Foreign Function Interface

NOTE: The current interface was designed for internal use in Lean and should be considered unstable. It will be refined and extended in the future.

As Lean is written partially in Lean itself and partially in C++, it offers efficient interoperability between the two languages (or rather, between Lean and any language supporting C interfaces). This support is however currently limited to transferring Lean data types; in particular, it is not possible yet to pass or return compound data structures such as C structs by value from or to Lean.

There are two primary attributes for interoperating with other languages:

  • @[extern "sym"] constant leanSym : ... binds a Lean declaration to the external symbol sym. It can also be used with def to provide an internal definition, but ensuring consistency of both definitions is up to the user.
  • @[export sym] def leanSym : ... exports leanSym under the unmangled symbol name sym.

The Lean ABI

The Lean Application Binary Interface (ABI) describes how the signature of a Lean declaration is encoded as a native calling convention. It is based on the standard C ABI and calling convention of the target platform. For a Lean declaration marked with either @[extern "sym"] or @[export sym] for some symbol name sym, let α₁ → ... → αₙ → β be the normalized declaration's type. If n is 0, the corresponding C declaration is

extern s sym;

where s is the C translation of β as specified in the next section. In the case of an @[extern] definition, the symbol's value is guaranteed to be initialized only after calling the Lean module's initializer or that of an importing module; see Initialization.

If n is greater than 0, the corresponding C declaration is

s sym(t₁, ..., tₘ);

where the parameter types tᵢ are the C translation of the αᵢ as in the next section. In the case of @[extern] all irrelevant types are removed first; see next section.

Translating Types from Lean to C

  • The integer types UInt8, ..., UInt64, USize are represented by the C types uint8_t, ..., uint64_t, usize_t, respectively

  • Char is represented by uint32_t

  • Float is represented by double

  • An enum inductive type of at least 2 and at most 2^32 constructors, each of which with no parameters, is represented by the first type of uint8_t, uint16_t, uint32_t that is sufficient to represent all constructor indices.

    For example, the type Bool is represented as uint8_t with values 0 for false and 1 for true.

  • Decidable α is represented the same way as Bool

  • An inductive type with a trivial structure, that is,

    • it is none of the types described above
    • it is not marked unsafe
    • it has a single constructor with a single parameter of relevant type is represented by the representation of that parameter's type.

    For example, { x : α // p }, the Subtype structure of a value of type α and an irrelevant proof, is represented by the representation of α.

  • Nat is represented by lean_object *. Its runtime value is either a pointer to an opaque bignum object or, if the lowest bit of the "pointer" is 1 (lean_is_scalar), an encoded unboxed natural number (lean_box/lean_unbox).

  • A universe Sort u, type constructor ... → Sort u, or proposition p : Prop is irrelevant and is either statically erased (see above) or represented as a lean_object * with the runtime value lean_box(0)

  • Any other type is represented by lean_object *. Its runtime value is a pointer to an object of a subtype of lean_object (see respective declarations in lean.h) or the unboxed value lean_box(cidx) for the cidxth constructor of an inductive type if this constructor does not have any relevant parameters.

    Example: the runtime value of u : Unit is always lean_box(0).


By default, all lean_object * parameters of an @[extern] function are considered owned, i.e. the external code is passed a "virtual RC token" and is responsible for passing this token along to another consuming function (exactly once) or freeing it via lean_dec. To reduce reference counting overhead, parameters can be marked as borrowed by prefixing their type with @&. Borrowed objects must only be passed to other non-consuming functions (arbitrarily often) or converted to owned values using lean_inc. In lean.h, the lean_object * aliases lean_obj_arg and b_lean_obj_arg are used to mark this difference on the C side.

Return values and @[export] parameters are always owned at the moment.


When including Lean code as part of a larger program, modules must be initialized before accessing any of their declarations. Module initialization entails

  • initialization of all "constants" (nullary functions), including closed terms lifted out of other functions
  • execution of all [init] functions
  • execution of all [builtinInit] functions, if the builtin parameter of the module initializer has been set

The module initializer is automatically run with the builtin flag for executables compiled from Lean code and for "plugins" loaded with lean --plugin. For all other modules imported by lean, the initializer is run without builtin. Thus [init] functions are run iff their module is imported, regardless of whether they have native code available or not, while [builtinInit] functions are only run for native executable or plugins, regardless of whether their module is imported or not. lean uses built-in initializers for e.g. registering basic parsers that should be available even without importing their module (which is necessary for bootstrapping).

The initializer for module A.B is called initialize_A_B and will automatically initialize any imported modules. Module initializers are idempotent (when run with the same builtin flag), but not thread-safe. Together with initialization of the Lean runtime, you should execute code like the following exactly once before accessing any Lean declarations:

void lean_initialize_runtime_module();
void lean_initialize();
lean_object * initialize_A_B(uint8_t builtin, lean_object *);
lean_object * initialize_C(uint8_t builtin, lean_object *);

//lean_initialize();  // necessary if you (indirectly) access the `Lean` package

lean_object * res;
// use same default as for Lean executables
uint8_t builtin = 1;
res = initialize_A_B(builtin, lean_io_mk_world());
if (lean_io_result_is_ok(res)) {
} else {
    return ...;  // do not access Lean declarations if initialization failed
res = initialize_C(builtin, lean_io_mk_world());
if (lean_io_result_is_ok(res)) {

//lean_init_task_manager();  // necessary if you (indirectly) use `Task`

@[extern] in the Interpreter

The interpreter can run Lean declarations for which symbols are available in loaded shared libraries, which includes @[extern] declarations. Thus to e.g. run #eval on such a declaration, you need to

  1. compile (at least) the module containing the declaration and its dependencies into a shared library, and then
  2. pass this library to lean --load-dynlib= to run code importing this module.

Note that it is not sufficient to load the foreign library containing the external symbol because the interpreter depends on code that is emitted for each @[extern] declaration. Thus it is not possible to interpret an @[extern] declaration in the same file.

See tests/compiler/foreign for an example.