Release notes - 7.0.1

SWI Prolog V7 maintains good source level compatibility with previous major versions, but there are significant changes to the term model and foreign language interface (FLI), and it is with these that JPL interfaces.

I have (eventually) prototyped a new version of JPL to serve SWI Prolog V7 in both its default and –traditional modes.

With Jan’s encouragement, I have taken the opportunity to modernise JPL’s APIs, without unnecessarily complicating the migration of user applications from V6 (and earlier).

The Prolog API (jpl_call/4 etc.), which presents JVM classes and objects to Prolog, has barely changed, as there have been no changes to the Java Native Interface (JNI) and programming model which affect JPL’s functionality. The Java language has evolved, but most new features are compiled out, and JPL is an interface to the JVM, not to Java.

The Java API (Query.oneSolution etc.) has changed in some important ways, and these are summarised below.

Modern vs. traditional syntax

summary: JPL7 supports –traditional mode too

When started from Prolog, e.g. as a side-effect of

jpl_call('java.lang.System', getProperty, ['os.arch'], X).

JPL7 respects the prevailing “syntax” option (modern or traditional) set when SWI Prolog was started; if you run

swipl --traditional ...

then JPL7 will automatically use traditional list syntax, otherwise it will use the (default) modern syntax.

If your application starts JPL from Java, e.g. as a side-effect of


then JPL7 initialises using modern syntax; you can override this by calling


before any JPL operation which starts or requires Prolog.

A new Java package name

JPL7’s Java API is in a new package, hence

import org.jpl7.*;

instead of JPL 3.x’s

import jpl.*;

I am confident that this renaming will prevent much more confusion than it causes, by clearly distinguishing the old and new API and applications which use them.

The new package name is more idiomatic, and the domain has been secured for documentation etc.

Unbounded integers

summary: new backwards-compatible support for arbitrarily large integers

JPL7 supports SWI Prolog’s unbounded integers, backwards-compatibly with JPL 3.x’s support for bounded (max 64-bit twos complement) integers.

An org.jpl7.Integer instance can be constructed from a java.math.BigInteger instance as well as from long, int, short, char or byte values.

If you do not expect Prolog to return “big” integer values (i.e. too large, +ve or -ve, to fit into a Java long), then you can continue to use Integer as in JPL 3.x.

Integer.longValue() will throw a JPLException if its value won’t fit in a long, and you can test for and retrieve big values with


Solutions as maps

summary: Hashtable is replaced by Map<String, Term>

In JPL7, a solution promises only to support this interface:

java.util.Map<java.lang.String, org.jpl7.Term>

whereas in JPL 3.x it was explicitly a java.util.Hashtable (from Object to Object).

JPL7 actually returns instances of the more modern HashMap, but this is not formally documented, it could (in principle) change, and you shouldn’t need to know this.

To migrate your JPL 3.x Java code to JPL7, you will need to replace



Map<String, Term>

if you use generics, or by


if you don’t. Use of generic types will eliminate many coercions and enhance static type checking.

Iterating one-at-a-time solutions

summary: hasNext replaces hasMoreSolutions etc.

org.jpl7.Query supports one-at-a-time solution fetching by implementing the Iterable and Iterator interfaces; a Query instance is its own Iterator, hence

for (Map soln : new Query("current_module(X)")) {
    ... soln.get("X").name() ...


Query q = new Query(current_module(X)"));
while (q.hasNext()) {
    ..."X").name() ...

The following JPL 3.x Query methods have been removed:



Use the corresponding methods from the Iterator interface instead:


In this respect, migrating JPL 3.x applications to JPL7 should require only shallow textual editing (flagged by the compiler), not structural changes.

Iterating aggregated solutions

summary: no change here

JPL’s Query.allSolutions() and .nSolutions() are analogous to SWI Prolog’s findall/3 and findnsolns/3.

Note that what JPL calls a solution is strictly a substitution, i.e. a map from variables to bindings. Nevertheless, JPL7 and its documentation will continue to use solution everywhere.

In JPL7, these Query methods (still) return an array of solutions; in current Java best practice they should arguably use java.util.List from the Collections Framework, but I can see no overall advantage in changing this.

Note that Java arrays can be iterated with the for-each construct:

for (Map soln : Query.allSolutions("current_module(X)")) {
    ... soln.get("X").name() ...

Atom is a direct subclass of Term

summary: Term class hierarchy flattened to reflect reality better

In JPL 3.x, Atom extended Compound; it was a specialised compound with no args.

 +-- Compound
 |    +-- Atom

in JPL7, Atom and Compound are sibling subclasses of Term, because SWI Prolog V7 compounds may have zero args yet be distinct from similarly named atoms, and JPL7 Atom instances have extra structure and functionality; there is no longer any inheritance worth representing.

