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Iris
Tutorial POPL18
Commits
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Jan 08, 2018
by
Robbert Krebbers
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Tactic overview
===============
Many of the tactics below apply to more goals than described in this document
since the behavior of these tactics can be tuned via instances of the type
classes in the file
[
proofmode/classes
](
proofmode/classes.v
)
. Most notable, many
of the tactics can be applied when the to be introduced or to be eliminated
connective appears under a later, an update modality, or in the conclusion of a
weakest precondition.
Applying hypotheses and lemmas


`iExact "H"`
: finish the goal if the conclusion matches the hypothesis
`H`

`iAssumption`
: finish the goal if the conclusion matches any hypothesis

`iApply pm_trm`
: match the conclusion of the current goal against the
conclusion of
`pm_trm`
and generates goals for the premises of
`pm_trm`
. See
proof mode terms below.
If the applied term has more premises than given specialization patterns, the
pattern is extended with
`[] ... []`
. As a consequence, all unused spatial
hypotheses move to the last premise.
Context management


`iIntros (x1 ... xn) "ipat1 ... ipatn"`
: introduce universal quantifiers
using Coq introduction patterns
`x1 ... xn`
and implications/wands using proof
mode introduction patterns
`ipat1 ... ipatn`
.

`iClear (x1 ... xn) "selpat"`
: clear the hypotheses given by the selection
pattern
`selpat`
and the Coq level hypotheses/variables
`x1 ... xn`
.

`iRevert (x1 ... xn) "selpat"`
: revert the hypotheses given by the selection
pattern
`selpat`
into wands, and the Coq level hypotheses/variables
`x1 ... xn`
into universal quantifiers. Persistent hypotheses are wrapped into
the persistence modality.

`iRename "H1" into "H2"`
: rename the hypothesis
`H1`
into
`H2`
.

`iSpecialize pm_trm`
: instantiate universal quantifiers and eliminate
implications/wands of a hypothesis
`pm_trm`
. See proof mode terms below.

`iSpecialize pm_trm as #`
: instantiate universal quantifiers and eliminate
implications/wands of a hypothesis whose conclusion is persistent. In this
case, all hypotheses can be used for proving the premises, as well as for
the resulting goal.

`iPoseProof pm_trm as (x1 ... xn) "ipat"`
: put
`pm_trm`
into the context and
eliminates it. This tactic is essentially the same as
`iDestruct`
with the
difference that when
`pm_trm`
is a nonuniverisally quantified spatial
hypothesis, it will not throw the hypothesis away.

`iAssert P with "spat" as "ipat"`
: generates a new subgoal
`P`
and adds the
hypothesis
`P`
to the current goal. The specialization pattern
`spat`
specifies which hypotheses will be consumed by proving
`P`
. The introduction
pattern
`ipat`
specifies how to eliminate
`P`
.
In case all branches of
`ipat`
start with a
`#`
(which causes
`P`
to be moved
to the persistent context) or with an
`%`
(which causes
`P`
to be moved to the
pure Coq context), then one can use all hypotheses for proving
`P`
as well as
for proving the current goal.

`iAssert P as %cpat`
: assert
`P`
and eliminate it using the Coq introduction
pattern
`cpat`
. All hypotheses can be used for proving
`P`
as well as for
proving the current goal.
Introduction of logical connectives


`iPureIntro`
: turn a pure goal into a Coq goal. This tactic works for goals
of the shape
`⌜φ⌝`
,
`a ≡ b`
on discrete COFEs, and
`✓ a`
on discrete CMRAs.

`iLeft`
: left introduction of disjunction.

`iRight`
: right introduction of disjunction.

`iSplit`
: introduction of a conjunction, or separating conjunction provided
one of the operands is persistent.

`iSplitL "H1 ... Hn"`
: introduction of a separating conjunction. The
hypotheses
`H1 ... Hn`
are used for the left conjunct, and the remaining ones
for the right conjunct. Persistent hypotheses are ignored, since these do not
need to be split.

`iSplitR "H0 ... Hn"`
: symmetric version of the above.

