Thanks, but the question was not about the empty() methods.
On Sunday, July 29, 2012 4:14:09 AM UTC+2, tbc++ wrote:
>
>
>
> On Sat, Jul 28, 2012 at 7:47 AM, Per Mildner wrote:
>
>> Looking at the persistent types in clojure.lang.* (PersistentVector.java
>> et al.) I see several occurrences of the idiom EMPTY.withMeta(meta()) where
>> EMPTY is a constant (static final) denoting an empty collection of the
>> appropriate type.
>>
>> What I can not understand, given that these types are all persistent, is
>> why the EMPTY constant is, in effect, copied at most places where it is
>> used. Why not use the same EMPTY instance instead, i.e.
>> replace EMPTY.withMeta(meta()) with just EMPTY. Unless I miss something
>> this could not hurt and would save some time and space.
>>
>>
> In this context, the empty() method does not mean "get an empty of this
> type", instead it means "empty this collection". So if we want to return a
> new collection that looks the same, except it is empty, then we need to
> pull in the metadata from the old collection.
>
> Example:
>
> => (meta (empty (with-meta [1 2 3] {:foo true})))
> {:foo true}
>
>
> Timothy
>
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