It would be good if I can define "Unknown function" in Arc. When Arc will try to call undefined function it shouldn't signal an error, but call this "nil function" instead.
I'm guessing this request was inspired by something similar to Ruby's method_missing method. method_missing is often used to implement behavior that involves the name of the message being passed to an object (the official example is a class that converts between Roman and Arabic numerals and uses method_missing to redirect messages such as "roman.III" to "roman.roman_to_arabic('III')"). Another usage is undefining all methods and then intercepting the calls, such as in RubyQuiz #95, where that approach was used to convert code blocks to S-expressions.
The former use isn't really that useful without an object system to restrict the scope of a single method_missing definition. And, of course, any object system could implement its own method_missing. And the second usage, in addition to being pathologically uncommon, could be implemented in Arc using macros, or, where function calls must be intercepted at runtime, by iterating through all values in the namespace (or even a closure as well) and overloading functions to add hooks that perform some other behavior if some boolean is set to true. (Of course, said boolean would probably need to be in a table keyed by the current thread.) Of course, that would require being able to retrieve a list of all values in a namespace or closure to iterate through (hmmm...now there's an idea).
