Advogato blog for cananian

A collection of Nell demos

Tue, 1 Nov 2011 23:07:12 GMT

Here are some banged-together demos of various pieces of One Laptop per Child's Project Nell. The ultimate goal is a Nell demo for CES in January 2012, but these bits should be considered as tech demos, benchmarks, and proofs of concept, not actual pieces of that demo (yet).

Most of these demos require WebGL support. Visit get.webgl.org for information about enabling WebGL in your browser; there is WebGL support in Chrome, Firefox, Safari, and Opera—although it often requires enabling experimental features in the browser preferences.

Tiles. Performance benchmark for a tile-based home screen. "Apps" are "locations" on your world map, which you can customize as you like. (Here's an interesting blog entry discussing world-creation for kids.) Day/night would ultimately reflect current time, although they've been greatly sped up in this demo. Lots of rough edges and missing UI, but all the textured triangles are present, so it should be an accurate benchmark.
(Drag with left mouse button to rotate, middle mouse button to zoom, right mouse button to pan.)
Nell at home. Basic idea (including transition) for activities which include dialog with Nell or story-telling.
Standalone model viewers: Castle (from blendswap), "Nell" (Sintel, from blendswap), Alternate (lightweight) Nell model, Alternate (heavyweight) house model (from blendswap).
In model viewers: drag with left mouse button to rotate, middle mouse button to zoom, right mouse button to pan.
Music maker. Uses WebGL and the Web Audio APIs to let you draw and perform music.
Inspired by André Michelle's ToneMatrix and Karplus-Strong Guitar (see also wiki and this 2008 Linux Audio Conference paper), as well as DinahMoe's ToneCraft and the Tenori-on.
Quake on XO-1.75 (video). Of course we need to actually run WebGL with good performance on XO hardware. Jon Nettleton has been working hard on our GL drivers, enabling the GPU on the XO-1.75 hardware for the first time. This Quake demo shows his progress—don't worry, Quake is not actually part of the Nell demo! (We have a GPU in the XO-1.5 as well, which hasn't yet been utilized.)
Codify—not one of our demos (it's a commercial iPad app) but it demonstrates the direction we'd like to push Pippy.

Coming soon: TurtleArt and Implode for the web. We've started converting them to GTK3 in preparation for hoisting them bodily onto the interwebs. Here's the source code repository for the TurtleArt port if you'd like to watch or participate in this hackage. (See repl.it for one of the more unusual ways to get Python running in the web context.) The rest of the demo source code is on github (or just "View Source" in your browser).

Introducing Nell

Sat, 1 Oct 2011 06:06:58 GMT

Between now and January CES, Chris Ball and I will be building Nell for the OLPC XO-3 tablet. Nell is a name, not an acronym, but if you want to pronounce it as "Narrative Environment for Learning Learning," I won't stop you.

Nell's development will be demo-oriented—we're going to try to write the most interesting bits first and learn as we go—so don't be upset if you don't see support right away for legacy Sugar activities ("Sweet Nell"), robust sharing support, mesh networking, or whatever your favorite existing feature is. They'll come, but the new crazy stuff is what we need to evaluate first.

Here are four of the big ideas behind Nell, along with pointers to some of our sources of inspiration.

Narrative. I probably don't need to restate that Neil Stephenson's "The Diamond Age" has been hugely influential, and we also owe a large debt to interactive fiction and the Boston IF group in particular. (Check out the talks from our "Narrative Interfaces day" at OLPC.) Wide Ruled (conference paper) and Mark Riedl at Georgia Tech have demonstrated interesting approaches to story representation. I'm also looking forward to the results of the Experimental Game Play group's September Story Game competition.

Emotion. The Radiolab podcast "Talking To Machines" crystallized my thinking about emotionally-attractive environments. The discussion with Caleb Chung, the creator of Furby, is particularly apropos. Caleb's goal is to make things which kids want to "play with for a long time," and he contributes his three rules for creating things which "feel alive": it must (1) feel and show emotions, (2) be aware of itself and its environment, and (3) have behaviors which change over time. Furby's pursuit of these goals include expressive eyes and ears, crying when held upside down, reacting to loud noises, and gradually switching from Furbish to English for its utterances. A living thing emits a constant stream of little surprises. Expect to see Nell put the XO-3's microphone and accelerometer to good use.

