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Formalisation of the "Dependent Ghosts Have a Reflection For Free" paper

This formalisation is a companion to the following paper to appear at ICFP. https://hal.science/hal-04163836

The corresponding Zenodo archive containing a VM to run the project is there: https://zenodo.org/records/11500966

Building the syntax (optional)

We build the syntax of our type theories using Autosubst 2 OCaml. We provide the generated files, but if you want to run generation again you can simply run make autosubst after having installed Autosubst 2 OCaml for Coq 8.19.

Just run

opam install coq-autosubst-ocaml

after you installed the other dependencies (see below).

It will generate GAST.v, CCAST.v, unscoped.v and core.v.

Building the project

We use Coq 8.19 and Equations 1.3. You can get Coq and Equations using opam as follows:

opam repo add coq-released https://coq.inria.fr/opam/released
opam update
opam pin add coq 8.19.0
opam install coq-equations coq-autosubst-ocaml

Running make at the root of the project is enough to build everything. It should compile without warnings or errors. The Param module takes a while to build.

Browsing the files

The formalisation can be found in the theories folder.

A rendered version of the files is given here.

Statistics

The formalisation (excluding Autosubst generated files) spans little under 18,000 lines of code. Autosubsts generates a little over 11,000 lines.

Assumptions

You will find only two axioms in the whole development. Both are found in the Model file. The first one assumes injectivity of the ctyval constructor of CC. This is a very natural assumption to make as we know it holds for Coq. The second is injectivity of Π-types for GTT. Sadly, our model is insufficient to derive it but we conjecture it holds as it concerns a conversion that is very close to that of CC which enjoys the property.

The only part of the development which may use those axioms is the admissible rules and the GRTT to GTT translation which uses those rules. We also conjecture it could be done without, albeit with longer proofs.

We provide a file Assumptions.v with reference to all lemmas of the submissions. It uses Print Assumptions on all of them which has the effect of printing the list of axioms used for each one. You may run make clean-assumptions and then make again if you wish to only check assumptions (after the project has been fully built).