bile phone inside a Web application. The contents of incoming SMSs are displayed in a browser window, letting users choose whether or not an SMS should be spoken aloud.

This application uses three separated tiers: a Web browser, a Web server, and a phone server. The phone server is responsible for receiving the SMSs and speaking them on demand. The Web server plays the role of orchestrator; when the phone notifies it that a new SMS has been received, it alerts the interested clients and, depending on the user configuration, speaks out the short message.

The application takes advantage of another useful feature of Hop: allowing a server to be a new source of program-generated Web events for clients or other servers. In the phone example, the server starts establishing a bridge with the phone (using the android variable), then waits for the notification of an SMS and accesses the phone text-to-speech facility ( using the tts variable).

The server code registers a function invoked each time a new SMS is received to forward a software Web event to interested Web clients.

implementation

Hop is an open source project whose source code can be downloaded from the Web site http://hop.inria.fr. All the actual source code shown in this article can also be downloaded from this Web site. The development kit contains the compilers, interactive documentation, various tools for creating and installing applications, and the runtime environment, which is a dedicated Web server that runs on all Un*x platforms—Linux, Mac OS X, and Android. It is operational on mainstream modern architectures such as x86/32, x86/64, PowerPC, and the ARM family.

The Hop runtime environment re-
quires only 3MB–4MB of RAM. This
small memory footprint makes it suit-
able for smartphones or even smaller
machines, such as the new ARM-
equipped computers (Phidget SBC2,
Raspberry Pi, among others) that offer
only a few megabytes to applications.
As far as speed is concerned, the Hop
Web server delivers dynamic Web
pages significantly faster than tradi-
tional servers such as Apache or Light-
tpd. The performance gain is a result
of the Hop-in-Hop implementation,
which enables the server to respond to
requests involving dynamic computa-
tions without costly execution context
switching. 4

A new Playground

Hop is the conjunction of a multitier programming language and a runtime environment for the Web. Its main goals are to unify the linguistic features needed for Web applications, to automate the physical distribution of code in a fully transparent way, and to help programmers in reusing and combining external resources. All of these are keys to program simplicity and conciseness.

This article has presented several complete Web application examples written in Hop. The first example illustrates how to raise the HTML abstraction level by using an algorithmic programming language approach. The second example shows how to reuse third-party code. The third example shows that combining Web technologies with open data opens the path for new application ideas. The last example presents a simple diffuse application on the Web. These examples have all been chosen for their simplicity; the same technique would be equally effective when dealing with much bigger applications in domains such as home automation or multimedia.

None of the applications presented here exceeds 30 lines of Hop source code. This is not accidental; it is the consequence of a perception shift where the Web is no longer regarded as a mere platform for sharing documents but as a revolutionary runtime environment. Note, however, that Hop is a realistic programming language, which implies a lot of additional bells and whistles. This article has ignored most of them, concentrating on multitier programming and the connection to official Web technologies.

Finally, note that the Hop core lan-
guage is a strict functional program-
ming language, with runtime type
checking. This design choice comes
mostly from a personal bias of the
first author. Arguably, it fits well with
the current programming trends of
the Web, but other flavors should be
possible, yielding other programming
languages. We hope to see such new
languages appearing, because the
multiplicity of approaches will foster
creativity and possibly open a new era
in language and tool design.

Related articles on queue.acm.org

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http://queue.acm.org/detail.cfm?id=1039534

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References

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3. kelsey, r., Clinger, W. and rees, j. the revised ( 5) report on the algorithmic language scheme. Higher-Order and Symbolic Computation 11, 1 (1998);

http://www.sop.inria.fr/indes/fp/bigloo/doc/r5rs.html/ 4. serrano, m. hop, a fast server for the diffuse Web.

In Proceedings of the 11th International Conference on Coordination Models and Languages (lisbon, portugal, 2009).

5. serrano, m., gallesio, e. and loitsch, f. hop, a language for programming the Web 2.0. In Proceedings of the First Dynamic Languages Symposium (portland, or, 2006).

 

Manuel Serrano is a senior scientist at InrIa, leading the InDes (Informatique Diffuse et sécurisée) team in sophia-antipolis. after completing his ph.D. at the pierre and marie Curie university (upmC) on the compilation of functional languages, he moved to nice and created the bigloo development environment for scheme. he joined InrIa in 2001 and has focused on development environments for the diffuse web since 2005.

Gérard Berry, director of research at InrIa, works on programming languages, their mathematical semantics, and program verification technologies. his focus has been on the lambda-calculus and its models, reactive and real-time programming and verification, and high-level digital circuits specification, synthesis, and verification. prior to joining InrIa he was chief scientist of esterel technologies and was the main author of the esterel language, which has been used in academia and industry for applications ranging from complex circuit synthesis to airplane control.