[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2373: adopt pyrsistent

[Tahoe-LAFS]

#2373: adopt pyrsistent
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     Reporter:  zooko        |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  normal       |  Milestone:  undecided
    Component:  unknown      |    Version:  1.10.0
   Resolution:               |   Keywords:
Launchpad Bug:               |
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Comment (by daira):

 +1 for the basic idea: I like persistent data structures and they're
 essentially as expressive as mutable ones, while being less error-prone.

 +1 for pyrsistent's API which appears, on a cursory reading, to be well-
 thought-out and reasonably complete. I particularly like the PRecord
 abstraction which we could make good use of in many places.

 +0.3 for pyrsistent's unit test suite. This is not a full +1 because the
 tests, although they cover the external interfaces reasonably well, are
 only "black-box" tests that do not attempt to check the internal
 representations of the persistent structures or to exercise any hard
 cases. In particular, they typically only test structures with a small
 number of elements, which does not properly cover all of the
 implementation, because most of the structures are based on trees with an
 arity of 32.

 -1 because pyrsistent has a C extension which is optionally used. This
 potentially introduces build problems if the build process has a problem
 compiling the extension and fails to correctly fall back to the pure-
 Python code.

 Note that I don't think we use frozenset and tuples in all the places we
 could. Where possible, it is better to use a truly immutable data
 structure rather than a persistent data structure, both for efficiency and
 understandability.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2373#comment:1>
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