Respond to an unknown method call
You are encouraged to solve this task according to the task description, using any language you may know.
- Task
Demonstrate how to make the object respond (sensibly/usefully) to an invocation of a method on it that it does not support through its class definitions.
Note that this is not the same as just invoking a defined method whose name is given dynamically; the method named at the point of invocation must not be defined.
This task is intended only for object systems that use a dynamic dispatch mechanism without static checking.
- Related task
AutoHotkey
Example by LinearSpoon <lang AutoHotkey>class example {
foo()
{
Msgbox Called example.foo()
}
__Call(method, params*)
{
funcRef := Func(funcName := this.__class "." method)
if !IsObject(funcRef)
{
str := "Called undefined method " funcName "() with these parameters:"
for k,v in params
str .= "`n" v
Msgbox %str%
}
else
{
return funcRef.(this, params*)
}
}
}
ex := new example ex.foo() ex.bar(1,2)</lang>
Brat
<lang brat>example = object.new
example.no_method = { meth_name, *args |
p "#{meth_name} was called with these arguments: #{args}"
}
example.this_does_not_exist "at all" #Prints "this_does_not_exist was called with these arguments: [at all]"</lang>
C#
<lang csharp>using System; using System.Dynamic;
class Example : DynamicObject {
public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
{
result = null;
Console.WriteLine("This is {0}.", binder.Name);
return true;
}
}
class Program {
static void Main(string[] args)
{
dynamic ex = new Example();
ex.Foo();
ex.Bar();
}
}</lang>
C++
Program terminates with diagnostic message, typically about a call to an "unimplemented pure virtual function". But in that situation, the function is actually known. The concept of a function not being known at all does not exist in C++, due to static name lookup checking. That is to say, the call a->b() causes the compiler to verify that the symbol b exists in the namespace associated with expression a, and refers to a function.
To avoid the pure virtual call, we can put some default function implementation into the abstract class which handles the situation (e.g. throws a custom exception or whatnot).
<lang cpp>class animal { public:
virtual void bark() // concrete virtual, not pure
{
throw "implement me: do not know how to bark";
}
};
class elephant : public animal // does not implement bark() { };
int main() {
elephant e; e.bark(); // throws exception
} </lang>
Caché ObjectScript
Caché classes can include support for what is called dynamic dispatch. If dynamic dispatch is in use and a program references a property or method that is not part of the class definition, then a method (called a dispatch method) is called that attempts to resolve the undefined method or property. For example, dynamic dispatch can return a value for a property that is not defined or it can invoke a method for a method that is not implemented. The dispatch destination is dynamic in that it does not appear in the class descriptor and is not resolved until runtime.
<lang cos>Class DynamicDispatch.Example Extends %RegisteredObject {
Method Foo() { Write "This is foo", ! }
Method Bar() { Write "This is bar", ! }
Method %DispatchMethod(Method As %String, Args...) { Write "Tried to handle unknown method '"_Method_"'" For i=1:1:$Get(Args) { Write ", " If i=1 Write "with arguments: " Write "'"_Args(i)_"'" } Write ! }
ClassMethod Test() { Set obj=##class(DynamicDispatch.Example).%New() Do obj.Foo() Do obj.Bar() Do obj.Grill() Do obj.Ding("Dong", 11) }
}</lang>
- Examples:
USER>Do ##class(DynamicDispatch.Example).Test() This is foo This is bar Tried to handle unknown method 'Grill' Tried to handle unknown method 'Ding', with arguments: 'Dong', '11'
Common Lisp
In Common Lisp, if a generic function is invoked on arguments for which there is no applicable specialized method, the method no-applicable-method is called with the generic function and the arguments.
<lang lisp>(defgeneric do-something (thing)
(:documentation "Do something to thing."))
(defmethod no-applicable-method ((method (eql #'do-something)) &rest args)
(format nil "No method for ~w on ~w." method args))
(defmethod do-something ((thing (eql 3)))
(format nil "Do something to ~w." thing))</lang>
Evaluating<lang lisp>(list (do-something 3) (do-something 4))</lang> produces
<lang lisp>("Do something to 3."
