Red Lobster

Red Lobster is a toolkit for working asynchronously on Node in ClojureScript, and is the mechanism through which Dog Fort gets things done. It wraps Node's EventEmitter and Stream types, and provides some useful abstractions; in particular, promises.

Promises

A Red Lobster promise is much like a promise in Clojure, except instead of running in its own thread, it can be realised from async code, and you can attach event listeners to it to respond to its realisation.

    (ns user
      (:require [redlobster.promise :as p]))

    (def my-promise (p/promise))

    (p/on-realised my-promise
      #(print (str "promise succeeded: " %))
      #(print (str "promise failed: " %)))

    (p/realise my-promise "cheezburger")
    ;; prints "promise succeeded: cheezburger"

The Short Form

There's also a promise macro that helps you write async code to realise a promise. The macro returns a new promise, and takes a set of forms that it executes immediately, and makes two functions realise and realise-error available inside the macro's scope for realising the promise. This lets you conveniently realise a promise through multiple levels of callbacks.

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [promise]]))

    (def fs (js/require "fs"))

    (defn read-file [path]
      (promise
        (.readFile fs path
          (fn [err data]
            (if err
              (realise-error err)
              (realise data))))))

    (def file-promise (read-file "/etc/passwd"))
    (p/on-realised file-promise
      #(print (str "File contents:\n" %))
      #(print "Error reading file!"))

Dereferencing Promises

Promises can also, obvoiusly, be dereferenced, but, unlike Clojure promises, this doesn't block until the promise has been realised. Notice that dereferencing doesn't distinguish between success or error states; you'll have to use the failed? function to determine whether the promise failed.

    (def my-promise (p/promise))
    @my-promise
    ; => :redlobster.promise/not-realised

    (p/realise my-promise "like a boss")
    @my-promise
    ; => "like a boss"

Chaining Promises

A promise can also be chained to another promise, either through simply calling realise with a new promise as the realised value, which will automatically realise the promise with the new promise's value once that promise realises, or through the waitp macro, which takes a promise, a success handler and an error handler, and returns a new promise bound to the original promise, realised through the same realise function the promise macro makes available:

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [promise waitp]]))

    (defn read-file-or-default [path]
      (let [file-promise (read-file path)]
        (waitp file-promise
          #(realise %)
          #(realise "default content"))))

Waiting For Promises

There's a when-realised macro which lets you create a promise that waits for a list of other promises to finish before evaluating its body and realising the new promise with the result of the evaluation. This is useful when waiting for a number of async operations to finish.

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [when-realised]]))

    (let [file-promise (read-file "/etc/passwd")]
      (when-realised [file-promise]
        (.write (.-stdout js/process) @file-promise)))
    ; writes the contents of /etc/passwd to stdout.
    ; returns a promise that will realise when the code has executed.

The let + when-realised construction above is a common pattern, so there's a let-realised macro for combining the two. The example above would have been better written like this:

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [let-realised]]))

    (let-realised
        [file-promise (read-file "/etc/passwd")]
      (.write (.-stdout js/process) @file-promise))

Waiting And Chaining

Because when-realised and let-realised return promises that are realised to the result of evaluating their bodies, and because you can chain promises together by realising a promise with another promise, you can easily create multi-step promises like this:

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [let-realised]]))

    (let-realised
        [filename-promise (read-file "/tmp/filename-inside.txt")]
      (let-realised
          [file-promise (read-file @filename-promise)]
        (.write (.-stdout js/process) @file-promise)))
    ; reads a filename from /tmp/filename-inside.txt, and then
    ; reads the contents of that file, printing the result to stdout.

Wrapping Node Callbacks

A very common idiom in Node is the error/result callback. A function takes a callback as its last argument, which in turn takes two arguments: an error argument, which will be null upon success, and a result argument. Callbacks start with handling any non-null error, and proceed with dealing with the result if there was no error.

    fs.readFile("/etc/passwd", function(error, result) {
      if (error) throw error;
      console.log(result);
    });

When we're using promises instead of callbacks, it's generally useful to wrap constructs like these in a promise. That's easily accomplished by using the defer-node macro. For instance, it lets us rewrite the read-file function from the previous examples very succinctly:

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [let-realised defer-node]]))

    (def fs (js/require "fs"))

    (defn read-file [path]
      (defer-node (.readFile fs path)))

You can pass a function as a second argument to defer-node, which will be applied to the result of the operation prior to realising the promise. An excellent candidate for this would be js->clj, but for the sake of example, let's make the read-file function more shouty.

    (ns user
      (:require [redlobster.promise :as p]
                [clojure.string :as str])
      (:use-macros [redlobster.macros :only [let-realised defer-node]]))

    (def fs (js/require "fs"))

    (defn read-file [path]
      (defer-node (.readFile fs path) str/upper-case))

At this point you're probably thinking, "wait, what if I use that function to transform the result into another promise?" Of course, that's an excellent way of chaining together Node API operations; let's rewrite the chaining example above using this technique.

    (ns user
      (:require [redlobster.promise :as p])
      (:use-macros [redlobster.macros :only [defer-node]]))

    (def fs (js/require "fs"))

    (defer-node (.readFile fs "/tmp/filename-inside.txt")
      (fn [result] (defer-node (.readFile fs result)
        (fn [result] (.write (.-stdout js/process) result)))))

License

Copyright 2012 Bodil Stokke and Matthew Molloy

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.