`iExist t1, .., tn`
: introduction of an existential quantifier.
Elimination of logical connectives


`iExFalso`
: Ex falso sequitur quod libet.

`iDestruct pm_trm as (x1 ... xn) "ipat"`
: elimination of a series of
existential quantifiers using Coq introduction patterns
`x1 ... xn`
, and
elimination of an object level connective using the proof mode introduction
pattern
`ipat`
.
In case all branches of
`ipat`
start with a
`#`
(which causes the hypothesis
to be moved to the persistent context) or with an
`%`
(which causes the
hypothesis to be moved to the pure Coq context), then one can use all
hypotheses for proving the premises of
`pm_trm`
, as well as for proving the
resulting goal. Note that in this case the hypotheses still need to be
subdivided among the spatial premises.

`iDestruct pm_trm as %cpat`
: elimination of a pure hypothesis using the Coq
introduction pattern
`cpat`
. When using this tactic, all hypotheses can be
used for proving the premises of
`pm_trm`
, as well as for proving the
resulting goal.
Separating logic specific tactics


`iFrame (t1 .. tn) "selpat"`
: cancel the Coq terms (or Coq hypotheses)
`t1 ... tn`
and Iris hypotheses given by
`selpat`
in the goal. The constructs
of the selection pattern have the following meaning:
+
`%`
: repeatedly frame hypotheses from the Coq context.
+
`#`
: repeatedly frame hypotheses from the persistent context.
+
`∗`
: frame as much of the spatial context as possible.
Notice that framing spatial hypotheses makes them disappear, but framing Coq
or persistent hypotheses does not make them disappear.
This tactic finishes the goal in case everything in the conclusion has been
framed.

`iCombine "H1" "H2" as "H"`
: turns
`H1 : P1`
and
`H2 : P2`
into
`H : P1 ∗ P2`
.
Modalities


`iModIntro`
: introduction of a modality that is an instance of the
`FromModal`
type class. Instances include: later, except 0, basic update and
fancy update.

`iMod pm_trm as (x1 ... xn) "ipat"`
: eliminate a modality
`pm_trm`
that is
an instance of the
`ElimModal`
type class. Instances include: later, except 0,
basic update and fancy update.
The persistence and plainness modalities


`iAlways`
: introduce a persistence or plainness modality and the spatial
context. In case of a plainness modality, the tactic will prune all persistent
hypotheses that are not plain.
The later modality


`iNext n`
: introduce
`n`
laters by stripping that number of laters from all
hypotheses. If the argument
`n`
is not given, it strips one later if the
leftmost conjunct is of the shape
`▷ P`
, or
`n`
laters if the leftmost
conjunct is of the shape
`▷^n P`
.

`iLöb as "IH" forall (x1 ... xn) "selpat"`
: perform Löb induction by
generating a hypothesis
`IH : ▷ goal`
. The tactic generalizes over the Coq
level variables
`x1 ... xn`
, the hypotheses given by the selection pattern
`selpat`
, and the spatial context.
Induction


`iInduction x as cpat "IH" forall (x1 ... xn) "selpat"`
: perform induction on
the Coq term
`x`
. The Coq introduction pattern is used to name the introduced
variables. The induction hypotheses are inserted into the persistent context
and given fresh names prefixed
`IH`
. The tactic generalizes over the Coq level
variables
`x1 ... xn`
, the hypotheses given by the selection pattern
`selpat`
,
and the spatial context.
Rewriting


`iRewrite pm_trm`
: rewrite an equality in the conclusion.

`iRewrite pm_trm in "H"`
: rewrite an equality in the hypothesis
`H`
.
Iris


`iInv N as (x1 ... xn) "ipat" "Hclose"`
: open the invariant
`N`
, the update
for closing the invariant is put in a hypothesis named
`Hclose`
.
Miscellaneous


The tactic
`done`
is extended so that it also performs
`iAssumption`
and
introduces pure connectives.

The proof mode adds hints to the core
`eauto`
database so that
`eauto`
automatically introduces: conjunctions and disjunctions, universal and
existential quantifiers, implications and wand, plainness, persistence, later
and update modalities, and pure connectives.
Selection patterns
==================
Selection patterns are used to select hypotheses in the tactics
`iRevert`
,
`iClear`
,
`iFrame`
,
`iLöb`
and
`iInduction`
. The proof mode supports the
following _selection patterns_:

`H`
: select the hypothesis named
`H`
.

