A monad for disjunctions. (e.g.: computations that may fail)
Either(a, b) structure represents the logical disjunction between
b. In other words,
Either may contain either a value of type
a or a value
b, at any given time. This particular implementation is biased on the
right value (
b), thus projections will take the right value over the left
A common use of this structure is to represent computations that may fail, when you want to provide additional information on the failure. This can force failures and their handling to be explicit, and avoid the problems associated with throwing exceptions — non locality, abnormal exits, etc.
Furthermore, being a monad,
Either(a, b) can be composed in manners similar
to other monads, by using the generic sequencing and composition operations
provided for the common interface in [Fantasy Land].
// Returns the contents of the file at `path`, if it exists.//// read : String -> Either(Error, String)return existspath? EitherRightfreadpath: /* otherwise */ EitherLeft"Non-existing file: " + pathvar intro = read'intro.txt' // => Right(...)var outro = read'outro.txt' // => Right(...)var nope = read'nope.txt' // => Left("Non-existing file: nope.txt")// We can concatenate things without knowing if they exist at all, and// failures are handled for usintrochainreturn outromapreturn a + borElselogFailure// => Right(...)introchainreturn nopemapreturn a + b// => Left("Non-existing file: nope.txt")
The easiest way is to grab it from NPM. If you're running in a Browser environment, you can use [Browserify]
$ npm install data.either
If you're not using NPM, [Download the latest release][release], and require
var Either = require'data.either'
[Download the latest release][release], and require the
[Download the latest release][release], and load the
file. The properties are exposed in the global
If you want to compile this library from the source, you'll need [Git], [Make], [Node.js], and run the following commands:
$ git clone git://github.com/folktale/data.either.git $ cd data.either $ npm install $ make bundle
This will generate the
dist/data.either.umd.js file, which you can load in
You can [read the documentation online][docs] or build it yourself:
$ git clone git://github.com/folktale/data.either.git $ cd data.either $ npm install $ make documentation
Then open the file
docs/index.html in your browser.
This library assumes an ES5 environment, but can be easily supported in ES3 platforms by the use of shims. Just include [es5-shim] :)
Copyright (c) 2013 Quildreen Motta.
Released under the MIT licence.[Fantasy Land]: https://github.com/fantasyland/fantasy-land [Browserify]: http://browserify.org/ [Git]: http://git-scm.com/ [Make]: http://www.gnu.org/software/make/ [Node.js]: http://nodejs.org/ [es5-shim]: https://github.com/kriskowal/es5-shim [docs]: http://docs.folktalejs.org/en/latest/api/data/either/index.html [release]: https://github.com/folktale/data.either/releases/download/v1.3.0/data.either-1.3.0.tar.gz