Platform-Specific Setup

Generic Build Instructions

Setting up a basic release build:

git clone
cd lean4
mkdir -p build/release
cd build/release
cmake ../..

Setting up a basic debug build:

git clone
cd lean4
mkdir -p build/debug
cd build/debug

This will compile the Lean library and binary into the stage1 subfolder; see below for details. Add -jN for an appropriate N to make for a parallel build.

Useful CMake Configuration Settings

Pass these along with the cmake ../.. command.

    Select the build type. Valid values are RELEASE (default), DEBUG, RELWITHDEBINFO, and MINSIZEREL.

    Select the C/C++ compilers to use. Official Lean releases currently use Clang; see also .github/workflows/ci.yml for the CI config.

Lean will automatically use CCache if available to avoid redundant builds, especially after stage 0 has been updated (see below).

Lean Build Pipeline

Since version 4, Lean is a partially bootstrapped program: most parts of the frontend and compiler are written in Lean itself and thus need to be built before building Lean itself - which is needed to again build those parts. This cycle is broken by using pre-built C files checked into the repository (which ultimately go back to a point where the Lean compiler was not written in Lean) in place of these Lean inputs and then compiling everything in multiple stages up to a fixed point. The build directory is organized in these stages:

  # Bootstrap binary built from stage0/src/.
  # We do not use any other files from this directory in further stages.
    config.h  # config variables used to build `lean` such as use allocator
    runtime/lean.h  # runtime headers, used by extracted C code, uses `config.h`
    Makefile  # used by `leanmake`
    lean/**/*.olean  # the Lean library (incl. the compiler) compiled by the previous stage's `lean`
    temp/**/*.{c,o}  # the library extracted to C and compiled by `leanc`
    libInit.a libStd.a libLean.a  # static libraries of the Lean library
    libleancpp.a  # a static library of the C++ sources of Lean
    lean  # the Lean compiler & server linked together from the above libraries
    leanc  # a wrapper around a C compiler supplying search paths etc
    leanmake  # a wrapper around `make` supplying the Makefile above

Stage 0 can be viewed as a blackbox since it does not depend on any local changes and is equivalent to downloading a bootstrapping binary as done in other compilers. The build for any other stage starts by building the runtime and standard library from src/, using the lean binary from the previous stage in the latter case, which are then assembled into a new bin/lean binary.

Each stage can be built by calling make stageN in the root build folder. Running just make will default to stage 1, which is usually sufficient for testing changes on the test suite or other files outside of the stdlib. However, it might happen that the stage 1 compiler is not able to load its own stdlib, e.g. when changing the .olean format: the stage 1 stdlib will use the format generated by the stage 0 compiler, but the stage 1 compiler will expect the new format. In this case, we should continue with building and testing stage 2 instead, which will both build and expect the new format. Note that this is only possible because when building a stage's stdlib, we use the previous compiler but never load the previous stdlib (since everything is prelude). We can also use stage 2 to test changes in the compiler or other "meta" parts, i.e. changes that affect the produced (.olean or .c) code, on the stdlib and compiler itself. We are not aware of any "meta-meta" parts that influence more than two stages of compilation, so stage 3 should always be identical to stage 2 and only exists as a sanity check.

In summary, doing a standard build via make involves these steps:

  1. compile the stage0/src archived sources into stage0/bin/lean
  2. use it to compile the current library (including your changes) into stage1/lib
  3. link that and the current C++ code from src/ into stage1/bin/lean

You now have a Lean binary and library that include your changes, though their own compilation was not influenced by them, that you can use to test your changes on test programs whose compilation will be influenced by the changes.

Finally, when we want to use new language features in the library, we need to update the stage 0 compiler, which can be done via make -C stageN update-stage0. make update-stage0 without -C defaults to stage1.

Development Setup

After building a stage, you can invoke make -C stageN test (or, even better, make -C stageN test ARGS=-jN to make ctest parallel) to run the Lean test suite. make test without -C defaults to stage1. While the Lean tests will automatically use that stage's corresponding Lean executables, for running tests or compiling Lean programs manually, you need to put them into your PATH yourself. A simple option for doing that is to use elan, see the next section.

You can use any of the supported editors for editing the Lean source code. If you set up elan as below, opening src/ as a workspace folder should ensure that stage 0 will be used for file in that directory. You should also set the LEAN_SRC_PATH environment variable to the path of the src/ directory to enable go-to-definition in the stdlib (automatically set when using nix-shell).

Dev setup using elan

You can use elan to easily switch between stages and build configurations based on the current directory, both for the lean/leanc/leanmake binaries in your shell's PATH and inside your editor.

If you haven't already installed elan, you can do so, without installing a default version of Lean, using

curl -sSf | sh -s -- --default-toolchain none

You can use elan toolchain link to give a specific stage build directory a reference name, then use elan override set to associate such a name to the current directory. We usually want to use stage0 for editing files in src and stage1 for everything else (e.g. tests).

# in the Lean rootdir
elan toolchain link lean4 build/release/stage1
elan toolchain link lean4-stage0 build/release/stage0
# make `lean` etc. point to stage1 in the rootdir and subdirs
elan override set lean4
cd src
# make `lean` etc. point to stage0 anywhere inside `src`
elan override set lean4-stage0

You can also use the +toolchain shorthand (e.g. lean +lean4-debug) to switch toolchains on the spot. lean4-mode will automatically use the lean executable associated with the directory of the current file as long as lean4-rootdir is unset and ~/.elan/bin is in your exec-path. Where Emacs sources the exec-path from can be a bit unclear depending on your configuration, so alternatively you can also set lean4-rootdir to "~/.elan" explicitly.

You might find that debugging through elan, e.g. via gdb lean, disables some things like symbol autocompletion because at first only the elan proxy binary is loaded. You can instead pass the explicit path to bin/lean in your build folder to gdb, or use gdb $(elan which lean).


  • Call make with an additional VERBOSE=1 argument to print executed commands.