You can!
Every time it comes up, I'm going to repeat this suggestion: Use Nu[1] as a REPL for iOS[2] and OS X[3].
[2] http://groups.google.com/group/programming-nu/browse_thread/...
[3] https://github.com/timburks/nu/blob/master/nu/console.nu
The big difference is heritage: F-Script is Smalltalk and Nu is Lisp.
Nu builds as a static library. You need to include it in your project and link it into your binary to make it available. Then you'll need to instantiate a NuRemoteHost object after your application has launched.
Once it's running you can launch a Nu interpreter and execute NuRemoteClient to connect to the host you created earlier.
Even though it's a Lisp dialect, Nu was designed from the beginning to be interoperable with Objective-C, so translating between them is easy. (Convert brackets to parentheses and you're 80% of the way there.)
As a contrived example, this:
if(![self.address startsWith:@"https://"])
self.address = [@"https://" stringByAppendingString:self.address]
(if (not (@address startsWith:"https://"))
(set @address ("https://" stringByAppendingString:@address)))