# Update on mathport (Dec 2021)

mathport is the tool we're planning on using to help us port mathlib to Lean 4. It has mostly been written by Mario Carneiro and Daniel Selsam, and Gabriel Ebner and I have been making some fixes.

To provide some context, mathlib is the primary library for mathematics in Lean 3, containing over 700k lines of code and growing fast! Lean 4 now has a preliminary release, and we would really like to transition to building a mathematics library in Lean 4. While the type theory and kernel in Lean 4 are quite similar from a user point of view to Lean 3, it is certainly not the case that we can run Lean 3 code in Lean 4. Our aspiration is to achieve a "semi-automated" port.

The mathport tool first of all provides a complete binary level port of mathlib (i.e. generating a Lean 4 .olean file for every .olean file in mathlib). This means that you will be able to import all files from mathlib, as long as you don't expect to be able to look at the actual sources! We have largely rejected the idea of a dual-compilation setup, in which existing code stays in Lean 3, while new code is written in Lean 4. The binary port could make this viable, but it seems complicated and fragile, and the community is excited to take full advantage of Lean 4. (A dual-compilation setup would not allow Lean 3 files to import from Lean 4 files, so the version transition would have to be a cut across the import graph.) Thus mathport further attempts a "best-effort" port of the source files, translating Lean 3 syntax to Lean 4 syntax. Right now, that "best effort" is not quite good enough!

Our goal is that this will steadily improve, until we reach a point at which it become viable for humans to finish the migration in an incremental fashion. Notably, because of the size of mathlib, and its growth rate, we have decided it is not desirable to "freeze" mathlib3 until very late in the process. (Throughout this document, mathlib refers to the current mathlib repository, but I'll try to consistently include the 3 to disambiguate!) Thus we will be regularly running mathport on a continuously evolving mathlib3 repository, and for an initial period not depositing the output in the mathlib4 repository. Nevertheless, mathport takes as input both mathlib3 and mathlib4, and attempts to automatically make use of the parts of the library which have already been ported, by "aligning" definitions. Thus once the output quality of mathport is sufficiently good, we expect to be able to move files across to the mathlib4 repository (starting from the bottom of the import hierarchy), while continuing to re-run mathport on the still evolving mathlib repository! Eventually, however, we will declare "flag day", at which point the mathlib3 repository will stop accepting PRs for new material, and we have a (hopefully brief!) intensive period of completing the migration.

Notably, mathport makes no attempt to automatically translate the tactics that have been written in mathlib3. Metaprogramming is sufficiently different in Lean 4 that this would be impossible, and moreover there are significant stylistic differences when writing tactics in Lean 4, so a literal translation would not be desirable. This means that we have a huge amount of work remaining to re-implement all the mathlib3 tactics in Lean 4. We've already done a few important ones (notably simp is implemented in Lean 4, and we have ring in mathlib4). We would really like to preserve feature parity as we make the transition, and so are hoping to re-implement tactics, rather than "dumb-down proofs" wherever possible.

The occasion for this blog post is that we now have continuous integration set up for mathport, and a reasonably easy to use setup that lets you work with the output of mathport without having to run it yourself.

I'll describe that setup up below, but first explain what sort of efforts are probably most useful right now to help the mathlib port.

They are approximately in priority order, in terms of my guess about what will hold up the port the most.

• Porting missing tactics from mathlib3 to mathlib4. This is still a huge task, and will not be automated in any way. If you've contributed tactics to mathlib3, please consider trying to port them to Lean 4. If you're interested in learning some Lean 4 metaprogramming, what better way to do that than porting tactics? We'll write more about this in a future post, with some pointers about places to get started, and how to hook up new tactic implementations to the existing tactic parsers that Mario has already ported to mathlib4.
• Resolve outstanding issues in mathport. (On some issues there's already an indicated fix but it needs implementing/testing. Other issues still need diagnosis.)
• Open the mathlib3port repository (instructions below), look at files (probably starting with "low-level" files), and identify things that mathport should be doing better. Check there isn't already an open issue, then open an issue.
• Note: at this stage I think it would be a bad idea to actually take a file from mathlib3port, clean it up, and PR it to mathlib4. That will hopefully come later, but we need to fix many mathport issues first.
• Instead, it is fine to make changes that you think mathport should be doing already and committing these on a branch, so that you can link to diffs when opening mathport issues.
• For now, don't worry too much about the state of proofs. We'd like to get to a state where the vast majority of statements are correctly translated as soon as possible.
• There are still many alignment problems between Lean 3 / mathlib3 declarations, and Lean 4 / mathlib4 declarations. Sometimes these can be fixed by adding #align commands in mathlib4. Sometimes they may turn out to be mathport bugs. Sometimes they will reflect deeper design problems we're going to need to talk about!

## Background: what is mathport?

mathport consists of two loosely coupled components: binport (largely Daniel's work), and synport (largely Mario's work).

• binport constructs Lean 4 .olean files, from Lean 3 .olean files. It largely works, and means that you can import mathlib3 content into Lean 4 (as long as you don't expect to have source files!) This is what lets us do things like:

import Mathbin

#lookup3 algebraic_geometry.Scheme
#check AlgebraicGeometry.Scheme


Yay, Lean 4 has schemes! :-) To see this file in action, you should check out a copy of the mathlib3port repository described below, and make a new file there.

• synport constructs Lean 4 .lean files, on a "best effort" basis. (It uses both the output from binport, and Lean 3's --ast output to guide it.) We should not expect that this will ever converge to a perfect translator. Instead the hope is that it gives us something that humans can plausibly improve to a complete translation of mathlib3.

