PLEXIL 4 Release Notes

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11 June 2015

PLEXIL 4 is a major release, with significant changes from previous releases. Here are some of the highlights.

Building PLEXIL 4

The PLEXIL C++ source code should be compatible with most modern and recent C++ compilers. It is regularly tested against gcc and g++ and clang and clang++ on both Linux and Mac OS X.

PLEXIL is now available for download as a gzip compressed tar archive file (or tarball). Prebuilt packages for Linux and Mac OS X are no longer available due to lack of resources.

Building PLEXIL from an SVN checkout now requires the GNU "autotools" suite: libtool, automake, and autoconf. These programs should not be required in the tarball release, as they have already been run prior to archiving. Please see Software Requirements for more information.

It is again possible to build the PLEXIL suite from the top level Makefile, whether downloaded as an archive or checked out from the repository:

cd plexil-4

This builds most of the tools that users are likely to want, plus several example applications.

PLEXIL Language

The language has had the following functions added.

Real-to-Integer Conversion Functions

The PLEXIL language has added the following functions for converting Real numbers to Integer values:

  • real_to_int(x) - Converts a Real value which is exactly an Integer to the corresponding Integer.
  • trunc(x) - Rounds the Real value towards 0.
  • round(x) - Rounds the Real value as defined in the C language standard.
  • ceil(x) - Returns the least positive Integer that is equal to or greater than x.
  • floor(x) - Returns the most positive Integer that is equal to or less than x.

When the resulting number would not fit into an Integer value, or for real_to_int a Real value is not exactly representable as an Integer, UNKNOWN is returned.

strlen Function

The strlen() function returns the length of a String value as an Integer.

Array Size Functions

Array sizes can be determined with the following new functions:

  • arraySize(a) returns the current size of the array a as an Integer.
  • arrayMaxSize(a) returns the maximum (declared) size of the array a as an Integer.

PLEXIL Executive

The PLEXIL Executive's performance has been significantly improved. Major areas of improvement:

  • The plan reader's intermediate representation has been removed. XML is now translated directly into internal representations.
  • Expression representations were reengineered from the ground up. See Expressions below for more information.
  • The innermost routines of the Executive have been streamlined.
  • The Id<T> "smart pointer" template has been eliminated.
  • The executive core no longer uses any hash tables.

The result is much faster overall execution time - typically 2-5x faster than PLEXIL 3 for medium sized plans - and a smaller but still significant improvement in memory usage. Many memory leaks have also been eliminated.


Previous versions of the Executive used double-precision floating point representation for all values in the language. PLEXIL 4 replaces this system with strongly typed Expression and Value objects.

As a result the PLEXIL Executive is now much stricter about expression types:

  • Boolean is now a distinct type, incompatible with numeric types.
  • The Executive will report an error if a Real value is assigned to an Integer variable.

Since the PLEXIL language was originally designed to be a strongly typed language, this should have no effect on conformant plans. However, non-conforming plans which used to run in earlier releases due to lax type checking by the Executive may no longer work as before.

String Representation

String values were interned (stored in a symbol table) in previous versions of the PLEXIL Executive. String values are now treated just as any other value. Intermediate values in string expressions are now discarded, rather than stored permanently. Plans which do string manipulation will use less memory as a result.

External Interfacing

The APIs for interfacing to external systems have changed significantly.

This PLEXIL wiki now has much larger, and we hope much improved, documentation on interfacing to the PLEXIL executive. See the sidebar menu at left. The PLEXIL team eagerly seeks feedback on this documentation.

The new Value class can represent any PLEXIL value for input or output. See the API Reference for details.

Extended PLEXIL

The Extended PLEXIL translator generates smaller and faster Core PLEXIL XML for several common language features.

Standard PLEXIL Compiler

The Standard PLEXIL compiler has added an optimization phase. Some moderately high-value optimizations have already been implemented. More will be forthcoming.

See PLEXIL Language above for other changes.

PLEXIL Checker

The PLEXIL Checker had not been updated in quite a while. It is now up to date with the current PLEXIL schema, and many bugs have been fixed. The Checker now performs a schema conformance check before any semantic checks.


The PLEXIL Viewer had also suffered from a lack of maintenance in previous releases. Many bugs have been fixed in the Viewer, and performance should be quite a bit better.

The PLEXIL Viewer now requires Java 8. Please let us know if you are unable to update from earlier versions, so we can develop a workaround.

The Viewer now defaults to TCP port 49100. Previously it used a port number in the ephemeral range. As before, users can specify a different port number.

Improved Shell Scripts

The plexil, plexilexec, plexiltest, and plexilsim shell scripts have been revised. Checking of TCP ports in use has been significantly improved.

Regression Tests and Examples

Most, if not all, of the PLEXIL plans in the regression test suites and the examples directory have been checked for conformance with the current schema and fixed where necessary. Many of these plans had never been updated since early in development.

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