`%`
: select the entire pure/Coq context.

`#`
: select the entire persistent context.

`∗`
: select the entire spatial context.
Introduction patterns
=====================
Introduction patterns are used to perform introductions and eliminations of
multiple connectives on the fly. The proof mode supports the following
_introduction patterns_
:

`H`
: create a hypothesis named
`H`
.

`?`
: create an anonymous hypothesis.

`_`
: remove the hypothesis.

`$`
: frame the hypothesis in the goal.

`[ipat1 ipat2]`
: (separating) conjunction elimination. In order to eliminate
conjunctions
`P ∧ Q`
in the spatial context, one of the following conditions
should hold:
+
Either the proposition
`P`
or
`Q`
should be persistent.
+
Either
`ipat1`
or
`ipat2`
should be
`_`
, which results in one of the
conjuncts to be thrown away.

`[ipat1ipat2]`
: disjunction elimination.

`[]`
: false elimination.

`%`
: move the hypothesis to the pure Coq context (anonymously).

`>`
and
`<`
: rewrite using a pure Coq equality

`# ipat`
: move the hypothesis to the persistent context.

`> ipat`
: eliminate a modality (if the goal permits).
Apart from this, there are the following introduction patterns that can only
appear at the top level:

`{selpat}`
: clear the hypotheses given by the selection pattern
`selpat`
.
Items of the selection pattern can be prefixed with
`$`
, which cause them to
be framed instead of cleared.

`!%`
: introduce a pure goal (and leave the proof mode).

`!#`
: introduce an persistence or plainness modality (by calling
`iAlways`
).

`!>`
: introduce a modality (by calling
`iModIntro`
).

`/=`
: perform
`simpl`
.

`//`
: perform
`try done`
on all goals.

`//=`
: syntactic sugar for
`/= //`

`*`
: introduce all universal quantifiers.

`**`
: introduce all universal quantifiers, as well as all arrows and wands.
For example, given:
∀ x, x = 0 ⊢ □ (P → False ∨ □ (Q ∧ ▷ R) ∗ P ∗ ▷ (R ∗ Q ∧ x = pred 2)).
You can write
iIntros (x) "% !# $ [[]  #[HQ HR]] /= !>".
which results in:
x : nat
H : x = 0
______________________________________(1/1)
"HQ" : Q
"HR" : R
□
R ∗ Q ∧ x = 1
Specialization patterns
=======================
Since we are reasoning in a spatial logic, when eliminating a lemma or
hypothesis of type
``P_0 ∗ ... ∗ P_n ∗ R``
, one has to specify how the
hypotheses are split between the premises. The proof mode supports the following
_specification patterns_
to express splitting of hypotheses:

`H`
: use the hypothesis
`H`
(it should match the premise exactly). If
`H`
is
spatial, it will be consumed.

`[H1 .. Hn]`
and
`[H1 .. Hn //]`
: generate a goal for the premise with the
(spatial) hypotheses
`H1 ... Hn`
and all persistent hypotheses. The spatial
hypotheses among
`H1 ... Hn`
will be consumed, and will not be available for
subsequent goals. Hypotheses prefixed with a
`$`
will be framed in the
goal for the premise. The pattern can be terminated with a
`//`
, which causes
`done`
to be called to close the goal (after framing).

`[H1 ... Hn]`
and
`[H1 ... Hn //]`
: the negated forms of the above
patterns, where the goal for the premise will have all spatial premises except
`H1 .. Hn`
.

`[> H1 ... Hn]`
and
`[> H1 ... Hn //]`
: like the above patterns, but these
patterns can only be used if the goal is a modality
`M`
, in which case
the goal for the premise will be wrapped in the modality
`M`
.

`[> H1 ... Hn]`
and
`[> H1 ... Hn //]`
: the negated forms of the above
patterns.

