1 Introduction

Python has a huge developer community that maintains a large set of resources, notably interfaces to just about anything one can imagine. Making such interfaces directly available to Prolog can surely be done. However, developing an interface typically requires programming in C or C++, a skill that is not widely available everywhere. Being able to access Python effortlessly from Prolog puts us in a much better position because Python experience is widely available in our target audience. This solution was proposed in Andersen & Swift, 2023, Swift & Andersen, 2023, initially developed for XSB.

Janus provides a bi-directional interface between Prolog and Python using the low-level C API of both languages. This makes using Python from Prolog as simple as taking the standard SWI-Prolog distribution and loading library library(janus). Using Prolog from Python is as simple as import janus_swi as janus and start making calls. Both Prolog and Python being dynamically typed languages, we can define an easy to use interface that provides a latency of about one μS.

The Python interface is modeled after the recent JavaScript interface developed for the WASM (Web Assembly) version. That is

• A di-directional data conversion is defined. See section 2.

• A Prolog predicate py_call/2 to call Python functions and methods, as well as access and set object attributes.

• A non-deterministic Prolog predicate py_iter/2 to enumerate a Python iterator.

• A Python function janus.query_once() to evaluate a Prolog query as once/1, providing input to Prolog variables using a Python dict and return a Python dict with bindings for each Prolog output variable.

• A python function janus.apply_once() to call a Prolog predicate with N input arguments followed by exactly one output argument. This provides a faster and easier to use interface to compliant predicates.

• Python iterators janus.query() and janus.apply() that provide access to non-deterministic Prolog predicates using the calling conventions of janus.query_once() and janus.apply_once().

The API of Janus is the result of discussions between the SWI-Prolog, XSB and Ciao lang teams. It will be reflected in a PIP (Prolog Improvement Proposal). Considering the large differences in designs and opinions in Prolog implementation, the PIP does not cover all aspects of the API. Many of the predicates and functions have a Compatibility note that explains the relation of the SWI-Prolog API and the PIP. We summarize the differences in section 14.