This webpage is about Lean 3, which is effectively obsolete; the community has migrated to Lean 4.

Jump to the corresponding page on the main Lean 4 website.

Get started with Lean #

You have several options for installing Lean 3, described below:

Regular install #

After this installation procedure, it is crucial to read how to start or get a Lean project. And of course you'll probably want to learn Lean!

A regular install following these instructions installs a few pieces of supporting software. You can read about this infrastructure if you are curious, but it is not necessary in order to use Lean.

Just a quick glance #

Lean can run in a web browser, although the performance is worse than a regular install. Nevertheless it is good for taking a quick look, for example at some first proofs in Lean. You need to wait for the orange bar at the top to turn green and say "Lean is ready!". Then you can click anywhere inside begin and end pairs to see the state of proofs evolving.

With a bit more time, you can play The Natural Number Game, in your browser.

You can also use Lean on CoCalc.

Maybe a couple of nights #

If you are more serious about trying Lean, or can't stand waiting for your web browser, but don't want to start installing too many things, then you can try one of our autonomous bundles for:

Each of these bundles contains a folder trylean from which you can run the program trylean without installing anything anywhere else on your computer (although MacOS users will need to tell their system they really want to run it). On Windows this is a batch file, and there will be a Microsoft Defender warning when running it. To allow the batch file to execute, click on "More Info" then "Run anyway". These bundles contain Lean itself, VScodium which is a distributable version of the VS Code editor, the Lean VS Code extension, the mathlib library, and a couple of Lean files to play with.

The downside is you won't be able to create your own projects or easily upgrade Lean and mathlib. You'll need a regular install for this.