One of Haskell's strengths is immutable data structures.  These
structures make it easier to reason about code, simplify concurrency and
parallelism, and in some cases can improve performance by allowing
sharing.  However, there are still classes of problems where mutable
data structures can both be more convenient, and provide a performance
boost.  This library is meant to provide such structures in a
performant, well tested way.  It also provides a simple abstraction over
such data structures via typeclasses.

WWW: https://github.com/fpco/mutable-containers