CHR.js

Compile and run Constraint Handling Rules (CHR) in JavaScript

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)   │
└────────────┘

More example CHR scripts are provided in the project's /examples directory.

Background

The implementation is based on the compilation schema presented in the paper CHR for imperative host languages (2208; Peter Van Weert, Pieter Wuille, Tom Schrijvers, Bart Demoen). As of yet basically none of the mentioned optimizations have been implemented.

Todo

• Provide an option to bundle the chr/runtime references to get a single-file package executable for example in browsers.
• Add event emitters for rule application etc to provide an easy logging and debugging mechanism.