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.
