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README.md

About

This is the code repository for a toy compiler that is proven fully abstract (preserves and reflects equivalences). It has a compiler for arithmetic expressions to a tiny stack machine (idea came from Adam Chlipala's CPDT).

The proofs follow a style that is essentially a modified version of the proof style advocated by Adam Chlipala in Certified Programming with Dependent Types -- modified slightly in that hints are always kept extremely locally scoped. The intention behind this (explained a little further in the library repository for the couple tactics that implement this: dbp/literatecoq) is to make reading proofs be as easy as possible, where on paper you would write "follows by induction using X, Y, and Z". When hints end up in global databases, you can end up writing proofs that say "follows by induction using X, Z", because you had hinted Y somewhere earlier.

Writeup

The accompanying writeup for this compiler is at https://dbp.io/essays/2018-04-19-how-to-prove-a-compiler-fully-abstract.html.

Setup

This requires opam, in order to install Coq (or install Coq some other way, but opam is recommended).

Download the submodules that contains the literatecoq tactics with:

git submodule init
git submodule update

Then you need to get Coq (tested with 8.6, 8.7.2, and 8.8.0: earlier/later may work):

opam repo add coq-released https://coq.inria.fr/opam/released
opam update && opam install coq.8.8.0

Once Coq has been installed, you can build the literatecoq library:

make -C literatecoq

Proving

The proofs are all in src/Proofs.v, which you can open up in your favorite editor for Coq (Proof General & Emacs being common).

Obviously, this tiny example is not going to teach you how to prove things in Coq! There are various resources how to do that. A good one that used to not be online but now seems to be is Bertot and Casteron's Coq'Art: Interactive Theorem Proving and Program Development. Classics that have always been online are Certified Programming with Dependent Types and Software Foundations. Theorem proving (of which I'm still quite new at) seems to be a skill that relies on three pretty different skills: deep understanding of typed functional programming concepts, normal paper-and-pencil proof strategy, and understanding the abilities / quirks of the particular system you are using.

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