Yield! Javascript Generators and Monocle.js

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Jonathan Lipps Senior Developer Yield! Javascript Generators and Monocle.js Utah JS • 8/29/2013

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In ES6, a bold new world awaits, ripe for the plunder of its sweet fruits!* * This is meant to be humorous. Remember kids: imperialism is bad.

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My favorite is the generator, a new type of function whose execution can be externally controlled \

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Generators are available in node 11 and up, by passing the --harmony ag to node \

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Let’s have a look, shall we? \

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Things to remember Calling a Generator returns an Iterator Iterators have a next() method Calling next() returns whatever is after the next yield statement in the generator

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Generators allow lazy construction of potentially in nite sequences \

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Code in generators can respond to behavior in the outside world! \

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Things to remember next() actually returns an object, with keys value and done Calling next() with a parameter sends that value in as the result of the generator’s last yield Calling next() always returns whatever is after the next yield statement in the generator, even if you are also sending in a value

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Hang on a minute! If we can arbitrarily pause function execution, and send values back in...

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...we can tell the generator to wait until we’ve received the result of an asynchronous function’s callback...

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...and avoid callback hell! We all love to hate callback hell.

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Rightward drift

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Repetitive error handling

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monocle.js is a library that lets you use a blocking-like syntax with async functions in generators

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https://github.com/jlipps/monocle-js

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monocle.js is a port of a Python project with the same basic goal

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https://github.com/saucelabs/monocle

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So long, callback hell!

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o_O takes a generator and converts it into what we call an ‘o-routine’

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Inside an o-routine, you can: yield to another o-routine: pause execution until the other o-routine has completed return the result of this o-routine: pass a response to an o-routine waiting on us

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monocle.launch takes an o-routine and starts executing it. You always need to kick things oﬀ this way!

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1. Launch the main o-routine

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2. main waits for myLibraryFunction to respond with a result

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3. we wait for result of request

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4. we wait for writing data to le

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5. we wait for contents of le

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6. we return contents of le to whoever was waiting on us

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7. main writes out the response and execution nishes

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But... yield? yield in generators usually acts like yield “as in crops”: it provides a result to next() yield in o-routines acts like yield “as in traﬃc”: it waits for something else to nish.

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It’s easy to convert existing asynchronous functions into o-routines

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monocle.callback gives you a special function you pass to an async function as its callback

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You can then yield to this special callback, and monocle will wait for it as if it were an o-routine

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monocle.monoclize() is a helper function that makes it easy to convert async libraries to o-routines

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We want to monoclize Node’s fs

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First we clone the fs object

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Then we make a list of methods

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Then we monoclize each one

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CASE STUDY Appium & Yiewd

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Appium is a mobile automation framework written in Node. It’s based on the WebDriver protocol.

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https://github.com/appium/appium

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WebDriver is an HTTP-based spec. Each automation command is sent over the wire.

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Wd is a Node.js client library that implements this protocol. Appium uses Wd to write all its tests.

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https://github.com/admc/wd

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Wd uses Node’s HTTP libraries, which are async functions. Appium test code thus looks like this:

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Yiewd is a monoclized version of Wd that converts all automation functions into o-routines

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https://github.com/jlipps/yiewd

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Thanks to monocle, 27 lines of Wd code turns into 16 lines of Yiewd

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In sum, monocle.js uses JS generators to open up a whole new world of code quality