With this meta-module you can write modules with chainable interfaces. Chainsaw takes care of all of the boring details and makes nested flow control super simple too.

Just call Chainsaw with a constructor function like in the examples below. In your methods, just do saw.next() to move along to the next event and saw.nest() to create a nested chain.

Examples

add_do.js

This silly example adds values with a chainsaw.

var Chainsaw = require('chainsaw');

function AddDo (sum) {
    return Chainsaw(function (saw) {
        this.add = function (n) {
            sum += n;
            saw.next();
        };
         
        this.do = function (cb) {
            saw.nest(cb, sum);
        };
    });
}

AddDo(0)
    .add(5)
    .add(10)
    .do(function (sum) {
        if (sum > 12) this.add(-10);
    })
    .do(function (sum) {
        console.log('Sum: ' + sum);
    })
;

Output: Sum: 5

prompt.js

This example provides a wrapper on top of stdin with the help of node-lazy for line-processing.

var Chainsaw = require('chainsaw');
var Lazy = require('lazy');

module.exports = Prompt;
function Prompt (stream) {
    var waiting = [];
    var lines = [];
    var lazy = Lazy(stream).lines.map(String)
        .forEach(function (line) {
            if (waiting.length) {
                var w = waiting.shift();
                w(line);
            }
            else lines.push(line);
        })
    ;
    
    var vars = {};
    return Chainsaw(function (saw) {
        this.getline = function (f) {
            var g = function (line) {
                saw.nest(f, line, vars);
            };
            
            if (lines.length) g(lines.shift());
            else waiting.push(g);
        };
        
        this.do = function (cb) {
            saw.nest(cb, vars);
        };
    });
}

And now for the new Prompt() module in action:

var util = require('util');
var stdin = process.openStdin();
 
Prompt(stdin)
    .do(function () {
        util.print('x = ');
    })
    .getline(function (line, vars) {
        vars.x = parseInt(line, 10);
    })
    .do(function () {
        util.print('y = ');
    })
    .getline(function (line, vars) {
        vars.y = parseInt(line, 10);
    })
    .do(function (vars) {
        if (vars.x + vars.y < 10) {
            util.print('z = ');
            this.getline(function (line) {
                vars.z = parseInt(line, 10);
            })
        }
        else {
            vars.z = 0;
        }
    })
    .do(function (vars) {
        console.log('x + y + z = ' + (vars.x + vars.y + vars.z));
        process.exit();
    })
;

Installation

With npm, just do: npm install chainsaw

or clone this project on github:

git clone http://github.com/substack/node-chainsaw.git

To run the tests with expresso, just do:

expresso

Light Mode vs Full Mode

node-chainsaw supports two different modes. In full mode, every action is recorded, which allows you to replay actions using the jump(), trap() and down() methods.

However, if your chainsaws are long-lived, recording every action can consume a tremendous amount of memory, so we also offer a "light" mode where actions are not recorded and the aforementioned methods are disabled.

To enable light mode simply use Chainsaw.light() to construct your saw, instead of Chainsaw().