We aim to provide an interface that is close enough to allow
developing Prolog code that uses Python and visa versa. Differences
between the two Prolog implementation make this non-trivial. SWI-Prolog
has native support for dicts, strings,
unbounded integers, rational numbers and
blobs that provide safe pointers to external objects that are
subject to (atom) garbage collection.
We try to find a compromise to make the data conversion as close as
possible while supporting both systems as good as possible. For this
reason we support creating a Python dict both from a SWI-Prolog dict and
from the Prolog term py({k1:v1, k2:v2, ...}). With
py defined as a prefix operator, this may be written
without parenthesis and is thus equivalent to the SWI-Prolog dict
syntax. The library(janus) library provides access
predicates that are supported by both systems and where the SWI-Prolog
version supports both SWI-Prolog dicts and the above Prolog
representation. See
items/2, values/3, key/2
and items/2.
Calling Python from Prolog provides a low-level and a more high level
interface. The high level interface is realized by py_call/[2,3]
and
py_iter/[2,3].
We realize the low level interfaces py_func/[3,4]
and
py_dot/[4,5]
on top of py_call/2.
The interface for calling Prolog from Python is settled on the five
primitives described in
section 5.
We are discussing to minimize the differences. Below we summarize the
known differences.
- SWI-Prolog represents Phyton dicts as Prolog dicts. XSB uses a term
py({k:v, ...}), where the
py() wrapper is optional. The
predicate py_is_dict/1
may be used to test that a Prolog term represents a Python dict. The
predicates values/3, keys/2, key/2
and items/2
can be used to access either representation.
- SWI-Prolog allows for
prolog(Term) to be sent to
Python, creating an instance of janus.Term().
- SWI-Prolog represents Python object references as a blob.
XSB uses a term. The predicate py_is_object/1
may be used to test that a Prolog term refers to a Python object. In
XSB, the user
must call py_free/1
when done with some object. In SWI-Prolog, either py_free/1
may be used or the object may be left to the Prolog (atom) garbage
collector.
- Prolog exceptions passed to Python are represented differently.
- When calling Prolog from Python and relying on well founded
semantics, only plain truth values (i.e.,
janus.undefined)
are supported in a portable way. Delay lists, providing details
on why the result is undefined, are represented differently.
This section will be written after the dust has settled. Topics
- Dealing with Python dicts
- Dealing with Prolog modules
- Dealing with Prolog references to Python objects
- More?
