CHR.js

Compile and run Constraint Handling Rules (CHR) in JavaScript.

Getting Started

The online version at chrjs.net is the easiest way to generate a constraint solver. Just enter your Constraint Handling Rules, try adding some constraints, and download the generated solver code.

Install

npm install chr

Usage as CLI

If you install CHR.js via npm it will create a new chrjs executable:

$ chrjs /path/to/your.chr > /created/constrainthandler.js

If no path is specified, chrjs reads from stdin.

Usage with node.js

The CHR.js module can be used programmatically:

var chrjs = require('chr')

var code = chrjs.transform('a ==> b')
chrjs.transformFile('path.chr', function(err, code) {
  console.log(code)
})

Example

Consider the following CHR source code, which generates all fibonacci numbers upto a given index N as constraints of the form fib(Number,Value):

upto(N), fib(A,AV), fib(B,BV) ==> B === A+1, B < N | fib(B+1,AV+BV);

The CHR source code can be compiled to JavaScript by the use of the chrjs command:

$ echo 'upto(N), fib(A,AV), fib(B,BV) ==> B === A+1, B < N | fib(B+1,AV+BV);' | chrjs > fib.js

The generated JavaScript code has references to chr/runtime components, so make sure it is available. Open a REPL to play with the generated fib.js:

$ node
> var konsole = require('chr/console') // for graphical representation
> var CHR = require('./fib.js')        // load the generated source
> CHR.fib(1,1)                         // the first fibonacci is 0
> CHR.fib(2,1)                         // the second is 1
> konsole(CHR.Store)                   // print the content of the
┌────────────┐                         //   constraint store
│ Constraint │
├────────────┤
│ fib(1,1)   │
├────────────┤
│ fib(2,1)   │
└────────────┘
> CHR.upto(5)                          // generate the first 5 fibs
> konsole(CHR.Store)                   // print the content of the
┌────────────┐                         //   constraint store again
│ Constraint │
├────────────┤
│ fib(1,1)   │
├────────────┤
│ fib(2,1)   │
├────────────┤
│ upto(5)    │
├────────────┤
│ fib(3,2)   │
├────────────┤
│ fib(4,3)   │
├────────────┤
│ fib(5,5)   │
└────────────┘
> CHR.fib(6,8).fib(7,13)               // constraints allow chaining
> CHR.Store.reset()                    // clear the constraint store

More example CHR scripts are provided in the project's /examples directory or at chrjs.net.

Background

CHR.js was realized as a project as part of the Masters programme in Computer Science at the University of Ulm. Its Project Report with additional information about its architecture can be found online: https://fnogatz.github.io/paper-now-chrjs/.

The implementation is based on the compilation scheme presented in the paper CHR for imperative host languages (2008; Peter Van Weert, Pieter Wuille, Tom Schrijvers, Bart Demoen). As of yet basically none of the mentioned optimizations have been implemented.

A list of open points for improving can be found in the wiki and the project report.