Section 3.3.1 of the iPhone SDK Agreement:
The thing to understand about Section 3.3.1. is that it is the legal letter of the SDK agreement, but not the desired intent.
Developers are allowed to use SQL for database operations. Developers are encouraged to use GLSL via OpenGL ES 2.0 for fast graphics. SQL and GLSL are not languages in the Objective-C family and they are compiled / interpreted. But they also don’t directly link against the Documented APIs (e.g., there’s no “SELECT subviews FROM UIView”).
Regular expressions and XSLT (ugh) are also similarly permitted.
How I intend to interpret Section 3.3.1., as an independent iPhone developer is as follows: Use Objective-C where it makes the most sense: directly interfacing with the iPhone SDK. When I need different tools to achieve something application specific, like an AI in Lua or printing commands in PostScript, I’m going to use them.
For me personally, I feel that this is what Apple permits and practices in the course of mobile software development.
Practices? Yeah. Have you looked at what iPhone OS itself uses? XML and JSON for data serialization. Nibs, property lists, XSLT in XML domain specific languages. TinyScheme to interpret Scheme code for sandboxed process permissions. hunspell‘s DSL drive the spell check dictionaries.
For sure, Apple has impure motives. But that doesn’t mean they’re wrong. Mobile constraints are different. Battery life and performance matter. A lot.
Use the right tools for the job: Objective-C when talking to iPhone OS. But whatever your application’s brain needs to think different.