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This is implemented in the next update.
Here's the relevant part of the developer agreement:

"An Application may not download or install executable code. Interpreted code may only be used in an Application if all scripts, code and interpreters are packaged in the Application and not downloaded. The only exception to the foregoing is scripts and code downloaded and run by Apple's built-in WebKit framework."

You also have to remember that everything has to be in the same binary. So, for example, GPLed code can't be included.

IMHO the only feasable way would be to have the compiler/tool run on a server (see http://feedback.textasticapp.com/feedback/7730-remote-source-code-compilation/).
Erlang only has a single line comment that starts with "%", right?

For some reason, the syntax definition file also had "%" as the start character of a multiline comment. I've removed that.
Textastic 1.1 i now available on the App Store.

New Features:
• Local web preview for html files
• It's now possible to hide the sidebar in landscape orientation
• The editor highlights the current line
• Saves and restores cursor position when switching files
• Supports TextExpander touch snippet expansion
• You can now manually select a syntax definition

Fixed:
• WebDAV server settings are now saved
• Additional keys are now automatically hidden when using a hardware keyboard
• Fixed issues with the Copy & Paste menu
• All supported file types are now registered for "Open In..."
• You can now delete the GettingStarted file
• Fixed some minor issues