Indeed.  Consider <a href="https:&#x2F;&#x2F;github.com&#x2F;chadaustin&#x2F;buffer-builder&#x2F;blob&#x2F;master&#x2F;src&#x2F;Data&#x2F;BufferBuilder.hs#L164" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;chadaustin&#x2F;buffer-builder&#x2F;blob&#x2F;master&#x2F;src...</a><p>The API is pure, but it&#x27;s implemented with a high-performance effectful monad, and none of the implementation details are exported.  A perfect use of unsafe[Dupable]PerformIO.