Revision history
----------------
v0.12.1, 2017-02-26
 * Fix #97, initialize CheckedPVector from iterator-
 * Fix #97, cache hash value on PMap. Thanks @sarum90 for this!

v0.12.0, 2017-01-06
 * Fix #87, add function get_in() for access to elements in deeply nested structures.
 * Fix #91, add method update() to pset and pbag.
 * Fix #92, incorrect discard of elements in transform on pvector
 * This is a release candidate for 1.0 as I now consider pyrsistent fairly stable.

v0.11.13, 2016-04-03
 * Fix #84, pvector segfault in CPython 3 when repr of contained object raises Exception.
 * Update README to cover for issue described in #83.

v0.11.12, 2016-02-06
 * Minor modifications of tests to allow testing as requested in #79 and #80.
 * Also run CI tests under python 3.5

v0.11.11, 2016-01-31
 * #78, include tests in pypi dist.

v0.11.10, 2015-12-27, NOTE! This release contains a backwards incompatible change
                      despite only stepping the patch version number. See below.
 * Implement #74, attribute access on PClass evolver
 * Implement #75, lazily evaluated invariant messages by providing a
   callable with no arguments.
 * Initial values on fields can now be evaluated on object creation
   by providing a callable with no arguments.

   NOTE! If you previously had callables as initial values this change means that those
         will be called upon object creation which may not be what you want. As
         a temporary workaround a callable returning a callable can be used. This
         feature and the concept of initial values will likely change slightly in the future.
         See #77 and and #76 for more information.

v0.11.9, 2015-11-01
 * Added PVector.remove(), thanks @radix for initiating this!

v0.11.8, 2015-10-18
 * Fix #66, UnicodeDecodeError when doing pip install in environments with ascii encoding as default.
   Thanks @foolswood!
 * Implement support for multiple types in pmap_field(), pvector_field() and pset_field(). Thanks @itamarst!

v0.11.7, 2015-10-03
 * Fix #52, occasional SEGFAULTs due to misplaced call to PyObject_GC_Track. Thanks @jkbjh for this!
 * Fix #42, complete support for delete. Now also on the C-implementation of the PVectorEvolver.
   Thanks @itamarst for contributing a whole bunch of Hypothesis test cases covering the evolver operations!

v0.11.6, 2015-09-30
 * Add +, -, & and | operations to PBag. Thanks @Futrell for this!

v0.11.5, 2015-09-29
 * Fix bug introduced in 0.11.4 that prevented multi level inheritance from PClass.
 * Make PClassMeta public for friendlier subclassing

v0.11.4, 2015-09-28
 * Fix #59, make it possible to create weakrefs to all collection types.
   Thanks @itamarst for reporting it.
 * Fix #58, add __str__ to InvariantException. Thanks @tomprince for reporting it.

v0.11.3, 2015-09-15
  * Fix #57, support pickling of PClasses and PRecords using pmap_field, pvector_field, and pset_field.
    Thanks @radix for reporting this and submitting a fix for it!

v0.11.2, 2015-09-09
  * Fix bug causing potential element loss when reallocating PMap. Thanks to @jml for finding
    this and submitting a PR with a fix!
  * Removed python 3.2 test build from Travis. There is nothing breaking 3.2 compatibility in this
    release but there will be no effort moving forward to keep the 3.2 compatibility.

v0.11.1, 2015-08-24
  * Fix #51, PClass.set() broken when used with string+value argument.
  * #50, make it possible to specify more than one assertion in an invariant
  * #48, make it possible to make recursive type references by using a string
    as type specification.

v0.11.0, 2015-07-11
  * #42, delete() function added to PVector to allow deletion of elements by index
    and range. Will perform a full copy of the vector, no structural sharing.
    Thanks @radix for helping out with this one!
  * Fix #39, explicitly disallow ordering for PMap and PBag, Python 3 style
  * Fix #37, PMap.values()/keys()/items() now returns PVectors instead of lists

v0.10.3, 2015-06-13
  * Fix #40, make it possible to disable the C extension by setting the
    PYRSISTENT_NO_C_EXTENSION environment variable.

v0.10.2, 2015-06-07
  * Fix #38, construction from serialized object for pvector/pset/pmap fields.

v0.10.1, 2015-04-27
  * Fix broken README.rst

v10.0.0, 2015-04-27
  * New type PClass, a persistent version of a Python object. Related to issues #30 and #32.
    Thanks @exarkun and @radix for input on this one!
  * Rename PRecordTypeError -> PTypeError, it is now also raised by PClass
  * New convenience functions, pvector_field, pmap_field and pset_field to create PRecord/PClass
    fields for checked collections. Issues #26 and #36. Thanks to @itamarst for this!
  * Removed deprecated function set_in() on PMap and PVector.
  * Removed deprecated factory function pclass.
  * Major internal restructuring breaking pyrsistent.py into multiple files. This should
    not affect those only using the public interface but if you experience problems please
    let me know.

v0.9.4, 2015-04-20
  * Fix #34, PVector now compares against built in list type

v0.9.3, 2015-04-06
  * Rename pclass back to immutable and deprecate the usage of the pclass function. PClass will be used by
    a new, different type in upcoming releases.
  * Documentation strings for the exceptions introduced in 0.9.2.