Talking and Listening. The "Talking To Machines" podcast also discusses ELIZA and Cleverbot, which dovetails with my interest in the popular Speak activity for Sugar and related toys like Talking Tomcat for mobile phones. The key insight here is that a little bit of "cheap trick" AI can go a long way toward making a personable and engaging system. We want Nell to feel like a friend. Recent work by the Common Sense Computing Initiative at MIT's Media Lab shows how we can reset this on a sounder basis and use mostly-unstructured input to allow the system to grow and learn (creating "behaviors changing over time"). In particular, I'll cite ConceptNet for its database and practical NLP tools, and inspiration from "Empathy Buddy," "StoryFighter," and the other projects described in their Beating Common Sense paper. It's also worth noting that open source speech tools are good and getting better (the VoxForge site points to most of them); also interesting is this technique for matching a synthesized voice to that of the user.

Collecting, nurturing, and rewarding. Collector games such as Pocket Frogs and Flower Garden are sticky activities which encourage kids to come back to the device and continue working toward a goal over a long period of time. Memrise is educational software illustrating this technique: its users tend a garden of flowers by mastering a set of flash cards. Nell will incorporate the sticky aspects of such games, possibly also integrating the Mozilla Open Badges infrastructure into an achievement/reward system.

I hope this has given you a general sense of the direction of our Nell project. In future blog posts I'll drill down into implementation details, demonstration storyboards, and other more concrete facets of Nell.

Narrative Interfaces

Wed, 15 Jun 2011 22:05:16 GMT

One Laptop per Child creates student-centric learning experiences. Our current software stack, however, is somewhat "shallow". When you turn on the XO, all the content is immediately available but there is no path or guidance provided. Nothing suggests what you should try first, or indicates an order to progress through the activities provided. Everything is available, but there's no built-in journey. No plot. How can we improve this?

This Friday (June 17) at 2pm Eastern we're inviting some folks over to OLPC's new offices at the American Twine building to discuss Narrative Interfaces, as part of the proposed XO-3 software stack. Nick Montfort will give a short talk on Curveship, his model-based interactive fiction system, and Chris Ball will present some related recent hacking. Angela Chang will present her Tinkerbooks early-literacy platform, which allows kids to interactively change the written story on the page. And I'll discuss Neal Stephenson's novel The Diamond Age (a recap of a short talk I gave at EduJAM in Uruguay), and give concrete suggestions for how Diamond Age's Primer might influence the software architecture for the XO-3. (I might even reveal how to make software testing semantically indistinguishable from writing a game!) Chris Ball and I have also been collecting best-of-breed "comic books that teach you something" as examples of educational narrative; we'll pass those around and post a reading list after the event.

The real point of this meeting isn't the talks, per se, but the discussions to follow. We're trying to gather folks who know a lot more about this stuff than we do, in order to learn from them and be inspired. We don't have a lot of space, unfortunately, so I'm going to have to ask for RSVPs from those who wish to attend. If you're in the Boston area and feel like you have something to contribute (and especially if you have created/could create Creative Commons-licensed content for education), drop me a line at cscott at laptop dot org. Describing what you can contribute to the discussion will help break ties if space is inadequate.

We will also live-stream the meeting at ustream.tv/channel/cscottnet. Afterwards we'll post higher-quality video and a list of cited works. Thanks in advance to everyone who will participate, online and off!

UPDATE: video now up; see this writeup on Chris Ball's blog.

Small systems... and distributed ones

Tue, 24 May 2011 23:50:32 GMT

Today I stumbled across some very interesting projects by Nickolay Platonov which I'd like to discuss in an OLPC context.

