Guarded Interaction Trees

This is the Coq formalization of guarded interaction trees, associated examples and case studies. Read the GITrees POPL paper describing our work.

Installation instructions

To install the formalization you will need Iris and std++ libraries. The dependencies can be easily installed using Opam with the following commands:

opam repo add coq-released https://coq.inria.fr/opam/released
opam update
opam install . --deps-only

Then the formalization can be compiled with make and installed with make install. You can pass the additional parameters to compile the formalization using multiple cores, e.g. make -j 3 for compiling using 3 threads.

Code Overview

All the code lives in the theories folder. Below is the quick guide to the code structure.

• gitree/ -- contains the core definitions related to guarded interaction trees
• lib/ -- derived combinators for gitrees
• effects/ -- concrete effects, their semantics, and program logic rules
• examples/input_lang/ -- formalization of the language with io, the soundness and adequacy
• examples/affine_lang/ -- formalization of the affine language, type safety of the language interoperability
• examples/input_lang_callcc/ -- formalization of the language with io, throw and call/cc, the soundness and adequacy
• examples/delim_lang/ -- formalization shift/reset effects, of a language with delimited continuations and its soundness
• prelude.v -- some stuff that is missing from Iris
• lang_generic.v -- generic facts about languages with binders and their interpretations, shared by input_lang and affine_lang

For the representation of binders we use a library implemented by Filip Sieczkowski and Piotr Polesiuk, located in the vendor/Binding/ folder.

References from the paper to the code

The version of the formalization that corresponds to the paper can be found under the tag popl24. Below we describe the correspondence per-section.

• Section 3
  • Definition of guarded interaction trees, constructors, the recursion principle, and the destructors are in gitree/core.v
  • Signtures for IO and higher-order store are in examples/store.v and input_lang/interp.v
  • The programming operations are in gitree/lambda.v and examples/while.v
  • The factorial example is in examples/factorial.v, and the pairs example is in examples/pairs.v
• Section 4
  • The definition of context-dependent versions of reifiers and the reify function are in gitree/reify.v
  • The reduction relation is in gitree/reductions.v
  • The specific reifiers for IO and state are in examples/store.v and input_lang/interp.v
• Section 5
  • The syntax for λrec,io is in input_lang/lang.v
  • The interpretation and the soundness proof are in input_lang/interp.v
• Section 6
  • The definition of the weakest precondition and the basic rules are in gitree/weakestpre.v
  • The additional weakest precondition rules are in program_logic.v and examples/store.v
  • The iter example is in examples/iter.v
• Section 7
  • The logical relation and the adequacy proof are in input_lang/logrel.v
• Section 8
  • The notion of a subeffect is in gitree/core.v
  • The notion of a subreifier and the associated definitions are in gitree/greifiers.v
  • The fact_io example is in examples/factorial.v
• Section 9
  • The syntax for λ⊸,ref is in affine_lang/lang.v
  • The logical relations for the type safety of λ⊸,ref and λrec,io are in affine_lang/logrel1.v and input_lang/logpred.v
  • The logical relation for the combined language is in affine_lang/logrel2.v

Notes

Disjunction property

Some results in the formalization make use of the disjunction property of Iris: if (P ∨ Q) is provable, then either P or Q are provable on their own. This propery is used to show safety of the weakest precondition, and it is related to the difference between internal and external reductions.

The internal reductions of GITrees is the relation istep, as defined in the paper, and it has type iProp as it is an internal relatin. There is also a similar external reduction relation sstep which lives in Coq's Prop. We use the istep relation in our definitions (since it is an internal relation), but we want to state the safety result w.r.t. the external relation sstep, which we take to be the 'proper definition' of the reductions for GITrees.

Showing that istep-safety implies sstep-safety (i.e. that if a GITree can do an istep then it can also do a sstep) requires the disjunction propety. The disjunction property for Iris can be shown assuming classical axioms (e.g. LEM) on the Prop-level.

In order not to introduce classical axioms into the whole formalization, we added the disjunction propety as an assumption to the safety theorem (wp_safety) and all of its instances (e.g. in logical relations).

Ground type of errors

One other difference with the paper worth mentioning, is that in the formalization we "hardcode" the type Err of errors, whereas in the paper we leave it parameterized. That is why in the affine_lang case study we use OtherError to represent linearity violations, instead of Err(Lin).