So the other day I had a list of (html) elements, and I wanted to get an array representing lines of text. The only problem being that some of the elements are displayed inline - so I needed to join those together. But only when they appeared next to each other.

I would call the generic way of doing so group_sequential, where an array is chunked into sub-arrays, and sequential elements satisfying some predicate are included in the same sub-array. That way, my predicate could be :inline?, and I could join the text of each grouped element together to get the lines out.

For example, using even numbers for simplicity:

[1,2,3,4,6,8,5,4,4].group_sequential(&:even?)
=> [[1],[2],[3],[4,6,8],[5],[4,4]]

Here’s the ruby code I came up with:

	class Array
		def group_sequential
			result = []
			group = []
			finish_group = lambda do
				unless group.empty?
					result << group
					group = []
				end
			end

			self.each do |elem|
				if yield elem
					group << elem
				else
					finish_group.call
					result << [elem]
				end
			end
			finish_group.call
			result
		end
	end
	

Things are slightly less noisy, but assignment is subtly awkward in python without using nonlocal scope keyword (only available in python3):

	def group_sequential(predicate, sequence):
		result = []
		group = []
		def finish_group():
			if group:
				result.append(group)
			return []

		for item in sequence:
			if predicate(item):
				group.append(item)
			else:
				group = finish_group()
				result.append([item])
		group = finish_group()
		return result
	

This feels like something that should be doable in a much more concise way than I came up with above. Any ideas? (In either ruby or python)

Update:

My friend Iain has posted a number of interesting solutions over yonder, which got me thinking differently about it (specifically, reminding me of dropwhile and takewhile). I applied python’s itertools to the problem to get this rather satisfactory result in python:

	from itertools import takewhile, tee, dropwhile
	def group_sequential(pred, sequence):
		taker, dropper = tee(iter(sequence))
		while True:
			group = list(takewhile(pred, taker))
			if group: yield group
			yield [dropwhile(pred, dropper).next()]
	

(note: this is a generator which is fine for my purposes - you can always wrap it in a call to list() to force it into an actual list).

The same approach is acceptable when done in ruby, but a bit more verbose because of the need to explicitly check for the end of the sequence, and to collect the results array:

	class Array
		def group_sequential(&pred)
			sequence = self
			results = []
			while true
				group = sequence.take_while &pred
				results << group if group.size > 0
				sequence = sequence.drop_while &pred
				return results if sequence.empty?
				results << [sequence.shift]
			end
		end
	end
	

Update (the second):

While poking around itertools, I managed to miss groupby. I assumed it did the same thing as ruby’s Enumerable#group_by, which is to say not at all what I want (though it’s surely useful at other times). So here is presumably the most concise version I’ll find, for the sake of closure:

	from itertools import groupby
	def group_sequential(pred, sequence):
		return [list(group) for key, group in groupby(sequence, pred)]
	

I had cause to repurpose this immediate feedback formatter to work with the SpecDoc rspec formatter. So here’s a minimal version that just monkey-patches the SpecdocFormatter class to provide immediate feedback. As long as it’s included somewhere before rspec starts running, it should do its thing…

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…a continuation (hah!) of defer: Taming asynchronous javascript with coffeescript.

In my last post, I outlined the ideas behind defer and its current state. One of the things I mentioned is that how to deal with return statements in asynchronous code is not yet decided. I’d like to explore a few of those ideas here. If you haven’t read the previous post, I suggest that you do.

defer is something I’ve been working towards in various fashions for many months now. It’s a way of writing asynchronous javascript in a more straightforward and synchronous-looking way. My current approach has been to modify the CoffeeScript compiler to introduce a defer keyword. There is currently some debate on the issue (parts one and two have even more history) as to whether the functionality is necessary, useful or appropriate. Here, I hope to show the reasons behind the idea, why it does what it does, and how it helps programmers write more concise and readable code.

So apparently iPhone OS 4 (or iOS, as it’s now awkwardly named), still has no way to sync data in the background. “Multitasking done right” indeed.

To be clear, that means that for every application that syncs data with the web, you must explicitly open it (and maybe tap some buttons) to initiate a sync. If you have a third-party mail client, an RSS reader, a todo list and instapaper, that’s quite a lot of tedium to go through every time you want to sync data. Heaven forbid you have more than a handful of apps that might need to talk to the internet without your explicit direction.

Get over your ego, Apple, and just copy android properly next time.

As an addition to my last post, I’d like to point out a great resource for getting started with android programming in scala. This article gives you a nice template to go from, and a brief rundown of the sbt build tool you can use to build and deploy your scala-based android app. Good stuff!

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