I've been hacking away at TurtleScript fueled partly by a drive for minimalism: a small system is a learnable system. To that end, the language is based on Douglas Crockford's "Simplified JavaScript" (as recognized by this top-down operator precedence parser) which tries hard to include only JavaScript's "Good Parts". For example, the initial code for the TurtleScript system uses prototype inheritance (via Object.create) rather than classical class-style inheritance. In fact, the new operator isn't even included in the Simplified JavaScript/TurtleScript language.

In a discussion of TurtleScript Alan Kay mentioned, "It is very tricky to retain/maintain readability (so the first Smalltalk was also an extensible language not just semantically but syntactically)." And today I stumbled across the Joose library, which gives a very readable syntax for traditional class-based inheritance. It backs this up with a robust meta-object protocol, introspection, and lots of nice features borrowed from Perl 6, CLOS, and Smalltalk. The syntax ought to work fine with the limited tile set of TurtleScript, although I might have to add a tile for the new operator.

However, adding Joose raises some questions. Is the increase in readability worth the addition of such a large library to the system? What impact will this have on understanding problems and debugging? Is a return to class-based inheritance a positive change? (There have been arguments that the make-a-clone-and-change-it practice of prototype inheritance is easier to understand for new learners.) Can a larger overall system actually be easier to understand?

And once we're looking at libraries... Nickolay Platonov is now working on Syncler, based on the Bayou system. Unobstructive replication mechanisms would certainly make it easier to construct the sorts of collaborative applications we've wanted for Sugar. I have two concerns with Syncler's current state. First, the use of explicit continuation-passing style greatly impairs readability. The JavaScript yield keyword helps a lot when writing asynchronous code. (It's not supported by all JavaScript engines, but yield wouldn't be hard to add to TurtleScript.) Second, Syncler's event model uses explicit callbacks. I've been greatly impressed with the Flapjax event model (and its strongly-typed functional cousin). Both of these changes ought to make asynchronous code much more readable—and isn't that an important part of grokking a system?

Turtles All The Way Down

Fri, 20 May 2011 06:07:54 GMT

Inspired by Bert Freudenberg, Ian Piumarta, and Walter Bender, I started hacking on "Turtles All The Way Down" (aka TurtleScript) on the plane back from Uruguay. Now there's a nice rendering demo to show what a tile-based editor for JavaScript might look like, as well as a bytecode compiler and interpreter for the language. The bytecode instruction set is still too large; encouraged by Craig Chambers' work on SELF I think I ought to be able to replace all the unary and binary operators, conditional jumps, and slot selectors by a single mapof operator. I can put a better object model on the interpreter, too; I've written some notes on the matter.

The question is: does this really have educational value? "Turtles all the way down" is a great slogan, and a fine way to teach a graduate-level class on compiler technology, but I feel that the higher-level UI for tile-based program editing is the really useful thing for tablet computing. I'm a compiler geek and love the grungy underbelly of this stuff, but I keep reminding myself I should really be spending more time building a beautiful fluffy surface.

Next Steps for New Technologies

Sat, 30 Apr 2011 02:13:01 GMT

I've reached the end of the month. I've accomplished my Android and NativeClient-related goals, but didn't get the time to do as much mesh and python investigation as I'd wanted. Here are some ideas for next month's work. (Next week I'll be in Uruguay for EduJAM.)

GObject Introspection (Android or NaCl)

Start by porting libffi. An android port would be straightforward, but since libffi involves code generation (ARM, x86), this is going to require a bit of assembly magic and the new "JIT"/"shared library" support in the NaCl plugin.
Then port gobject-introspection. GObject-Introspection relies on libffi for its guts, but the hard part of this port will be refactoring g-i's build process, which is not cross-compilation friendly. Might need to rewrite some tools. If targeting NaCl, you might consider finishing the code allowing execution of unsandboxed NaCl binaries.
Turn gobject-introspection on its head: generate GIR and a C binding for the platform "native" interface. For NaCl, this would be a GObject-using C-level binding of the browser-native DOM; for Android, this would be a GIR binding of the native Android APIs. These bindings should be mostly automatically generated, since they will need to continue tracking successive native platform releases/HTML5 features.
Demos! Change browser DOM from Python, write native Android apps in Python. Add a gobject-introspection binding to cforth, then do the same from forth. (Forth might be a simpler place to start than Python. Or not.)

