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In this changelog, we document "large-ish" changes to Iris that affect even the
way the logic is used on paper.  We also mention some significant changes in the
Coq development, but not every API-breaking change is listed.  Changes marked
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`[#]` still need to be ported to the Iris Documentation LaTeX file(s).
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## Iris 3.1.0 (unfinished)
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Changes in and extensions of the theory:

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* Add new modality: ■ ("plainly").
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* Camera morphisms have to be homomorphisms, not just monotone functions.
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* Add a proof that `f` has a fixed point if `f^k` is contractive.
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* Constructions for least and greatest fixed points over monotone predicates
  (defined in the logic of Iris using impredicative quantification).
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* Add a proof of the inverse of `wp_bind`.
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* Support verifying code that might get stuck by distinguishing "non-stuck"
  vs. "(potentially) stuck" weakest preconditions. (See
  [Swasey et al., OOPSLA '17] for examples.) The non-stuck `WP e @ E {{ Φ }}`
  ensures that, as `e` runs, it does not get stuck. The stuck `WP e @ E ?{{ Φ
  }}` ensures that, as usual, all invariants are preserved while `e` runs, but
  it permits execution to get stuck. The former implies the latter. The full
  judgment is `WP e @ s; E {{ Φ }}`, where non-stuck WP uses *stuckness bit* `s
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  = NotStuck` while stuck WP uses `s = MaybeStuck`.
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Changes in Coq:

