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Remotely debugging your iPhone Scheme apps with a REPL
July 5, 2009
View the remote debugger source
In an earlier blog post, I showed you how to develop on the iPhone
with Gambit Scheme. Although that is cool, what about debugging?
Haven't we just cut ourselves off from using either GDB (we aren't
using Objective-C) or Gambit's debugger (we aren't running the program
from a console)?
A debugger is an essential tool, especially in dynamic languages like
Scheme. It may not be as important in languages like Haskell because
of type-checking, but even then you want to watch "data flow" as the
program is running. I think we can say that using Scheme on the iPhone
is impractical until we can use Gambit's debugger for our iPhone apps.
Fortunately, Marc Feeley, implementer of Gambit Scheme, recently wrote
a "proof of concept" remote debugger. It allows you to push REPLs from
a running Gambit application (the client) to a different machine (the
server).
You guessed it. The client will be our iPhone application and the
server will be our main computer, allowing us access to REPLs on our
computer which are running inside the iPhone application.
I think this is best explained in the screencast below. First, let me
explain the philosophy of the remote debugger a little more.
Marc wrote the remote debugger in response to someone asking about
debugging multiple Gambit Scheme threads at once on the Gambit mailing
list (view original message). With the remote debugger, you can tell
Gambit to create a remote REPL for errors on each thread. So if two
threads errored, two separate debugging REPLs would open up on the
server.
Now we have facilities for debugging Scheme iPhone apps
remotely. We can run Scheme apps on either the iPhone Simulator or
the iPhone itself, and when an errors occurs we get a real Gambit REPL
on our computer for debugging the problem.
What's even cooler is that we have everything we need for running
a full Scheme REPL to *interact with our iPhone app real-time*! Check
it out in the screencast below.
Please note that this library is still "proof of concept," and there are
more things to figure out and bugs to fix. I am not publishing it standalone
yet, but you can find it included in the gambit iPhone example program on github or in
Marc's reply.
You can also see how I integrated the debugger library in the example
iPhone app here. A couple lines further down and you can see how I
fire up a main REPL.
(I apologize for all the static. It was raining and I didn't realize
it would pick it up.)
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Next up is support for touch events and access to the accelerometer!
View the remote debugger source