"No method for #<STANDARD-GENERIC-FUNCTION DO-SOMETHING 214FC042> on (4).")</lang>
D
Similar to the Python entry, but D performs this statically. <lang d>import std.stdio;
struct Catcher {
void foo() { writeln("This is foo"); }
void bar() { writeln("This is bar"); }
void opDispatch(string name, ArgsTypes...)(ArgsTypes args) {
writef("Tried to handle unknown method '%s'", name);
if (ArgsTypes.length) {
write(", with arguments: ");
foreach (arg; args)
write(arg, " ");
}
writeln();
}
}
void main() {
Catcher ca;
ca.foo();
ca.bar();
ca.grill();
ca.ding("dong", 11);
}</lang>
- Output:
This is foo This is bar Tried to handle unknown method 'grill' Tried to handle unknown method 'ding', with arguments: dong 11
Déjà Vu
The function set-default is useful here:
<lang dejavu>}
labda:
print "One!"
- one
labda: print "Two!"
- two
local :obj {
labda: print "Nope, doesn't exist." set-default obj
obj!one obj!two obj!three </lang>
- Output:
One! Two! Nope, doesn't exist.
E
In E, a message consists of a verb (arbitrary string) and arguments (sequence of arbitrary objects). It is conceptually entirely up to any given object how it dispatches incoming messages.
Practically, the object definition syntax provides a matcher clause to handle unrecognized messages. This example has the same behavior as the Python example.
<lang e>def example {
to foo() { println("this is foo") }
to bar() { println("this is bar") }
match [verb, args] {
println(`got unrecognized message $verb`)
if (args.size() > 0) {
println(`it had arguments: $args`)
}
}
}</lang>
Elena
Using generic handler (ELENA 3.2): <lang elena>import extensions.
class Example {
generic
[
console printLine($subject," was invoked").
]
generic : x
[
console printLine($subject,"(",x,") was invoked").
]
generic : x : y
[
console printLine($subject,"(",x,",",y,") was invoked").
]
}
program = [
var o := Example new. o foo. o bar(1). o someMethod(1,2).
].</lang>
- Output:
foo was invoked bar(1) was invoked someMethod(1,2) was invoked
Fancy
<lang fancy> class CatchThemAll {
def foo {
"foo received" println
}
def bar {
"bar received" println
}
def unknown_message: msg with_params: params {
"message: " ++ msg print
"arguments: " ++ (params join: ", ") println
}
}
a = CatchThemAll new a foo a bar a we_can_do_it a they_can_too: "eat" and: "walk" </lang>
Fantom
In Fantom, you can call methods statically or dynamically. Static calls to methods will be checked at compile time. Dynamic method calls (indicated by an instance->method syntax) are run through a "trap" method at run time. This method looks up the given method name, and throws an exception if the method/field is not known. This exception can be caught, and processed specially:
<lang fantom> class A {
public Void doit (Int n)
{
echo ("known function called on $n")
}
// override the 'trap' method, which catches dynamic invocations of methods
override Obj? trap(Str name, Obj?[]? args := null)
{
try
{
return super.trap(name, args)
}
catch (UnknownSlotErr err)
{
echo ("In trap, you called: " + name + " with args " + args.join(","))
return null
}
}
}
class Main {
public static Void main ()
{
a := A()
// note the dynamic dispatch
a->doit (1)
a->methodName (1, 2, 3)
}
} </lang>
Output:
$ fan unknown-method.fan known function called on 1 In trap, you called: methodName with args 1,2,3
Forth
Works with any ANS Forth
Needs the FMS-SI (single inheritance) library code located here: http://soton.mpeforth.com/flag/fms/index.html <lang forth>include FMS-SI.f include FMS-SILib.f
var x \ instantiate a class var object named x x add: \ => "aborted: message not understood" </lang>
Go
This uses reflection as in Send an unknown method call#Go, but goes one more step to put the reflection code in a method. This allows an unknown method call to be handled by this method of the receiving object. <lang go>package main
import (
"fmt" "reflect"
)
type example struct{}
func (example) Foo() int {
return 42
}
// a method to call another method by name func (e example) CallMethod(n string) int {
if m := reflect.ValueOf(e).MethodByName(n); m.IsValid() {
// it's known. call it.
return int(m.Call(nil)[0].Int())
}
// otherwise it's unknown. handle as needed.
fmt.Println("Unknown method:", n)
return 0
}
func main() {
var e example
fmt.Println(e.CallMethod("Foo"))
fmt.Println(e.CallMethod("Bar"))
}</lang>
- Output:
42 Unknown method: Bar 0
Groovy
Groovy allows us to capture all unknown method calls using the methodMissing function <lang groovy>class MyObject {
def foo() {
println 'Invoked foo'
}
def methodMissing(String name, args) {
println "Invoked missing method $name$args"
}
}</lang>
Testing: <lang groovy>def o = new MyObject() o.foo() o.bar() o.bar(1, 2, 'Test')</lang>
- Output:
Invoked foo Invoked missing method bar[] Invoked missing method bar[1, 2, Test]
Icon and Unicon
Unicon implements objects via a translator that emits native code. While Icon does not support objects, the native code could potentially be translated and run under Icon provided other Unicon extensions are not used.