GTK (Android or NaCl)

Build on the cairo/pango port to proceed to a full GTK backend for Android/NaCl. These backends ought to be upstreamable. The NaCl port should be based on the broadway work: the cairo canvas would be drawn to more directly, but a lot of the mechanism which captures JavaScript events and translates them into the GTK event loop could probably be reused.
Demo: "Hello GTK world" in Android/NaCl.

Sugar partitioning.

Bring Sugar closer to being a true multi-language multi-library platform.

Refactor sugar modules (for example, sugar toolbar widget) as standalone C libraries. Basic idea is to embed Python and export a C API, while preserving as much of the code as possible. Python libraries now invoke this library via g-i-r instead of directly. The python embedding tool is probably a useful standalone product.
Rewrite "Hello, Sugar" activity in C (or vala), using #include for import and GObject inheritance instead of python inheritance. Use this as a guide to pull apart sugar into modules (as above) to make this code actually work as written.

Miscellanous topics

ChromeOS w/ touch support.
Find an appropriate machine, do an installation, what are the roadblocks/rough spots? Can we install on XO-1.75 as a testbed?
TurtleArt as JavaScript viewer/editor.
Revisit TurtleScript work, but skip over the time-consuming "construct an editor" step by reusing the (excellent) TurtleArt code.
Mesh: android olsrd frontend, build testbed, research 802.11 DCF issues.

Summary

There are four rough topics here; I might try to continue the breadth-first search by spending a week on each. It might be more satisfying to downselect two of these issues and spend two weeks on each.

Pango/Android -vs- Pango/NaCl

Sat, 30 Apr 2011 02:13:01 GMT

At the end of my Sugar/Android week, I had a simple Pango-on-Cairo demo running. This was built on a stack of ported libraries, including gettext, pixman, freetype, libxml2, fontconfig, and glib, as well as cairo and pango. You can run the demo yourself by sideloading pango-demo.apk onto your Android device (tested on a Motorola Xoom), and you can browse the source code to see what it entailed (here's the scariest part). (I was inspired by Akita Noek's android-cairo project, but I ended up reworking the build scheme and redoing most of the ports.)

It made sense to start my Sugar/NaCl investigation by porting the same demo application to Native Client. The same stack of ported libraries was involved, although it was easy to include more functionality in the Native Client ports, including threading and PNG/PS/PDF support in cairo. The source code is a fork from the upstream naclports project, and the process was generally much cleaner. (But see my previous post for some caveats regarding naclports.) If you're using Chrome 10 or 11, you can run the demo in your browser (follow the instructions on that page). The Wesnoth team has a parallel project which ported some of these libraries as well, but not in an upstreamable manner.

The demo app uses cairo to draw the background, an animated X, and some basic text in the center; it uses Pango's advanced international text support to draw properly-shaped Persian text in a circle around it. The center text is the "proper" bilingual Greek/Japanese written form of "pango"; the text around the edges is the Persian name of the internationalization library, "harfbuzz". Note that the Persian text is written right-to-left—and that I didn't put a full CJK font in the NaCl app, so the Japanese "go" character is missing. The Android port rebuilds the font cache at each startup, so it loads rather slowly; the NaCl port contains a prebuilt font cache so it starts more quickly.

Both ports took about two weeks. I blew my original schedule, partly due to the Patriot's day holiday, and partly because I'd given Android about a week's head start by tinkering on it before my original schedule post. The framerate of the demo is much better on NaCl (so fast that the edges of the animated X look choppy in the screenshot), but the hardware isn't easily comparable, so the comparison doesn't really tell us much. The porting effort was certainly more pleasant on NaCl, since newlib is a much more complete libc than Android's "Bionic"—but having gdb available made debugging on Android easier. (There is an unintegrated NaCl branch that integrates NaCl gdb in the browser, though!)

