Eclair runtime API

Eclair is meant to be embedded and controlled from inside another host language. It exposes a tiny API that allows you to do all the high level operations:

  1. Initializing the Eclair runtime,
  2. Serializing data from host language to Eclair,
  3. Running the Eclair program,
  4. Deserializing results from Eclair back to the host language,
  5. Cleanup / teardown of the Eclair runtime.

Note that the types of the API functions mentioned below are written down in C just as an indication, it’s also completely possible to call these functions from JavaScript-based languages if Eclair code is compiled to WebAssembly for example.

Eclair exposes a lot of low-level internal details in it’s runtime API, giving a developer a lot of control over how they want to use Eclair. This comes at a cost though: it’s easier to make mistakes and introduce bugs. For this reason, high-level language bindings exist that make integrating with Eclair code a much easier task. Or you can use the low level API directly.

Initializing the Eclair runtime

Before you can call any of the other functions in Eclair, it’s important that you first initialize the runtime. This can be done by invoking the eclair_program_init function:

struct program;

struct program* eclair_program_init();

The function returns an opaque pointer (a “handle” to the Eclair program). The rest of the API always requires this pointer to be passed in as the first argument.

Serializing data from host language to Eclair

Now that the runtime is initialized, you can start serializing data from your host language to Eclair. The language provides a couple of functions to add one or more facts:

void eclair_add_fact(
  struct program* program,
  uint32_t fact_type,
  uint32_t* fact_data

void eclair_add_facts(
  struct program* program,
  uint32_t fact_type,
  uint32_t* fact_data,
  uint32_t fact_count

One thing might immediately stand out: Eclair only accepts an array of uint32_t data. This is done for two reasons:

  1. Eclair only uses uint32_t internally to represent data (for both performance and simplicity),
  2. It’s better for performance when communicating data back and forth, since we only need to do a single function call into Eclair with a single contiguous array.

The fact array should be filled with data of one fact directly followed by the next. For example, given this Eclair program:

@def edge(u32, u32) input.

If you wanted to push edge(1, 2) and edge(2, 3) into Eclair, you would need to create an array of this shape:

uint32_t fact_data[] = {
  1, 2,
  3, 4

If you have facts that contain data that is not a uint32_t (e.g. a string value), you will need to first encode the string in Eclair. This can be done with the eclair_encode_string function that takes both the length of the string and a pointer to a byte-array (Eclair assumes UTF-8 encoding):

uint32_t eclair_encode_string(
  struct program*,
  uint32_t string_length,
  const char* string_data

The function returns a uint32_t value that represents the string, that you can insert at the right location into the fact array.

One final thing to point out about eclair_add_fact and eclair_add_facts is the fact_type argument. In order to get the value for this argument, you will need to call eclair_encode_string with the name of the relation, to get the corresponding fact type value back.

Running an Eclair program

Once you have serialized all your input facts into Eclair, you can now run your program. This is done by invoking the eclair_program_run function:

void eclair_program_run(struct program*);

This will run the Eclair Datalog program from start to end and calculate all derived facts.

Deserializing data from Eclair back to the host language

Deserializing the data back into the host language is similar to serialization of data. Eclair works here also with a single uint32_t array of data. For deserialization a few more helper functions are required to process the returned array of data. The functions related to deserialization are the following:

uint32_t eclair_fact_count(
  struct program*,
  uint32_t fact_type

uint32_t* eclair_get_facts(
  struct program*,
  uint32_t fact_type

void eclair_free_buffer(uint32_t* data);

eclair_fact_count returns the total amount of facts returned for a relation, while eclair_get_facts returns the actual array containing fact data. eclair_free_buffer is needed since the returned uint32_t-array is dynamically allocated on the heap and needs to be manually freed by the host language after it is no longer needed.

These 3 functions allow you to write the following code for processing fact results in the host language:

uint32_t fact_count = eclair_fact_count(program, fact_type);
uint32_t* data = eclair_get_facts(program, fact_type);

for (uint32_t i = 0; i < fact_count; i++) {
   // Process each fact in the array here.
   // Be sure to index the facts in the array properly!


If you have string values inside your fact data, you will manually need to call eclair_decode_string to go from the uint32_t value back to the underlying string byte-array. The signature of this function is as follows:

struct symbol {
  uint32_t length;
  const char* data;

struct symbol* eclair_decode_string(
  struct program*,
  uint32_t string_index

The returned symbol does not need to be freed, this will happen automatically when the Eclair program is shutdown (see next section).

If you pass in a string index that is not internally used by Eclair, you will get back a NULL pointer because the symbol could not be found. Always check if the returned symbol is valid.

Cleanup of the Eclair runtime

Once you are finished using Eclair, you need to shutdown the runtime. This is a required manual operation since Eclair has no garbage collector and performs manual memory management under the hood.

Eclair provides a single function for this:

void eclair_program_destroy(struct program*);

Calling this function will free up any memory still in use by Eclair, so that the system can use it for other purposes. After this point it’s no longer valid to call other functions of the API.

Allocating memory

When compiling for the WebAssembly target, Eclair also needs to expose malloc and free for allocating memory inside the WebAssembly.Memory buffer. These functions are exposed as eclair_malloc and eclair_free and have the same function signatures as the libc counterpart:

void* eclair_malloc(uint32_t num_bytes);
void  eclair_free(void* pointer);

Each time you need to push an array of data into Eclair (e.g. when adding facts or serializing a string), you first need to do a call to eclair_malloc and use the returned address to fill the WebAssembly memory buffer with data. After the data is pushed into Eclair and is no longer in use, the data needs to be freed with eclair_free.

© 2021-2023 Luc Tielen