[Haskell] ANN: JMacro 0.6 and JMacro-RPC, featuring cross-platform json-rpc, easy AJAX applications, evented reactive panels and more

Gershom Bazerman gershomb at gmail.com
Mon Dec 17 02:36:49 CET 2012

jmacro: http://hackage.haskell.org/package/jmacro
jmacro-rpc: http://hackage.haskell.org/package/jmacro-rpc
jmacro-rpc-snap: http://hackage.haskell.org/package/jmacro-rpc-snap

I'm happy to announce the release of JMacro 0.6 (0.6.3 actually) which 
brings a host of improvements. JMacro is a quasiquoted library for the 
programmatic generation of JavaScript code. It lets you write in a 
superset of JavaScript with haskell-like syntax and compile-time 
guarantees of syntactic correctness, and is intended to be used for both 
embedded javascript and compilers and libraries targeting 
JavaScript--where hygienic naming, proper scoping, and anti-quotation 
are especially important. Documentation is on the Haskell Wiki: 

This release brings a host of improvements to efficiency, driven largely 
by user-submitted patches and requests. Text is used more uniformly, as 
opposed to String, and JSON integration is provided via aeson rather 
than the json package. Additionally, nearly all of JavaScript is 
supported, including more obscure operators, as well as labels, and 
labeled break and continue.

The big news, however, is JMacro-RPC, which provides multi-server tools 
for serving JSON-RPC 2.0 [1] remote procedure calls, as well as binding 
to them from both Haskell and JavaScript. On the Haskell side, rpc 
server functions and caller stubs are automatically generated by 
typeclasses. On the JavaScript side, which is less typed, a function is 
exposed to marshall javascript function arguments directly into RPC 
calls to a located URL.

[1] http://jsonrpc.org/specification

On top of these RPCs, JMacro-RPC provides the conversation page 
abstraction, which allows the easy creation of web-applications with 
server side state. The user simply provides a function from JavaScript 
-> Page that splices in some JavaScript, and a list of functions they 
wish to be accessible from the page. All JavaScript within the page then 
has access to these functions, which run, transparently, server-side, 
with access to page-invocation-local shared state. Backends exist for 
JMacro-RPC for Happstack and Snap, and can be easily written for any 
other server or framework.

JMacro-RPC now also provides the Panels library, which is even 
higher-level, providing the convenience of continuation-style web 
development with no scaling hassle. On the server side, Panels are 
entirely stateless, storing no client-specific state (so requests can be 
sharded to multiple servers as well without worrying about session 
replication). Code is written with a set of combinators over Panels, 
which package up display and behavior simultaneously. Panels, inspired 
by FRP, can provide Signals, which are sampleable, Events, which are 
discrete and can trigger updates, and Sinks which can be bound to 
Signals (behaving similarly to FRP wormholes). Semantics, by virtue of 
client-server interaction, are necessarily evented rather than 
continuous. As such, this is properly a system for declarative, 
event-driven reactivity rather than genuine FRP. Such a system is 
impoverished compared to the richer continuous-time or fully 
stream-based discrete-time semantics provided by of real FRP libraries. 
However, it is more than powerful enough for the sorts of user 
interfaces Panels are designed to build, and holds the added advantage 
of being straightforward, tractable, and a tight fit for client-server 

While Signals are applicative and Sinks are contravariant functors, 
which exposes at least some flavor of FRP, simple Signals and Sinks 
which don't require server-side processing (and are safely distinguished 
as such by types) can be bound to one-another purely on the client-side, 
avoiding unnecessary round-trips.

The Panels code combines concepts from FRP [1][2], Self-Adjusting 
Computation [3], ASP.NET update panels [4], and research in stable 
naming schemes for bound variables [5]. The JavaScript "rts" driving 
evented updates is designed to be reasonably efficient and leak-free.

Between basic RPCs, Conversations, and Panels, JMacro-RPC is intended to 
provide the full "power scale" for writing robust AJAX web applications 
with complex server-side logic. Because it is designed to be usable from 
any Haskell web framework or server, it is suitable for widespread 

Patches, bug-reports, contributions, feature requests, documentation, 
etc. all are very welcome.

[1] http://www.haskell.org/haskellwiki/Functional_Reactive_Programming
[3] http://www.umut-acar.org/self-adjusting-computation
[4] http://msdn.microsoft.com/en-us/library/bb399001(v=vs.100).aspx
[5] http://citeseerx.ist.psu.edu/viewdoc/summary?doi= ("I 
am not a number: I am a free variable", McBride and McKinna. See section 
four, on hierarchical names.)


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