Unicon does not natively allow a class to intercept unknown method calls and this task was originally marked as an omit. However, there are several ways that this functionality could be added to Unicon including:
- Using a try/catch (as per The Unicon Code Library) to catch the error (invalid method call). This is not very general and adds a lot of syntax for each method call.
- Using execution monitoring to catch and handle the error in a parallel co-expression. This would be very general and transparent requiring no extra syntax. It would be an excellent choice for a debugger.
- Using a procedure to invoke the method and catch any error. While not as general as the execution monitor approach, it doesn't require as much extra syntax as a try/catch and is focused specifically on method calls.
The example below is based upon the last case. The procedure 'DynMethod' would be used in place of normal method invocation. A special method 'UndefinedMethod' can be defined to handle unknown methods. The procedure 'DynMethod' requires knowledge of the internals of the code emitted to support objects.
<lang Unicon>procedure DynMethod(obj,meth,arglist[])
local m
if not (type(obj) ? ( tab(find("__")), ="__state", pos(0))) then
runerr(205,obj) # invalid value - not an object
if meth == ("initially"|"UndefinedMethod") then fail # avoid protected
m := obj.__m # get methods list
if fieldnames(m) == meth then # method exists?
return m[meth]!push(copy(arglist),obj) # ... call it
else
if fieldnames(m) == "UndefinedMethod" then # handler exists?
return obj.UndefinedMethod!arglist # ... call it
else runerr(207,obj) # error invalid method (i.e. field)
end</lang>
<lang Unicon>procedure main()
x := foo() y := foo2()
x.a() # example of normal method call DynMethod(x,"a") # using DynMethod DynMethod(x,"simplydoesntexist") # results in error DynMethod(y,"simplydoesntexist") # catches error DynMethod(y,"simplydoesntexist",1,2,3,4,5) # with parameters
end
class foo(A) # sample class and methods
method a(p1)
l1 := p1
return
end
method b(p2)
l2 := p2
return
end
initially
i1 := 0
return
end
class foo2 : foo (A)
method UndefinedMethod(x[]) # Undefined Method handler
write(&errout,"You called an undefinded method of this object.")
return
end
end</lang>
Io
<lang io>Example := Object clone do(
foo := method(writeln("this is foo"))
bar := method(writeln("this is bar"))
forward := method(
writeln("tried to handle unknown method ",call message name)
if( call hasArgs,
writeln("it had arguments: ",call evalArgs)
)
)
)
example := Example clone
example foo // prints "this is foo" example bar // prints "this is bar" example grill // prints "tried to handle unknown method grill" example ding("dong") // prints "tried to handle unknown method ding"
// prints "it had arguments: list("dong")"</lang>
J
In J, you can define a method at invocation time, and this definition can be in z which is the parent from which all objects and classes inherit.
For example, we could define <lang J>example=:3 :0
doSomething_z_=: assert&0 bind 'doSomething was not implemented' doSomething__y
)
doSomething_adhoc1_=: smoutput bind 'hello world' dSomethingElse_adhoc2_=: smoutput bind 'hello world'</lang>
With those definitions in a fresh session (where adhoc2 has not been given a definition for doSomething), we get the following behavior:
<lang J> example <'adhoc1' hello world
example<'adhoc2'
|doSomething was not implemented: assert</lang>
(Note that need to have a cover verb (or adverb or conjunction) for the method call if you want to abstract it or dynamically intercept previously undefined methods.)
JavaScript
There is a way (a bit unconfortable if you compare it to php´s way), involves using Proxy interface defined on ES 6, it isn´t supported still on all vendors, but for updated info, view MDN proxy site
<lang javascript> obj = new Proxy({},
{ get : function(target, prop)
{
if(target[prop] === undefined)
return function() {
console.log('an otherwise undefined function!!');
};
else
return target[prop];
}
});
obj.f() ///'an otherwise undefined function!!' obj.l = function() {console.log(45);}; obj.l(); ///45 </lang> Where {} is the object to wrap in the proxy, but it could be a reference to another object, and the second argument to proxy´s constructor are the handlers that supersede the behavior of the target ({}), the 'get' function gets executed every time a property from target ({}) is needed, in this case, only if the property didn´t existed before, it returns the dynamically generated function.