Much of the GNOME/POSIX library stack assumes access to a filesystem tree and does file-based configuration. In our demo application, fontconfig was the most culpable party: it wanted to load a configuration file describing font locations and naming, then to load the fonts themselves from the file system, and finally to write a cache file describing what it found back to the file system. Most ported software is going to want similar access—even if you store the user's own documents in a Journal, software still expects to find configuration, caches, and other data in a filesystem.

Android provides the POSIX filesystem APIs, but the filesystem an app can touch is segmented and sandboxed. As discussed previously, Android's Opaque Binary Blob feature may allow you to create a app-specific filesystem, but this doesn't let you share (for example) fonts and font configuration between activities. NaCl might eventually provide a similar unshared mechanism based on the HTML5 AppCache.

The preferred solution is more limited, but more flexible: no built-in filesystem APIs are used (or in NaCl's case, provided!) at all. Instead, you provide your own implementation of the POSIX file APIs (either via the --wrap linker indirection or through an appropriate backend to newlib/glibc/glib). For the NaCl demo app, I wrote a rather-elaborate in-memory filesystem --- only to find that an even-more-elaborate one already existed in naclports. But the longer-term solution uses message-passing (SRPC in NaCl, intents in Android) to implement these POSIX APIs. In Native Client, the implementation would be in browser-side JavaScript, which would then allow you to share parts of the filesystem tree between activities and/or map it into (cached) web-addressed resources. In either case, your application still sees the bog-standard POSIX API it expects.

More problematic are the networking APIs. Here Android provides a pretty standard socket library, while Native Client provides nothing at all. Using a browser-based implementation, as for the file APIs, will work fine for HTTP, WebSockets and even P2P via the HTML5 P2P APIs. But it's not clear that (for example) glib's elaborate asynchronous DNS name resolver implementation can (or should!) be implemented in a NaCl port.

In the end, the porting effort and abstraction shifts needed for Native Client and Android are roughly comparable. I expect Native Client will hold a strong edge in allowing close integration with web standards and web technologies. Android will probably continue to hold an edge in third-party application support and platform maturity.

Sugar-on-Native Client investigation

Sat, 30 Apr 2011 02:13:01 GMT

This post will describe the state of Native Client in general, based on week 2 of my original four week plan. In the next post, I'll link to my work so far, and compare the Native Client and the Android efforts. Recapping, the end goal of these explorations is a platform for the next generation of the Sugar learning environment.

To begin, the Native Client (NaCl) plugin is fairly mature in a number of areas. Version 0.2 of the NaCl SDK was recently released (a version number which substantiates the "fairly" in my previous sentence), and the NativeClient plugin is currently shipping in Chrome (versions 10 and 11), although you have to manually turn on a preference in the about:flags dialog to enable it. The NaCl toolchain is much more standard than the Android NDK toolchain I discussed previously, and the robust naclports tree shows that the patches required for NaCl ports of common packages tend not to be too evil. The Tcl interpreter and Qt tookit port demos show that fairly complex pieces of code can be deployed today on NaCl.

On the other hand, there are three main difficulties:

The default NaCl toolchain uses newlib as its standard C library. This is consistent with Google's preference for BSD-licensed code in SDKs they provide to the public (see the discussion of Bionic in the Android SDK). However, there also exists a branch of the SDK which uses glibc. The glibc branch supports several additional features, like shared library support. However, it is unclear whether this will ever be a "supported" part of the SDK. If glibc does become supported, it is unlikely ever to be the only supported libc; the BSD-licensed newlib will need to remain available as an option. (Yes, the LGPL license of glibc shouldn't inspire such paranoia, but Google has elected not to undertake the education of all prospective third-party developers.)
The naclports project, although fairly robust, is driven between the Scylla and Charybdis of compatibility. The goal is that all the code in naclports be buildable at all times on all three major platforms: Windows, Mac, and Linux. Further, it should support both x86_32 and x86_64 backends, and ideally ARM and pNaCl as well. It's auto-built against the latest SDK sources, but should also work on the latest released SDK. And with the addition of the glibc/newlib split discussed above, the possible build targets are multiplied further. Needless to say, keeping the tree building against such a large number of variants is not an easy task, and naclports is usually broken in some way. In practice, most developers seem to pay attention to some subset (say, x86_32/newlib/Linux host), but it's hard to push patches upstream without worrying about breaking some obscure target. It might be best to base future work on a proper package technology, like (say) dpkg-cross.
In general, a lot of interesting NaCl development has occurred on branches that are not easily integrated. I've already mentioned glibc support, which is a toolchain branch; shared library support is on another branch that requires a new chromium plugin as well. At various times different means have been implemented to run NaCl binaries "natively" outside the sandbox (for example, in order to test some feature at build time, or auto-generate some piece of code via introspection). These efforts live on abandoned branches, while the "official" means to do this is incomplete. Similarly, a lot of interesting NaCl work used the now-abandoned legacy "NPAPI" plugin interface to interact with the browser. It was followed by the "Pepper" plugin interface, which was itself abandoned. Current work uses the Pepper2 browser plugin APIs, which (unfortunately) have not yet been implemented in non-Chrome browsers and continue to flux about. Many interesting browser interactions exist only in deprecated Pepper APIs, not having yet been built into Pepper2. ARM and pNaCl work also appears to be on unintegrated branches. There are a number of different gdb support strategies.

