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To Be Continued

To Be Continued is a library that simplifies asynchronous programming. It allows synchronous and asynchronous code to be blended together, handling the generation of the necessary callback functions automatically.


Add the following dependency to your project's project.clj file:

[to-be-continued "0.1.0"]

Then add the appropriate require statements to the namespaces in which you want to use To Be Continued. Even if you are not directly calling the functions defined in the to-be-continued.fns namespace, it still must be required for use by the macros.


(:require [to-be-continued.macros :as tbc]
          [to-be-continued.fns :as tbc-fns])

For Clojure only, the to-be-continued.core namespace is provided as a convenience. It includes both the macros and the functions.


(:require-macros [to-be-continued.macros :as tbc])
(:require [to-be-continued.fns :as tbc-fns])

Async-Aware Threading Macros

The -+-> and -+->> macros are asynchronous-aware equivalents of Clojure's -> and ->> macros, respectively. When any of the intermediate forms in the threading expression ends in ..., it signals that the form is invoking asynchronous code and requires a callback. To Be Continued will replace the ... with an automatically generated callback function that will continue with the threading expression where the asynchronous call left off.

For example:

(defn fetch-hobbies
  "Given a person map, fetches the person's hobbies and passes them
   to the specified continuation."
  [person callback]
  (tbc/-+-> person
            (fetch-profile ...)

In this case, fetch-profile is a hypothetical function that asynchronously fetches the profile of a person. It takes two arguments: (1) the login name of the person and (2) a callback to invoke with the profile is available. The -+-> macro generates a callback function representing the continuation of the thread (i.e., resuming with :hobbies).

Note that the return values of -+-> and -+->> are undefined. It is expected that the final form in the thread will be the invocation of a callback to hand over control to whatever code is expecting the result.

Parallel Binding of Asynchronous Results

The let-par macro is the asynchronous equivalent of Clojure's let macro. It allows the results of multiple asynchronous functions, executed in parallel, to be bound to variables that can be referenced the body expression.

For example:

(defn make-breakfast
  (tbc/let-par [ham  (get-ham ...)
                eggs (get-eggs ...)]
    (tbc/-+-> (cook ham eggs)

The get-ham and get-eggs asynchronous functions will be executed in parallel. When both have completed, the body of the let-par expression will be evaluated with ham and eggs bound to their results.

Note that the use of the -+-> macro above is not strictly necessary because the cook function is synchronous. It use is encouraged, however, because--once error handling is implemented; see the Roadmap section below--it will properly handle any errors that occur.

Like -+-> and -+->>, the value of a let-par expression is undefined. It is expected to invoked a callback with its result.

Parallel Mapping an Asynchronous Function over a Collection

The map-par function is the asynchronous equivalent of Clojure's map function. It takes an asynchronous function of two arguments--an input value and a callback--and invokes it for each member of a collection. When all of the results are available, they are passed as a vector to the specified callback.

For example:

(tbc-fns/map-par fetch-profile some-callback)

Like any other asynchronous function that takes a callback as its last argument, map-par expressions may be included in -+-> and -+->> threading expressions or used as let-par binding forms.

Example Application

Please see tbc-node-example for an example ClojureScript + Node.js application that uses To Be Continued to asynchronously query the GitHub API.


  • Error handling
  • Additional means of parallel execution


Copyright (c) 2012 Greg Spurrier

Distributed under the MIT license. See LICENSE.txt for the details.


Simplified Asynchronous Programming for Clojure and ClojureScript







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