 +-- Compound
 +-- Atom

Migrating your JPL 3.x Java code to JPL7 requires recoding situations (if any) which exploit this inheritance. The Java compiler should flag these for you, and the impact is local (albeit not a simple global edit).

New types of Atom

summary: atoms are backwards-compatibly typed, embracing strings and (some) blobs

Classic Prolog atoms are equal iff their string names are equal, end of story (almost).

SWI Prolog has long had strings and blobs as well as classic atoms, and V7 builds on this.

JPL7 atoms have two internal components:

String name; // as in JPL 3.x
String type; // new in JPL7

For traditional atoms, the type is "text".

For out-of-band special atoms, such as the V7 empty list, the type is "reserved_symbol", as in JPL7 source code

protected static final Atom LIST_NIL_MODERN = new Atom("[]", "reserved_symbol");
public static Atom LIST_NIL = LIST_NIL_MODERN; // unless we switch to --traditional

SWI Prolog V7 strings are represented in JPL7 as Atom instances whose type is "string"; if you don’t bother to check this, strings appear to be returned from Prolog as “text” atoms which may be convenient (or misleading).

Note that type is currently a java.lang.String, not a symbolic constant or enum. This is because SWI Prolog supports user-defined blob types with arbitrary user-defined names, although JPL7 doesn’t (yet) handle these.

List support

summary: update explicit uses of ./2 and []/0, preferably using LIST_PAIR and LIST_NIL

In SWI Prolog V7, lists are not an ADT. Although the empty list is denoted by an out-of-band atom

new Atom("[]", "reserved_symbol")

the list pair constructor is a regular functor

new Compound("[|]", new Term[] {....})

which (to me) means that V7 lists are still just a term encoding convention, as in classic Prolog.

Note also that


are valid structures which do not map comfortably to instances of (say) java.util.List<Term>.

For these reasons taken together, JPL7 passes lists as regular terms, and provides utilities to convert to and from various Java classes.

If your Java code manipulates lists, use the new symbols and methods e.g.

new Compound(JPL.LIST_PAIR, new Term[],{ myTerm, JPL.LIST_NIL })

if (soln.get("X").isListNil()) {

Dict support

summary: not yet implemented or finally designed; aiming to be future-proof

The SWI Prolog V7 dict is an abstract data type (ADT), currently implemented using an out-of-band functor and a conventional, may-change internal structure.

JPL7 proposes to support dicts with a new subclass of Term containing a name and wrapping (or otherwise impersonating) a map, hence (in pseudo code)

new Dict(Map<Term, Term> map) // construct an anonymous dict
new Dict(String name, Map<Term, Term> map) // construct a named dict // null if dict is anonymous
dict.get(Term key)

Note that: dict keys must currently be either (textual) atoms or (small) integers; the map is necessarily from Term to Term; JPL7 must check (at run-time) that all keys are legitimate.

This is not yet implemented, and constructive discussion is welcomed.

Replaced in JPL7

summary: a few names and types have changed

gone replacement
import jpl.*; import org.jpl7.*;
Hashtable Map<String, Term>
Query.hasMoreElements() Query.hasNext() or for-each
Query.nextElement() or for-each
Query.hasMoreSolutions() Query.hasNext() or for-each
Query.nextSolution() or for-each

Absent from JPL7, deprecated in JPL 3.x

summary: several deprecated methods have been removed

gone replacement Query.goal().name()
Query.args() Query.goal().args()
Query.rewind() Query.close()
Query.query() Query.hasSolution()
Term.debugString() (feel free to improvise)
Float.value() Float.doubleValue()
Integer.value() Integer.longValue()

Absent from JPL7, disfunctional in JPL 3.x

summary: Query.abort() has gone (for now)

Some JPL 3.x features have simply been dropped, when arguably they should have been fixed.

gone rationale
Query.abort() it never worked

Term method orthogonality

summary: all Term subclasses support all methods, reducing need for coercions

For coding convenience, the major methods of Term’s concrete subclasses (Atom, Compound, Integer, Float, Variable and (soon) Dict) are supported in JPL7 by all these classes; this reduces the need for coercions.

method Atom Compound Float Integer Variable
hasFunctor(double, int)          
hasFunctor(String, int)          
hasFunctor(int, int)          
typeName() Atom Compound Float Integer Variable

Some things which haven’t been modernised

summary: a few, harmless legacy oddities have been preserved

JPL7’s Query.allSolutions() should perhaps return a java.util.List of solutions (following the way of the Collections Framework) but I’ve decided to maintain the JPL 1.x tradition of returning an array. Note that arrays can be iterated with for-each

for (Map soln : Query.allSolutions("p(X)")) {
    ... soln.get("X") ...

Term accessors should perhaps all have method names beginning with “get”,


but for now we keep the (concise but un-Java-idiomatic) naming from prior JPL.

an API between SWI-Prolog and the Java Virtual Machine - hosted on GitHub