None of these difficulties is insurmountable—and in fact, some are side-effects of the desirable active development and productization of Native Client. To date I've done my work on the (more compatible) SDK v0.1 and the (more upstreamable) newlib library. So far newlib has not been a huge obstacle, and this basis allows my patches and ports to be more broadly useful. This might change in the future—certainly at some point we need to move to ARM and/or pNaCl for XO-3, which will probably require building chrome and the NaCl toolchain from scratch. At that point, it may be worth further exploring the non-mainstream branches.

How does the iPad "use the iPhone's GPS"?

Wed, 27 Apr 2011 19:16:08 GMT

A few months ago, a number of stories came out covering the iPad's remarkable-seeming ability to share the GPS of a tethered iPhone. Apple's latest location database FAQ confirms the suspicions I voiced at the time: there's no actual GPS sharing involved. Instead, Apple is using the simultaneous GPS and Wifi radios on your iPhone to "crowd-source" what I'll call a "skyhook" database (after the first company to publicly use the technique). This correlates Wifi base station identifiers with their GPS locations in real time -- including (most likely) the real time location of the "base station" created by the iPhone when it is in tethering mode. All nearby Apple devices use this database to compute their location (based on all visible wifi base stations). Since the nearby device sees the iPhone's "base station" and the iPhone is busily updating the position of that "base station" in real time (along with all the other base stations the iPhone can see), the iPad (lacking a GPS of its own) gains the apparent magical ability to compute a very accurate position for itself.

The real interesting part of this story involves user consent and privacy—do iPhone users generally know that their devices are registering their location in Apple's database in real time whenever tethering is turned on? Any device which can query Apple's location database for the MAC address of your iPhone can track the position of your iPhone whenever you are tethering. That's basically what the magical ability of the iPad/iPhone pair tells us. Did you know that?

Sugar-on-Android, week one

Tue, 12 Apr 2011 21:14:03 GMT

Last week I described a four-week plan to survey key technologies for One Laptop Per Child's forthcoming XO-3 tablet computer. Here I'll describe the results of the first week of work, which dove into Google's Android operating system. Warning: technical content ahead...