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* Rename some things and change notation:
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  - The unit of a camera: `empty` -> `unit`, `∅` -> `ε`
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  - Disjointness: `⊥` -> `##`
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  - A proof mode type class `IntoOp` -> `IsOp`
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  - OFEs with all elements being discrete: `Discrete` -> `OfeDiscrete`
  - OFE elements whose equality is discrete: `Timeless` -> `Discrete`
  - Timeless propositions: `TimelessP` -> `Timeless`
  - Camera elements such that `core x = x`: `Persistent` -> `CoreId`
  - Persistent propositions: `PersistentP` -> `Persistent`
  - The persistent modality: `always` -> `persistently`
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  - Consistently SnakeCase identifiers:
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    + `CMRAMixin` -> `CmraMixin`
    + `CMRAT` -> `CmraT`
    + `CMRATotal` -> `CmraTotal`
    + `CMRAMorphism` -> `CmraMorphism`
    + `CMRADiscrete` -> `CmraDiscrete`
    + `UCMRAMixin` -> `UcmraMixin`
    + `UCMRAT` -> `UcmraT`
    + `DRAMixin` -> `DraMixin`
    + `DRAT` -> `DraT`
    + `STS` -> `Sts`
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  - Many lemmas also changed their name.  `always_*` became `persistently_*`,
    and furthermore: (the following list is not complete)
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    + `impl_wand` -> `impl_wand_1` (it only involves one direction of the
      equivalent)
    + `always_impl_wand` -> `impl_wand`
    + `always_and_sep_l` -> `and_sep_l`
    + `always_and_sep_r` -> `and_sep_r`
    + `always_sep_dup` -> `sep_dup`
    + `wand_impl_always` -> `impl_wand_persistently` (additionally,
      the direction of this equivalence got swapped for consistency's sake)
    + `always_wand_impl` -> `persistently_impl_wand` (additionally, the
      direction of this equivalence got swapped for consistency's sake)
  The following `sed` snippet should get you most of the way:
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```
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sed 's/\bPersistentP\b/Persistent/g; s/\bTimelessP\b/Timeless/g; s/\bCMRADiscrete\b/CmraDiscrete/g; s/\bCMRAT\b/CmraT/g; s/\bCMRAMixin\b/CmraMixin/g; s/\bUCMRAT\b/UcmraT/g; s/\bUCMRAMixin\b/UcmraMixin/g; s/\bSTS\b/Sts/g' -i $(find -name "*.v")
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```
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* Fix a bunch of consistency issues in the proof mode, and make it overall more
  usable.  In particular:
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  - All proof mode tactics start the proof mode if necessary; `iStartProof` is
    no longer needed and should only be used for building custom proof mode
    tactics.
  - Change in the grammar of specialization patterns: `>[...]` -> `[> ...]`
  - Various new specification patterns for `done` and framing.
  - There is common machinery for symbolic execution of pure reductions. This
    is provided by the type classes `PureExec` and `IntoVal`.
  - There is a new connective `tc_opaque`, which can be used to make definitions
    opaque for type classes, and thus opaque for most tactics of the proof
    mode.
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  - Define Many missing type class instances for distributing connectives.
  - Implement the tactics `iIntros (?)` and `iIntros "!#"` (i.e. `iAlways`)
    using type classes. This makes them more generic, e.g., `iIntros (?)` also
    works when the universal quantifier is below a modality, and `iAlways` also
    works for the plainness modality.  A breaking change, however, is that these
    tactics now no longer work when the universal quantifier or modality is
    behind a type class opaque definition.  Furthermore, this can change the
    name of anonymous identifiers introduced with the "%" pattern.
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* Make `ofe_fun` dependently typed, subsuming `iprod`.  The latter got removed.
* Generalize `saved_prop` to let the user choose the location of the type-level
  later.  Rename the general form to `saved_anything`.  Provide `saved_prop` and
  `saved_pred` as special cases.
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* Define the generic `fill` operation of the `ectxi_language` construct in terms
  of a left fold instead of a right fold. This gives rise to more definitional
  equalities.
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* The language hierarchy (`language`, `ectx_language`, `ectxi_language`) is now
  fully formalized using canonical structures instead of using a mixture of
  type classes and canonical structures. Also, it now uses explicit mixins. The
  file `program_logic/ectxi_language` contains some documentation on how to
  setup Iris for your language.
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* Improved big operators:
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  + They are no longer tied to cameras, but work on any monoid
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  + The version of big operations over lists was redefined so that it enjoys
    more definitional equalities.
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* Various improvements to `solve_ndisj`.
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* Improve handling of pure (non-state-dependent) reductions in heap_lang.
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* Use `Hint Mode` to prevent Coq from making arbitrary guesses in the presence
  of evars, which often led to divergence. There are a few places where type
  annotations are now needed.
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* Move the `prelude` folder to its own project: std++
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* The rules `internal_eq_rewrite` and `internal_eq_rewrite_contractive` are now
  stated in the logic, i.e. they are `iApply` friendly.
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* Restore the original, stronger notion of atomicity alongside the weaker
  notion. These are `Atomic a e` where the stuckness bit `s` indicates whether
  expression `e` is weakly (`a = WeaklyAtomic`) or strongly (`a =
  StronglyAtomic`) atomic.
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## Iris 3.0.0 (released 2017-01-11)
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* There now is a deprecation process.  The modules `*.deprecated` contain
  deprecated notations and definitions that are provided for backwards
  compatibility and will be removed in a future version of Iris.
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* View shifts are radically simplified to just internalize frame-preserving
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  updates.  Weakestpre is defined inside the logic, and invariants and view
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  shifts with masks are also coded up inside Iris.  Adequacy of weakestpre is
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  proven in the logic. The old ownership of the entire physical state is
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  replaced by a user-selected predicate over physical state that is maintained
  by weakestpre.
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* Use OFEs instead of COFEs everywhere.  COFEs are only used for solving the
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  recursive domain equation.  As a consequence, CMRAs no longer need a proof of
  completeness.  (The old `cofeT` is provided by `algebra.deprecated`.)
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* Implement a new agreement construction.  Unlike the old one, this one
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  preserves discreteness.  dec_agree is thus no longer needed and has been moved
  to algebra.deprecated.
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* Renaming and moving things around: uPred and the rest of the base logic are in
  `base_logic`, while `program_logic` is for everything involving the general
  Iris notion of a language.
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* Renaming in prelude.list: Rename `prefix_of` -> `prefix` and `suffix_of` ->
  `suffix` in lemma names, but keep notation ``l1 `prefix_of` l2`` and ``l1
  `suffix_of` l2``.  `` l1 `sublist` l2`` becomes ``l1 `sublist_of` l2``. Rename
  `contains` -> `submseteq` and change `` l1 `contains` l2`` to ``l1 ⊆+ l2``.
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* Slightly weaker notion of atomicity: an expression is atomic if it reduces in
  one step to something that does not reduce further.
* Changed notation for embedding Coq assertions into Iris.  The new notation is
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  ⌜φ⌝.  Also removed `=` and `⊥` from the Iris scope.  (The old notations are
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  provided in `base_logic.deprecated`.)
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* Up-closure of namespaces is now a notation (↑) instead of a coercion.
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* With invariants and the physical state being handled in the logic, there is no
  longer any reason to demand the CMRA unit to be discrete.
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* The language can now fork off multiple threads at once.
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* Local Updates (for the authoritative monoid) are now a 4-way relation with
  syntax-directed lemmas proving them.
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## Iris 2.0

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* [heap_lang] No longer use dependent types for expressions.  Instead, values
  carry a proof of closedness.  Substitution, closedness and value-ness proofs
  are performed by computation after reflecting into a term langauge that knows
  about values and closed expressions.
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* [program_logic/language] The language does not define its own "atomic"
  predicate.  Instead, atomicity is defined as reducing in one step to a value.
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* [program_logic] Due to a lack of maintenance and usefulness, lifting lemmas
  for Hoare triples are removed.

## Iris 2.0-rc2

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This version matches the final ICFP 2016 paper.
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* [algebra] Make the core of an RA or CMRA a partial function.
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* [program_logic/lifting] Lifting lemmas no longer round-trip through a
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  user-chosen predicate to define the configurations we can reduce to; they
  directly relate to the operational semantics.  This is equivalent and
  much simpler to read.
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## Iris 2.0-rc1

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This is the Coq development and Iris Documentation as submitted to ICFP 2016.