Are there equivalents to Ruby's method_missing in other languages?

Question

In Ruby, objects have a handy method called method_missing which allows one to handle method calls for methods that have not even been (explicitly) defined:

Answer 1

Actionscript 3.0 has a Proxy class that can be extended to provide this functionality.

dynamic class MyProxy extends Proxy {
  flash_proxy override function callProperty(name:*, ...rest):* {
    try {
      // custom code here
    }
    catch (e:Error) {
      // respond to error here
    }
}  

Answer 2

Its equivalent in Io is using the forward method.

From the docs:

If an object doesn't respond to a message, it will invoke its "forward" method if it has one....

Here is a simple example:

Shout := Object clone do (
    forward := method (
        method_missing := call message name
        method_missing asUppercase
    )
)

Shout baz println     # => BAZ

/I3az/

Answer 3

I was looking for this before, and found a useful list (quickly being overtaken here) as part of the Merd project on SourceForge.

---

 Construct                          Language
-----------                        ----------
 AUTOLOAD                           Perl
 AUTOSCALAR, AUTOMETH, AUTOLOAD...  Perl6
 __getattr__                        Python
 method_missing                     Ruby
 doesNotUnderstand                  Smalltalk
 __noSuchMethod__(17)               CoffeeScript, JavaScript
 unknown                            Tcl
 no-applicable-method               Common Lisp
 doesNotRecognizeSelector           Objective-C
 TryInvokeMember(18)                C#
 match [name, args] { ... }         E
 the predicate fail                 Prolog
 forward                            Io

With footnotes:

• (17) firefox
• (18) C# 4, only for "dynamic" objects

Answer 4

Smalltalk has the doesNotUnderstand message, which is probably the original implementation of this idea, given that Smalltalk is one of Ruby's parents. The default implementation displays an error window, but it can be overridden to do something more interesting.

Answer 5

This is accomplished in Lua by setting the __index key of a metatable.

t = {}
meta = {__index = function(_, idx) return function() print(idx) end end}
setmetatable(t, meta)

t.foo()
t.bar()

This code will output:

foo
bar

Answer 6

Tcl has something similar. Any time you call any command that can't be found, the procedure unknown will be called. While it's not something you normally use, it can be handy at times.