Basic design of Sugar-on-Android

Cross-compile gobject/GTK/gobject-introspection/cairo/dbus for Android; distribute these key libraries as NDK libraries. This is what I spent most of my time on this week: I've managed so far to port libiconv, gettext, glib, pixman, freetype, fontconfig, cairo, libxml2, and pango. (Source code)
Use cairo or OpenGL backends of GTK3 to render legacy Sugar activities directly to Android canvas.
Modularize sugar; use D-Bus for inter-module communication. Interprocess communication mechanism is Android 'intents'; these can redirect to the web or the Android Market for missing dependencies. (Collabora reportedly already has a D-Bus implementation for Android.) Sugar components can also become Android Services.
Implement Sugar Home/Groups/Neighborhood views and Journal as four separate Android App Widgets. These could also be implemented by providing a new Android home application, but I think the finer-grained modularity afforded by splitting these functions would yield a better design and make it easier to incorporate upstream improvements to the Android launcher.(Android Live Wallpaper is also similar in function, but not as good a fit.)
The Sugar Journal becomes an Android "Content Provider", which stores/retrieves content for other Sugar activities. (There is special Android support for "collection-based Widgets" and Live Folders which may be helpful.)
Use gobject-introspection to build a multi-language environment. Use JGIR to expose Sugar APIs to "native" Dalvik apps; use something like the Android Scripting Environment to expose Android native APIs to GIR languages (Python, JavaScript, C, etc).
[opportunity] Use the Android port of OLSRd to implement a Neighborhood view. Alternatively, investigate AODV routing on Android and/or AllJoyn (which also requires root access, see pg 24-25 of the manual).

Key Benefits

Sugar is integrated into Android environment; use native Android education apps, or apps like Movie Studio which have no Sugar equivalents yet.
Android APIs and customization hooks are good, and provide a more-extensible framework for development.

Open challenges (general)

The web integration story is cloudy. Java and JavaScript can call each other inside a bundled WebView widget, but this isn't supported in standard Browser app. Browser plugin interface would help.
No good story for building 'native' Java/Dalvik or C apps on the device. Writing a simple Dalvik compiler would help. Dalvik specs are available, and people have written Dalvik compilers for toy languages.
"View source" requests can be implemented as an Android 'intent' message, but no good story for implementing this functionality other than on a case-by-case basis in each activity.
Although the Amazon Marketplace for Android indicates that it can be done, it appears that there is no "blessed" mechanism for creating .apk files on the device and installing them. (Android bug, discussion)

Current technical issues/bugs

Cross-compiling for Android is currently a miserable experience. The Android NDK appears to have been put together by a team which had never seen a proper cross-compiler before. Since I only had a week for this exploration, I mostly kludged things together to get past this, but any serious work with Android should start by defining and upstreaming proper autoconf "target triplets" for Android-on-{ARMv5, ARMv7, x86} and building a proper cross-compiler. Then patches to various tools and libraries could start being upstreamed. Using the bespoke Android.mk build system of the NDK is a non-starter. No serious obstacles here, just work to do.
Xoom hardware is ARMv7, but Android emulator is ARMv5 only. Unfortunately, gdb is broken on the Xoom. So we're building for ARMv5 at the moment, so we can debug in the (slow) emulator.
No good support for shared libraries may cause activity bloat. May be able to be worked around using the new Opaque Binary Blob (OBB) feature.
Much existing code (fontconfig, gettext, gtk, etc) expects to read configuration files from the filesystem. Currently we are using the default fall-back configurations. OBB support may help here as well. There are a number of different storage APIs in Android, but none seems quite right.
It would be nice to implement a ring-style XO home screen without completing replacing the android Launcher. No clear way to constrain app layout on home screen w/o completely replacing the Launcher. Is it worth hacking the Launcher source?
Mesh on Android using OLSRd current requires root access. In order to run on unrooted Android devices, we need (a) proper power management for Ad Hoc mode wifi, (b) APIs to enable Ad Hoc mode, and (c) APIs to manipulate kernel routes.
We're building libraries without thread support because Android's "Bionic" libc has an eccentric thread library. Linking with -lpthread fails because the thread functionality is bundled into -lc. Probably just providing an empty libpthread.so would help a lot.
Some work has been done to build GNU libc for Android. This bloats activities even further, but might help ease library porting.
Porting gobject-introspection will be painful because its makefiles are not set up for cross-compiling. Some steps want to run on the target hardware, which is difficult in the Android environment.

Bottom line

I can see how the whole Sugar stack can be put together on the Android platform. The hardest part is probably just setting up packaging and a good and repeatable build environment for the different components, and getting enough adoption of this that patches to support Android can be pushed upstream. Many of the important pieces can be developed in parallel (Theme, Journal, Mesh, Friends, Home, library porting, etc). A little early to tell how hard it will be to port existing Sugar activities to the new Python/pygobject/GTK3 framework.