On Sat, Jul 28, 2012 at 7:47 AM, Per Mildner
<[email protected]>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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