Do is a library that adds higher level abstraction and continuables. What I mean by a continuable is explained by the following:
function divide(a, b) { return function (callback, errback) {
// Use nextTick to prove that we're working asynchronously
process.nextTick(function () {
if (b === 0) {
errback(new Error("Cannot divide by 0"));
} else {
callback(a / b);
}
});
}}
Do expects async functions to not require the callback in the initial invocation, but instead return a continuable which can then be called with the callback and errback. This is done by manually currying the function. The "continuable" is the partially applied version of the function returned by the outer function. The body of the function won't be executed until you finish the application by attaching a callback.
divide(100, 10)(function (result) {
puts("the result is " + result);
}, function (error) {
throw error;
});
This style is extremely simple (doesn't require an external library like promises to use), and is fairly powerful.
- The initial function can have variable arguments.
- The continuable itself is portable until it's invoked by attaching callbacks.
The Do library makes doing higher-level abstractions easy. All of these helpers are themselves continuables so you can attach callbacks by calling the returned, curried function.
Takes an array of actions and runs them all in parallel. You can either pass in an array of actions, or several actions as function arguments.
- If you pass in an array, then the output will be an array of all the results
- If you pass in separate arguments, then the output will have several arguments.
Example:
// Multiple arguments
Do.parallel(
Do.read("/etc/passwd"),
Do.read(__filename)
)(function (passwd, self) {
// Do something
}, errorHandler);
// Single argument
var actions = [
Do.read("/etc/passwd"),
Do.read("__filename")
];
Do.parallel(actions)(function (results) {
// Do something
}, errorHandler);
Chains together several actions feeding the output of the first to the input of the second and the final output to the continuables callback.
Example:
// Multiple arguments
Do.chain(
Do.read(__filename),
function (text) {
return Do.save("newfile", text);
},
function () {
return Do.stat("newfile");
}
)(function (stat) {
// Do something
}, errorHandler);
// Single argument
var actions = [
Do.read(__filename),
function (text) {
return Do.save("newfile", text);
},
function () {
return Do.stat("newfile");
}
];
Do.chain(actions)(function (stat) {
// Do something
}, errorHandler);
Takes an array and does an array map over it using the async callback fn. The signature of fn is function fn(item, callback, errback) or any regular continuable.
Example:
// Direct callback filter
var files = ['users.json', 'pages.json', 'products.json'];
function loadFile(filename, callback, errback) {
fs.read(filename)(function (data) {
callback([filename, data]);
}, errback);
}
Do.map(files, loadFile)(function (contents) {
// Do something
}, errorHandler);
// continuable based filter
var files = ['users.json', 'pages.json', 'products.json'];
Do.map(files, fs.read)(function (contents) {
// Do something
}, errorHandler);
Takes an array and does an array filter over it using the async callback fn. The signature of fn is function fn(item, callback, errback) or any regular continuable.
Example:
// Direct callback filter
var files = ['users.json', 'pages.json', 'products.json'];
function isFile(filename, callback, errback) {
fs.stat(filename)(function (stat) {
callback(stat.isFile());
}, errback);
}
Do.filter(files, isFile)(function (filtered_files) {
// Do something
}, errorHandler);
// Continuable based filter
var files = ['users.json', 'pages.json', 'products.json'];
function isFile(filename) { return function (callback, errback) {
fs.stat(filename)(function (stat) {
callback(stat.isFile());
}, errback);
}}
Do.filter(files, isFile)(function (filtered_files) {
// Do something
}, errorHandler);
Takes an array and does a combined filter and map over it. If the result
of an item is undefined, then it's filtered out, otherwise it's mapped in.
The signature of fn is function fn(item, callback, errback) or any regular continuable.
Example:
// Direct callback filter
var files = ['users.json', 'pages.json', 'products.json'];
function check_and_load(filename, callback, errback) {
fs.stat(filename)(function (stat) {
if (stat.isFile()) {
loadFile(filename, callback, errback);
} else {
callback();
}
}, errback);
}
Do.filterMap(files, check_and_load)(function (filtered_files_with_data) {
// Do something
}, errorHandler);
// Continuable based filter
var files = ['users.json', 'pages.json', 'products.json'];
function check_and_load(filename) { return function (callback, errback) {
fs.stat(filename)(function (stat) {
if (stat.isFile()) {
loadFile(filename, callback, errback);
} else {
callback();
}
}, errback);
}}
Do.filterMap(files, check_and_load)(function (filtered_files_with_data) {
// Do something
}, errorHandler);
Do has a super nifty Do.convert function that takes a library and converts it to use Do style continuables. For example, if you wanted to use fs.readFile and fs.writeFile, then you would do this:
var fs = Do.convert(require('fs'), ['readFile', 'writeFile']);
Do will give you a copy of fs that has readFile and writeFile converted to Do style. It's that easy!
All async functions in node follow a common interface:
method(arg1, arg2, arg3, ..., callback)
Where callback is of the form:
callback(err, result1, result2, ...)
This is done to keep node simple and to allow for interoperability between the various async abstractions like Do continuables and CommonJS promises.
If you're writing a library, make sure to export all your async functions following the node interface. Then anyone using your library can know what format to expect.
- Make some sort of helper that makes it easy to call any function regardless of it's sync or async status. This is tricky vs. promises since our return value is just a regular function, not an instance of something.
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