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# Controlling Flow: callbacks are easy ## What's actually hard? - Doing a bunch of things in a specific order. - Knowing when stuff is done. - Handling failures. - Breaking up functionality into parts (avoid nested inline callbacks) ## Common Mistakes - Abandoning convention and consistency. - Putting all callbacks inline. - Using libraries without grokking them. - Trying to make async code look sync. ## Define Conventions - Two kinds of functions: *actors* take action, *callbacks* get results. - Essentially the continuation pattern. Resulting code *looks* similar to fibers, but is *much* simpler to implement. - Node works this way in the lowlevel APIs already, and it's very flexible. ## Callbacks - Simple responders - Must always be prepared to handle errors, that's why it's the first argument. - Often inline anonymous, but not always. - Can trap and call other callbacks with modified data, or pass errors upwards. ## Actors - Last argument is a callback. - If any error occurs, and can't be handled, pass it to the callback and return. - Must not throw. Return value ignored. - return x ==> return cb(null, x) - throw er ==> return cb(er) ```javascript // return true if a path is either // a symlink or a directory. function isLinkOrDir (path, cb) { fs.lstat(path, function (er, s) { if (er) return cb(er) return cb(null, s.isDirectory() || s.isSymbolicLink()) }) } ``` # asyncMap ## Usecases - I have a list of 10 files, and need to read all of them, and then continue when they're all done. - I have a dozen URLs, and need to fetch them all, and then continue when they're all done. - I have 4 connected users, and need to send a message to all of them, and then continue when that's done. - I have a list of n things, and I need to dosomething with all of them, in parallel, and get the results once they're all complete. ## Solution ```javascript var asyncMap = require("slide").asyncMap function writeFiles (files, what, cb) { asyncMap(files, function (f, cb) { fs.writeFile(f, what, cb) }, cb) } writeFiles([my, file, list], "foo", cb) ``` # chain ## Usecases - I have to do a bunch of things, in order. Get db credentials out of a file, read the data from the db, write that data to another file. - If anything fails, do not continue. - I still have to provide an array of functions, which is a lot of boilerplate, and a pita if your functions take args like ```javascript function (cb) { blah(a, b, c, cb) } ``` - Results are discarded, which is a bit lame. - No way to branch. ## Solution - reduces boilerplate by converting an array of [fn, args] to an actor that takes no arguments (except cb) - A bit like Function#bind, but tailored for our use-case. - bindActor(obj, "method", a, b, c) - bindActor(fn, a, b, c) - bindActor(obj, fn, a, b, c) - branching, skipping over falsey arguments ```javascript chain([ doThing && [thing, a, b, c] , isFoo && [doFoo, "foo"] , subChain && [chain, [one, two]] ], cb) ``` - tracking results: results are stored in an optional array passed as argument, last result is always in results[results.length - 1]. - treat chain.first and chain.last as placeholders for the first/last result up until that point. ## Non-trivial example - Read number files in a directory - Add the results together - Ping a web service with the result - Write the response to a file - Delete the number files ```javascript var chain = require("slide").chain function myProgram (cb) { var res = [], last = chain.last, first = chain.first chain([ [fs, "readdir", "the-directory"] , [readFiles, "the-directory", last] , [sum, last] , [ping, "POST", "example.com", 80, "/foo", last] , [fs, "writeFile", "result.txt", last] , [rmFiles, "./the-directory", first] ], res, cb) } ``` # Conclusion: Convention Profits - Consistent API from top to bottom. - Sneak in at any point to inject functionality. Testable, reusable, ... - When ruby and python users whine, you can smile condescendingly.