`[# $H1 .. $Hn]`
and
`[# $H1 .. $Hn //]`
: generate a goal for a persistent
premise in which all hypotheses are available. This pattern does not consume
any hypotheses; all hypotheses are available in the goal for the premise, as
well in the subsequent goal. The hypotheses
`$H1 ... $Hn`
will be framed in
the goal for the premise. These patterns can be terminated with a
`//`
, which
causes
`done`
to be called to close the goal (after framing).

`[%]`
and
`[% //]`
: generate a Coq goal for a pure premise. This pattern
does not consume any hypotheses. The pattern can be terminated with a
`//`
,
which causes
`done`
to be called to close the goal.

`[$]`
: solve the premise by framing. It will first repeatedly frame the
spatial hypotheses, and then repeatedly frame the persistent hypotheses.
Spatial hypothesis that are not framed are carried over to the subsequent
goal.

`[> $]`
: like the above pattern, but this pattern can only be used if the
goal is a modality
`M`
, in which case the goal for the premise will be wrapped
in the modality
`M`
before framing.

`[# $]`
: solve the persistent premise by framing. It will first repeatedly
frame the spatial hypotheses, and then repeatedly frame the persistent
hypotheses. This pattern does not consume any hypotheses.
For example, given:
H : □ P ∗ P 2 ∗ R ∗ x = 0 ∗ Q1 ∗ Q2
One can write:
iDestruct ("H" with "[#] [H1 $H2] [$] [% //]") as "[H4 H5]".
Proof mode terms
================
Many of the proof mode tactics (such as
`iDestruct`
,
`iApply`
,
`iRewrite`
) can
take both hypotheses and lemmas, and allow one to instantiate universal
quantifiers and implications/wands of these hypotheses/lemmas on the fly.
The syntax for the arguments of these tactics, called _proof mode terms_, is:
(H $! t1 ... tn with "spat1 .. spatn")
Here,
`H`
can be both a hypothesis, as well as a Coq lemma whose conclusion is
of the shape
`P ⊢ Q`
. In the above,
`t1 ... tn`
are arbitrary Coq terms used
for instantiation of universal quantifiers, and
`spat1 .. spatn`
are
specialization patterns to eliminate implications and wands.
Proof mode terms can be written down using the following short hands too:
(H with "spat1 .. spatn")
(H $! t1 ... tn)
H
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# The Iris tutorial @ POPL'18
For the tutorial material you need to have the following dependencies installed:

Coq 8.6.1 / 8.7.0 / 8.7.1

Ssreflect 1.6.4

Coqstd++ 1.1

Iris 3.1
*Note:*
the tutorial material will not work with earlier versions of Iris, it
is important to install the exact versions as given above.
## Installing Iris via opam
The easiest, and recommend, way of installing Iris and its dependencies is via
the OCaml package manager opam (1.2.2 or newer). You first have to add the Coq
opam repository:
opam repo add coqreleased https://coq.inria.fr/opam/released
Then you can do
`opam install coqiris`
.
## Installing Iris from source
If you are unable to use opam, you can also build Iris from source. For this,
make sure to
`git checkout`
the correct versions, and run
`make; make install`
for Ssreflect, Coqstd++ and Iris.
## Compiling the exercises
Run
`make`
to compile the exercises. You need to have exercise 3 compiled to
work on exercise 4 and 5.
## Documentation
The file
`ProofMode.md`
in the tutorial material (which can also be found in the
root of the Iris repository) contains a list of the Iris Proof Mode tactics.
If you would like to know more about Iris, we recommend to take a look at:

http://irisproject.org/tutorialmaterial.html
Lecture Notes on Iris: HigherOrder Concurrent Separation Logic
Lars Birkedal and Aleš Bizjak
Used for an MSc course on concurrent separation logic at Aarhus University

https://www.mpisws.org/~dreyer/papers/irisgroundup/paper.pdf
Iris from the Ground Up: A Modular Foundation for HigherOrder Concurrent
Separation Logic
Ralf Jung, Robbert Krebbers, JacquesHenri Jourdan, Aleš Bizjak, Lars
Birkedal, Derek Dreyer.
A detailed description of the Iris logic and its model
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