v0.9.2, 2015-04-03
  * More informative type errors from checked types, issue #30
  * Support multiple optional types, issue #28

v0.9.1, 2015-02-25
  * Multi level serialization for checked types

v0.9.0, 2015-02-25, Lots of new stuff in this release!
  * Checked types, checked versions of PVector, PMap, PSet that support type and invariant specification.
    Currently lacking proper documentation but I'm working on it.
  * set_in() on PVector and PMap are now deprecated and will be removed in the next release.
    Use transform() instead. set_in() has been updated to use transform() for this release
    this means that some corner error cases behave slightly different than before.
  * Refactoring of the PVector to unify the type. Should not have any user impact as long as
    only the public interface of pyrsistent has been used. PVector is now an abstract base class
    with which the different implementations are registered.
  * Evolvers have been updated to return themselves for evolving operations to allow function chaining.
  * Richer exception messages for KeyErrors and IndexErrors specifying the key/index that caused the failure.
    Thanks @radix for this.
  * Missing attribute on PMaps when accessing with dot-notation now raises an AttributeError instead of a
    KeyError. Issue #21.
  * New function decorator @mutant that freezes all input arguments to a function and the return value.
  * Add __version__ to pyrsistent.py. Issue #23.
  * Fix pickling for pset. Issue #24.

v0.8.0, 2015-01-21
  * New type PRecord. Subtype of PMap that allows explicit, declarative field specification. Thanks @boxed
    for inspiration!
  * Efficient transformations of arbitrary complexity on PMap and PVector. Thanks @boxed for inspiration!
  * Breaking change to the evolver interface. What used to be .pvector(), .pmap() and .pset()
    on the different evolvers has now been unified so that all evolvers have one method .persistent()
    to produce the persistent counterpart. Sorry for any inconvenience.
  * Removed the tests directory from the package.
  * PMap and PSet now contains a copy-function to closer mimic the interface of the dict and set. These
    functions will simply return a reference to self.
  * Removed deprecated alias 'immutable' from pclass.

v0.7.1, 2015-01-17
  * Fixes #14 where a file executed (unexpectedly) during installation was not python 3 compatible.

v0.7.0, 2015-01-04, No 1.0, instead a bunch of new stuff and one API breaking change to PMap.remove().
  * Evolvers for pvector, pmap and pset to allow simple and efficient updates of multiple elements
    in the collection. See the documentation for a closer description.
  * New method mset on pvector to update multiple values in one operation
  * Remove deprecated methods merge and merge_with on PMap
  * Change behavior of PMap.remove, it will now raise a KeyError if the element is not present.
    New method PMap.discard will instead return the original pmap if the element is not present.
    This aligns the PMap with how things are done in the PSet and is closer to the behavior of the
    built in counterparts.

v0.6.3, 2014-11-27
  * Python 2.6 support, thanks @wrmsr!
  * PMap.merge/merge_with renamed to update/update_with. merge/merge_with remains but will be
    removed for 1.0.
  * This is a release candidate for 1.0! Please be aware that PMap.merge/merge_with and immutable()
    will be removed for 1.0.

v0.6.2, 2014-11-03
  * Fix typo causing the pure python vector to be used even if the C implementation was
    available. Thanks @zerc for finding it!

v0.6.1, 2014-10-31
  * Renamed 'immutable' to 'pclass' for consistency but left immutable for compatibility.

v0.6.0, 2014-10-25
  * New data structure, persistent linked list
  * New data structure, persistent double ended queue

v0.5.0, 2014-09-24
  * New data structure, persistent bag / multiset
  * New functions freeze and thaw to recursively convert between python
    built in data types and corresponding pyrsistent data types.
  * All data structures can now be pickled
  * New function merge_in on persistent map which allows a user
    supplied function to implement the merge strategy.

v0.4.0, 2014-09-20
  * Full Python 3 support.
  * Immutable object implemented.
  * Bug fixes in PVector.__repr__() and PMap.__hash__() and index check of PVector.
  * Repr changed to be fully cut and paste compatible
  * Changed assoc() -> set(), assoc_in() -> set_in(), massoc() -> mset().
    Sorry for the API breaking change but I think those names are more pythonic.
  * Improved documentation.

v0.3.1, 2014-06-29
  * assoc() on PSet renamed back to add()

v0.3.0, 2014-06-28
  * Full Sequence protocol support for PVector
  * Full Mapping protocol support for PMap
  * Full Set protocol support for PSet
  * assoc_in() support for both PMap and PVector
  * merge() support for PMap
  * Performance improvements to the PVector C extension speed up allocation

v0.2.1, 2014-06-21
  * Supply the tests with the distribution

v0.2.0, 2014-06-21
  * New C extension with an optimized version of the persistent vector
  * Updated API slightly

v0.1.0, 2013-11-10
  * Initial release.


TODO (in no particular order)
-----------------------------
- Versioned data structure where the different versions can be accessed by index?
- Ordered sets and maps
- A good performance measurement suite
