Alloy is a Lean 4 library that allows one to embed external FFI code (currently just C) directly within Lean. For example, we can define an external C add function like so:

alloy c extern def myAdd (x y : UInt32) : UInt32 := {
  return x + y;

And Alloy will generate the corresponding C function:

LEAN_EXPORT uint32_t _alloy_c_l_myAdd ( uint32_t x , uint32_t y ) {
  return x + y;

Building Shims

Alloy exploits Lake's module facets feature to automagically build the shim it produces when compiling the module. Combined with the new precompileModules feature, this allows the shim code to be directly used by importers in a interpreted context (e.g., for #eval or when editing).

To use Alloy with your project and build shims for a library, add the following to your Lakefile:

require alloy from git ""

module_data alloy.c.o.export : BuildJob FilePath
module_data alloy.c.o.noexport : BuildJob FilePath
lean_lib <your-lib> where
  precompileModules := true
  nativeFacets := fun shouldExport =>
    if shouldExport then
      #[Module.oExportFacet, `alloy.c.o.export]
      #[Module.oNoExportFacet, `alloy.c.o.noexport]
  -- and whatever other configuration options you wish to add

Take a look at the examples to see how all of this works. The my_add example provides a minimal setup whereas the S example provides a more complete demonstration of Alloy's power.


Alloy is still a work-in-progress. However, it is now at the point where it can be feasible used to build FFIs. Its biggest TODOs are LSP support for the embedded C code and adding more utilities to help with common code patterns (e.g., defining wrapped C structures).