The Extended CHR Implementation

 

A new, extended, chr library has been developed, with the intention of providing the basis for a system that will allow more optimisations than the previous implementation. At the same time, some of the syntax of the CHR has been changed to conform better to standard Prolog.

The system is still experimental, and provides no special support for debugging CHR code. Please report any problems encountered while using this system.

The main user visible differences from the original chr library are as follows:

• The extended library produces code that generally runs about twice as fast as the old non-debugging code. It is expected that further improvements should be possible.
• CHR code is no longer compiled with a special command - the normal compile command will now recognise and compile CHR code when the extended chr library is loaded. No intermediate Prolog file is produced. The .chr extension is no longer supported implicitly.
• Syntax of some operators have been changed to conform better to standard Prolog.
• A framework for supporting more than two head constraints has been introduced. However, support for propagation rules with more than two heads have not yet been added. Simplification and simpagation rules with more than two heads are currently supported.
• The compiler does not try to reorder the CHR any more. Instead, they are ordered in the way they are written by the user.
• label_with is no longer supported. It can be replaced with user defined labelling.
• The operational semantics of rules have been clarified.
• There is no special support for debugging yet. The CHR code would be seen by the debugger as the transformed Prolog code that is generated by the compiler.

• Invoking the extended CHR library
• Syntactic Differences
• Compiling
• Semantics
  • Addition and removal of constraints
  • Executing Propagation and simpagation rules
• Options and Built-In Predicates
• Compiler generated predicates