The documentation is rather misleading, as it implies that "obj" can be a
symbol. However, because ^ is a reader macro, it is applied to "obj" before
it is evaluated.
Clojure maps, vectors and sets all evaluate to themselves, so attaching
metadata to the unevaluated expression via the ^ reader macro, is the same
as attaching it to the evaluated expression via with-meta.
So:
^:abc foo
^:abc [1 2 3]
Will both attach the metadata directly to the literal value following it,
but because 'foo' evaluates to something else, the reader metadata is lost
after the symbol is evaluated.
- James
On 5 May 2015 at 22:01, Andrey Antukh <n...@niwi.be> wrote:
> Thanks to both for the responses, but I stil not clearly understand.
>
> The documentation says very clearly that:
>
> In addition to with-meta, there are a number of reader macros (The Reader:
> Macro Characters) for applying metadata to the expression following it:
> ^{:doc "How obj works!"} obj - Sets the metadata of obj to the provided
> map.
> Equivalent to (with-meta obj {:doc "How obj works!"})
>
>
> (def foo ^:abc [1 2 3]) -> (meta foo) -> {:abc true}
> (def foo ^:abc some-func) -> (meta foo) -> nil
> (def foo ^:abc 'some-symbol) -> (meta foo) -> nil (In clojure programming
> book uses example attaching metadata using the reader to the symbol, but
> seems it not works as expected)
>
> Is a little bit confusing. The metadata documentation says clearly that
> are equivalent, but are not equivalent.
>
>
> Thank you very much again.
>
> Regards.
> Andrey
>
> 2015-05-05 21:49 GMT+02:00 Andy- <andre.r...@gmail.com>:
>
>> In addition to James comment: IMO clojure.org/metadata should be clearer
>> about this. It's mentioned more clearly on the reader page:
>> http://clojure.org/reader#The%20Reader--Macro%20characters
>> "The metadata reader macro first reads the metadata and attaches it to
>> the next form read (see with-meta
>> <http://clojure.github.io/clojure/clojure.core-api.html#clojure.core/with-meta>
>> to
>> attach meta to an object):"
>>
>> Stress on *next form read*.
>>
>> On Tuesday, May 5, 2015 at 2:31:40 PM UTC-4, Andrey Antukh wrote:
>>>
>>> Hi!
>>>
>>> I have some trouble with clojure metadata / reader and I do not know if
>>> I'm doing something wrong.
>>>
>>> I have this code:
>>>
>>> (defn some-func [])
>>>
>>> (def func ^:abc some-func)
>>>
>>> (assert (= (meta func) {:abc true}))
>>>
>>> (def data [[:bar (with-meta some-func {:abc true})]
>>> [:baz ^:abc some-func]])
>>>
>>> (assert (= (meta (get-in data [0 1])) {:abc true}))
>>> (assert (= (meta (get-in data [1 1])) {:abc true}))
>>>
>>> It fails in the first assert and in the last (if I comment the first one
>>> obviously). I do not understand why that form of metadata does not works
>>> as I expect (http://clojure.org/metadata)
>>>
>>> Thank you very much.
>>>
>>> Regards.
>>> Andrey
>>>
>>> --
>>> Andrey Antukh - Андрей Антух - <andrei....@kaleidos.net> / <
>>> ni...@niwi.be>
>>> http://www.niwi.be <http://www.niwi.be/page/about/>
>>> https://github.com/niwibe
>>>
>>
>
>
> --
> Andrey Antukh - Андрей Антух - <andrei.anto...@kaleidos.net> / <
> n...@niwi.be>
> http://www.niwi.be <http://www.niwi.be/page/about/>
> https://github.com/niwibe
>
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