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Date: Mon, 01 Feb 2010 16:22:07 -0700
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To: nodejs@googlegroups.com
Subject: Re: [nodejs] Re: Promises, callbacks, Continuables and more
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On 2010-02-01 14:51, Jan Sch�tze wrote:
> 2010/2/1 Felix Geisend�rfer <fe...@debuggable.com>:
>> * Having a single callback for both error/success
> +1
> 
> This has the advantage that you don't have to write the logic for
> chained callbacks two times and it's not up to the library developer
> to decide what an "error"/"success" is.

One issue with most of the suggestions I've seen for this 
so far is that they require type testing to distinguish 
success or failure.

If you're going to have a single callback, make it 
unambiguous, so that the same pattern can be used regardless 
of the range of values that might be returned.

In Felix's[1] example:

function loadConfig() {
  return posix.cat('config.json')
    (function(config, error) {
      return config || posix.cat('configuration.json')
    })
    (function(config, error) {
      return error || JSON.parse(config);
    })
}

[1]: https://gist.github.com/85c313e588a3fc9d8636

This works only until you need to return undefined, (or 0, 
"", null, or Boolean false...) as a successful asynchronous 
result.

I like to use values which are either [0,<error>] or [1,<data>], 
which gives[2]:

function loadConfig() {
  return posix.cat('config.json')
    (function(either) {
      return either[0] ? either[1] : posix.cat('configuration.json')
    })
    (function(either) {
      return either[0] ? JSON.parse(either[1]) : "Could not read conf file: "+either[1]
    })
}

[2]: https://gist.github.com/67cbf71c79fd5b6bf3a1

Or if you prefer separate arguments, make the first indicate 
success and the second be either the successful result or the 
error:

function loadConfig() {
  return posix.cat('config.json')
    (function(succeeded,value) {
      return succeeded ? value : posix.cat('configuration.json')
    })
    (function(succeeded,value) {
      return succeeded ? JSON.parse(value) : "Could not read conf file: "+value
    })
}

-- 
http://inimino.org/~inimino/blog/