Add undefined-function, which will be called, if a function is undefined

[porky11]

undefined-function may be predefined like this

(def undefined-function (macro extern (name rest))
  (def args (var auto \ (get-varargs-list mc (- (arg-count mc) 1) rest)))
  (report-undefined-function-error name args)
  (exit 1)))

This only is useful, if in different contextes different version of this function may be used. Normally in some special package (for example the current package for debug reasons). This would require, that functions of some package will use the version of undefined-function as it is defined in this package, even if it is not imported to the current package, which the functions would belong to. (for example when you call (pkg.fn args) (pkg.undefined-function pkg.fn args) will be called. This will enable some nice features I have in mind. See referencing issue

[tomhrr]

I think there are other ways to handle this in general, and particularly in the context of #155. For example, rather than expanding argument forms to safe.{name}, safe could instead expand them to convert-to-safe {name}, which could use the various exists functions from the introspection module to determine what to do.

[porky11]

yes, in most cases this can be done in different ways. And if there is really a case, where undefined-function is really helpful, there could be written a macro, which converts all function calls inside into (call-or-undefined fn …), a macro which expands to undefined-function or to the function call. (which is similar to safe) But it's currently not that easy to write such macros (doesn't work correctly for safe), since macro expansion seems not to work correctly and there is no way to manually macroexpand something (see #157)