To understand how mathport (mostly talking about the synport part from here on) works, it's important to understand that it is translating mathlib3 to Lean 4 source code, "modulo" the current content of mathlib4. That is, the premise is that as we progressively construct mathlib4 (whether by translating by hand, moving content from mathport's output to mathlib4, or adding #align statements) the output from running mathport on mathlib3 will change. In particular, as mathport is translating each declaration, it checks to see if a corresponding declaration in mathlib4 already exists, and is defeq. If so, mathport will instead just use that declaration. If not, mathport will make a copy, appending a × to the name. Sometimes these misalignments are due to an unintentional non-defeq, that can be fixed in mathlib4. Other times, we genuinely want to change something in mathlib4 (e.g. to use Lean 4's multiple parent structures, which are better than old_structure_cmd). As a result, we expect that some misalignments will persist throughout the mathport-assisted stage of the port, and only afterwards will we polish these away.

A detailed account of how binport and synport are working is beyond the scope of this blog post. Hopefully we'll have one eventually, but in the meantime Mario's talk is very helpful.

## What should I look at?

Please note that mathport takes considerable resources to run on mathlib3: approximately 3.5 hours, and 32gb of RAM. So you'll probably want to look at artifacts generated by CI rather than running it yourself. There are four GitHub repositories you can look at:

• mathlib3port should be most people's first stop. This contains a copy of the .lean files produced by a recent run of synport, in the Mathbin directory. You should just be able to check out a copy of this repository, and open the folder in VS Code, to see the current state of mathport output. You can also try out the above example with #check AlgebraicGeometry.Scheme in a fresh file here.
• Remember the synported files are expected to be horribly broken; most tactics aren't implemented, and there are bugs around parenthesization of arguments, and name resolution!
• Good luck finding even a single file that compiles cleanly right now.
• lean3port is the corresponding repository containing a copy of a recent run of mathport on the Lean 3 core library. It's less interesting perhaps, but also smaller and easier to inspect.
• mathport contains the code for mathport itself, as well as the continuous integration set up that runs mathport on Lean 3 core and mathlib3 every time there is either a PR to mathport, or a commit to master. The artifacts produced by CI appear at https://github.com/leanprover-community/mathport/releases, and the two repositories listed above have a lakefile.lean that will download and unpack these artifacts.
• mathlib4 is the current preliminary port of mathlib to Lean 4. Later, when mathport begins to stabilize, we will begin moving files from the output of mathport into this repository, but not yet. For now, this repository serves several purposes:

• Primarily, it is the home for ports of tactics implemented in mathlib3 to Lean 4.
• In order to build these tactics, a certain minimal library is necessary (e.g. for the ring tactic, we need the notion of a ring!) and we are constructing this by hand.
• There is scope for experimental developments, but please don't just port parts of mathlib3 to Lean 4 for the sake of doing so. If you're investigating how some new Lean 4 feature might be used in the eventual port of mathlib, it's okay to do so in this repository.

For now, the mathlib4 repository has fairly low standards: we don't expect the full review process used in mathlib3. Notable parts of the mathlib4 repository relevant for mathport are:

• Mathlib/Mathport/SpecialNames.lean contains a sequence of #align statements, for cases where a definition in Lean 4 core has a different name from the name that would be produced automatically by changing case conventions from Lean 4 or mathlib3.
• Mathlib/Mathport/Syntax.lean contains definitions of all the syntaxes of tactics currently implemented in mathlib3. These have been written by hand by Mario, and we should take care to keep these up to date. Please be careful editing this file, and we also need to remember to update it if any further changes to tactics land in mathlib3. Because in this file we only have the syntax statements, if you try to use the tactics here you will get Tactic not implemented yet errors. If you would like to work on porting tactics, essentially this file is the TODO list! You should take the relevant syntax definitions, move them to their own file in the Mathlib/Tactic/ directory, and provide an implementation. Conversely, please do not start porting a mathlib3 tactic without faithfully reproducing the syntax, as this will then cause problems for mathport. It's completely fine to provide an interim port, that throws errors when encountering some of the bells and whistles specified by a syntax declaration.

## How do I run mathport?

The Makefile in https://github.com/leanprover-community/mathport is currently the best available documentation for running mathport. You will need to make sure you have curl, git, cmake, and elan installed on your system. Basic usage is make build source predata port.

These stages are:

• build: compile mathport (which is written in Lean 4) itself
• source: pull the relevant commits of Lean 3 and mathlib3, and do a little preparatory work in those directories
• predata: recompile the Lean 3 library and mathlib3, with lean --ast --tlean, to generate the auxiliary files mathport needs.
• port: run mathport on Lean 3 and mathlib3.

Running all of them in sequence is necessary if you're starting from scratch, but is painfully slow.

You don't really want to run make predata yourself. Typically you don't want to run make port on the entire library either: you'd prefer to download an artifact containing the results, but then re-run mathport on a single file, in order to try out a bugfix to mathport.

We provide artifacts for various stages of the build on the releases page of the mathport repository. The script ./download-release.sh nightly-YYYY-MM-DD downloads one of these, after which you can skip the make predata and/or make port steps (you will still need to run make build and make source).

If you've already got a local copy of the output of make port (either by running it yourself, or using ./download-release.sh) you can also use the make TARGET=data.nat.bitwise port-mathbin-single target (similarly for port-lean-single) to run mathport on a single file. This is useful if you are testing a change to mathport.