that returned that specific type. in cases where the compiler determined that the types would always match the more specific version would be used.

This was later made fully automatic so you don't need to add this anymore.

edited May 07 '16 at 20:41

Ondrej Slinták

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answered Sep 26 '11 at 17:35

Arthur Ulfeldt

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When you're saying *fully-automatic*, do you mean that in recent versions of clojure compiler produces twins and performs such optimization *for every function* no matter marked with ^static or not? – om-nom-nom Feb 27 '13 at 16:50
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when type annotations cause the compiler to determine that a function can produce something other than Object, then the other forms will be generated. – Arthur Ulfeldt Feb 27 '13 at 18:58

score 11 · Answer 2 · edited Sep 29 '14 at 04:17

11

According to the Google Groups thread “Type hinting inconsistencies in 1.3.0”, it’s a no-op.

^:static has been a no-op for a while AFAIK, made unnecessary after changes to vars a while back.

— a May 2011 post by Chas Emerick

edited Sep 29 '14 at 04:17

Rory O'Kane

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answered Sep 26 '11 at 12:52

bmillare

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score -4 · Answer 3 · answered Sep 26 '11 at 09:33

Seems it's a new metadata attribute in clojure 1.3. And you can compare the source between 1.3 and 1.2:

So I think it has something to do with ^:dynamic which indicates whether the var is allowed for dynamic binding. Just my guess. Not sure until I see document about this attribute.

What does "^:static" do in Clojure?

3 Answers3

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