I'll let other people comment on this wrt Python (even though I think it looks cool and useful to me).  Out of curiosity, why does frozenmap not have the same runtime and ordering guarantees as dict?

On Thursday, September 12, 2019 at 7:47:25 AM UTC-4, Yury Selivanov wrote:

I've just published a PEP to add a frozenmap type to Python; it should
be online shortly.

Read it here: https://discuss.python.org/t/pep-603-adding-a-frozenmap-type-to-collections/2318

Here's an excerpt from the Abstract:

A *persistent data structure* is defined as a data structure that
preserves the previous version of the data when the data is modified.
Such data structures are effectively *immutable*, as operations on
them do not update the structure in-place, but instead always yield
a new updated structure (see [0]_ for more details.)

This PEP proposes to add a new fully persistent and immutable mapping
type called ``frozenmap`` to the ``collections`` module.

The bulk of ``frozenmap``'s reference implementation is already
used in CPython to implement the ``contextvars`` module.

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