CHANGELOG.md

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 [#] still need to be ported to the Iris Documentation LaTeX file(s).

Iris master

Changes in and extensions of the theory:

• [#] Change in the definition of WP, so that there is a fancy update between the quantification over the next states and the later modality. This makes it possible to prove more powerful lifting lemmas: The new versions feature an "update that takes a step".
• [#] Weaken the semantics of CAS in heap_lang to be efficiently implementable: CAS may only be used to compare "unboxed" values that can be represented in a single machine word.
• [#] Add weakest preconditions for total program correctness.
• [#] "(Potentially) stuck" weakest preconditions are no longer considered experimental.
• [#] The adequacy statement for weakest preconditions now also involves the final state.
• [#] Add the notion of an "observation" to the language interface, so that every reduction step can optionally be marked with an event, and an execution trace has a matching list of events. Change WP so that it is told the entire future trace of observations from the beginning. Use this in heap_lang to implement prophecy variables.
• [#] The Löb rule is now a derived rule; it follows from later-intro, later being contractive and the fact that we can take fixpoints of contractive functions.
• [#] Add atomic updates and logically atomic triples, including tactic support. See heap_lang/lib/increment.v for an example.
• [#] heap_lang now uses right-to-left evaluation order. This makes it significantly easier to write specifications of curried functions.
• [#] heap_lang values are now injected in heap_lang expressions via a specific constructor of the expr inductive type. This simplifies much the tactical infrastructure around the language. In particular, this allow us to get rid the reflection mechanism that was needed for proving closedness, atomicity and "valueness" of a term. The price to pay is the addition of new "administrative" reductions in the operational semantics of the language.
• [#] Extend the state interpretation with a natural number that keeps track of the number of forked-off threads, and have a global fixed proposition that describes the postcondition of each forked-off thread (instead of it being True). Additionally, there is a stronger variant of the adequacy theorem that allows to make use of the postconditions of the forked-off threads.

Changes in Coq:

• An all-new generalized proof mode that abstracts away from Iris! See http://iris-project.org/mosel/ for the corresponding paper. Major new features:

  • The proof mode can now be used with logics derived from Iris (like iGPS), with non-step-indexed logics and even with non-affine (i.e., linear) logics.

  • iModIntro is more flexible and more powerful, it now also subsumes iNext and iAlways.

  • General infrastructure for deriving a logic for monotone predicates over an existing logic (see the paper for more details).

    Developments instantiating the proof mode typeclasses may need significant changes. For developments just using the proof mode tactics, porting should not be too much effort. Notable things to port are:

  • All the BI laws moved from the uPred module to the bi module. For example, uPred.later_equivI became bi.later_equivI.

  • Big-ops are automatically imported, imports of iris.base_logic.big_op have to be removed.

  • The ⊢ notation can sometimes infer different (but convertible) terms when seraching for the BI to use, which (due to Coq limitations) can lead to failing rewrites, in particular when rewriting at function types.

• The iInv tactic can now be used without the second argument (the name for the closing update). It will then instead add the obligation to close the invariant to the goal.
• Added support for defining derived connectives involving n-ary binders using telescopes.
• The proof mode now more consistently "prettifies" the goal after each tactic. Prettification also simplifies some BI connectives, like conditional modalities and telescope quantifiers.
• Improved pretty-printing of Iris connectives (in particular WP and fancy updates) when Coq has to line-wrap the output. This goes hand-in-hand with an improved test suite that also tests pretty-printing.
• Added a gmultiset RA.
• Rename timelessP -> timeless (projection of the Timeless class)
• The CMRA axiom cmra_extend is now stated in Type, using sigT instead of in Prop using exists. This makes it possible to define the function space CMRA even for an infinite domain.
• Rename proof mode type classes for laters:
  • IntoLaterN → MaybeIntoLaterN (this one may strip a later)
  • IntoLaterN' → IntoLaterN (this one should strip a later)
  • IntoLaterNEnv → MaybeIntoLaterNEnv
  • IntoLaterNEnvs → MaybeIntoLaterNEnvs
• Rename:
  • frag_auth_op → frac_auth_frag_op
  • cmra_opM_assoc → cmra_op_opM_assoc
  • cmra_opM_assoc_L → cmra_op_opM_assoc_L
  • cmra_opM_assoc' → cmra_opM_opM_assoc
• namespaces has been moved to std++.
• Changed IntoVal to be directly usable for rewriting e into of_val v, and changed AsVal to be usable for rewriting via the [v <-] destruct pattern.
• wp_fork is now written in curried form.
• PureExec/wp_pure now supports taking multiple steps at once.
• A new tactic, wp_pures, executes as many pure steps as possible, excluding steps that would require unlocking subterms. Every impure wp_ tactic executes this tactic before doing anything else.

