Earlier in this chapter, we discussed the virtues of immutability and the pitfalls of mutability. However, even though mutability is fundamentally unsafe, it also has very good single-threaded performance. Now, what if there was a way to restrict the mutable operation in a local context in order to provide safety guarantees? That would be equivalent to combining the performance advantage and local safety guarantees. That is exactly the abstraction called transients, which is provided by Clojure.

Firstly, let's verify that it is safe (up to Clojure 1.6 only):

user=> (let [t (transient [:a])]
  @(future (conj! t :b)))
IllegalAccessError Transient used by non-owner thread  clojure.lang.PersistentVector$TransientVector.ensureEditable (PersistentVector.java:463)

As we can see previously, up to Clojure 1.6, a transient created in one thread cannot be accessed by another. However, this operation is allowed in Clojure 1.7 in order for transducers to play well with the core.async (https...