Iris 3.1.0 (released 2017-12-19)

Changes in and extensions of the theory:

• Define uPred as a quotient on monotone predicates M -> SProp.
• Get rid of some primitive laws; they can be derived: True ⊢ □ True and □ (P ∧ Q) ⊢ □ (P ∗ Q)
• Camera morphisms have to be homomorphisms, not just monotone functions.
• Add a proof that f has a fixed point if f^k is contractive.
• Constructions for least and greatest fixed points over monotone predicates (defined in the logic of Iris using impredicative quantification).
• Add a proof of the inverse of wp_bind.
• [Experimental feature] Add new modality: ■ ("plainly").
• [Experimental feature] 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 = NotStuck while stuck WP uses s = MaybeStuck.

Changes in Coq:

• Move the prelude folder to its own project: coq-std++
• Some extensions/improvements of heap_lang:
  • Improve handling of pure (non-state-dependent) reductions.
  • Add fetch-and-add (FAA) operation.
  • Add syntax for all Coq's binary operations on Z.
• 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.
• Improved big operators:
  • They are no longer tied to cameras, but work on any monoid
  • The version of big operations over lists was redefined so that it enjoys more definitional equalities.
• Rename some things and change notation:
  • The unit of a camera: empty -> unit, ∅ -> ε
  • Disjointness: ⊥ -> ##
  • A proof mode type class IntoOp -> IsOp
  • 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
  • Adequacy for non-stuck weakestpre: adequate_safe -> adequate_not_stuck
  • Consistently SnakeCase identifiers:
    • CMRAMixin -> CmraMixin
    • CMRAT -> CmraT
    • CMRATotal -> CmraTotal
    • CMRAMorphism -> CmraMorphism
    • CMRADiscrete -> CmraDiscrete
    • UCMRAMixin -> UcmraMixin
    • UCMRAT -> UcmraT
    • DRAMixin -> DraMixin
    • DRAT -> DraT
    • STS -> Sts
  • Many lemmas also changed their name. always_* became persistently_*, and furthermore: (the following list is not complete)
    • 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:

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")

• PersistentL and TimelessL (persistence and timelessness of lists of propositions) are replaces by TCForall from std++.
• Fix a bunch of consistency issues in the proof mode, and make it overall more usable. In particular:
  • 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.
  • 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.
• Make ofe_fun dependently typed, subsuming iprod. The latter got removed.
• 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.
• 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.
• 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.
• Various improvements to solve_ndisj.
• 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.
• The rules internal_eq_rewrite and internal_eq_rewrite_contractive are now stated in the logic, i.e., they are iApply-friendly.

Iris 3.0.0 (released 2017-01-11)

• 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.
• View shifts are radically simplified to just internalize frame-preserving updates. Weakestpre is defined inside the logic, and invariants and view shifts with masks are also coded up inside Iris. Adequacy of weakestpre is proven in the logic. The old ownership of the entire physical state is replaced by a user-selected predicate over physical state that is maintained by weakestpre.
• Use OFEs instead of COFEs everywhere. COFEs are only used for solving the recursive domain equation. As a consequence, CMRAs no longer need a proof of completeness. (The old cofeT is provided by algebra.deprecated.)
• Implement a new agreement construction. Unlike the old one, this one preserves discreteness. dec_agree is thus no longer needed and has been moved to algebra.deprecated.
• 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.
• 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.
• 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 ⌜φ⌝. Also removed = and ⊥ from the Iris scope. (The old notations are provided in base_logic.deprecated.)
• Up-closure of namespaces is now a notation (↑) instead of a coercion.
• 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.
• The language can now fork off multiple threads at once.
• Local Updates (for the authoritative monoid) are now a 4-way relation with syntax-directed lemmas proving them.

Iris 2.0

• [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.
• [program_logic/language] The language does not define its own "atomic" predicate. Instead, atomicity is defined as reducing in one step to a value.
• [program_logic] Due to a lack of maintenance and usefulness, lifting lemmas for Hoare triples are removed.

Iris 2.0-rc2

This version matches the final ICFP 2016 paper.

• [algebra] Make the core of an RA or CMRA a partial function.
• [program_logic/lifting] Lifting lemmas no longer round-trip through a 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.

Iris 2.0-rc1

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