There are more handlers, that go from modify the way properties are added, to how to retrieve __proto__, just read the docs!.
Before Proxy interface, firefox (only) supported a suitable trap:
<lang javascript>var example = new Object;
example.foo = function () {
alert("this is foo");
} example.bar = function () {
alert("this is bar");
} example.__noSuchMethod__ = function (id, args) {
alert("tried to handle unknown method " + id);
if (args.length != 0)
alert("it had arguments: " + args);
}
example.foo(); // alerts "this is foo" example.bar(); // alerts "this is bar" example.grill(); // alerts "tried to handle unknown method grill" example.ding("dong"); // alerts "tried to handle unknown method ding"
// alerts "it had arguments: dong</lang>
Julia
Julia will throw a MethodError exception if its multiple dispatch cannot find a proper function for a given set of arguments. This exception can be caught and resolved within a generically typed function that is not specific as to the object type of its arguments. <lang julia> function add(a, b)
try
a + b
catch
println("caught exception")
a * b
end
end
println(add(2, 6))
println(add(1//2, 1//2))
println(add("Hello ", "world"))
</lang>
Kotlin
Kotlin JS does not currently target ECMAScript 2015 and so the Proxy object cannot be used for this task. The only way it can currently be accomplished is to use the Mozilla extension __noSuchMethod__ property which works with Firefox 43 but is no longer supported by more up to date versions: <lang scala>// Kotlin JS version 1.2.0 (Firefox 43)
class C {
// this method prevents a TypeError being thrown if an unknown method is called
fun __noSuchMethod__(id: String, args: Array<Any>) {
println("Class C does not have a method called $id")
if (args.size > 0) println("which takes arguments: ${args.asList()}")
}
}
fun main(args: Array<String>) {
val c: dynamic = C() // 'dynamic' turns off compile time checks c.foo() // the compiler now allows this call even though foo() is undefined
}</lang>
- Output:
Class C does not have a method called foo
Lasso
Unknown methods are handled by the inclusion of the special "_unknownTag" method.
If this is not included in the type, an error will result that may terminate processing unless otherwise handled. <lang Lasso>define exampletype => type { public foo() => { return 'this is foo\r' } public bar() => { return 'this is bar\r' } public _unknownTag(...) => { local(n = method_name->asString) return 'tried to handle unknown method called "'+#n+'"'+ (#rest->size ? ' with args: "'+#rest->join(',')+'"')+'\r' } }
local(e = exampletype)
- e->foo()
// outputs 'this is foo'
- e->bar()
// outputs 'this is bar'
- e->stuff()
// outputs 'tried to handle unknown method called "stuff"'
- e->dothis('here',12,'there','nowhere')
// outputs 'tried to handle unknown method called "dothis" with args: "here,12,there,nowhere"'</lang>
Logtalk
There are two ways to handle unknown messages. From the sender side, we can catch the exception that is generated when an object doesn't understand a message:
<lang logtalk>
- - object(foo).
:- public(try/0). try :- catch(bar::message, Error, handler(Error)).
handler(error(existence_error(predicate_declaration,message/0),_)) :- % handle the unknown message ...
- - end_object.
</lang>
From the receiver side, an object can implement the built-in protocol "forwarding", defining the handler for messages that it doesn't understand (e.g. by forwarding or delegating it to another object):
<lang logtalk>
- - object(foo,
implements(forwarding)).
forward(Message) :- % handle the unknown message ...
- - end_object.
</lang>
Lua
This is specifically the purpose of the __index metamethod:
<lang Lua> local object={print=print} setmetatable(object,{__index=function(t,k)return function() print("You called the method",k)end end}) object.print("Hi") -->Hi object.hello() -->You called the method hello </lang>
Mathematica
By default, when evaluating a symbol's DownValues, Mathematica picks the most specific.
<lang Mathematica>obj[foo] = "This is foo.";
obj[bar] = "This is bar.";
obj[f_Symbol] := "What is " <> SymbolName[f] <> "?";
Print[obj@foo];
Print[obj@bar];
Print[obj@baz];</lang>
- Output:
This is foo. This is bar. What is baz?
Object Pascal
An exception about calling a virtual method will be raised and interrupt the current code flow. But in that situation, the function is actually known. The concept of a function not being known at all does not exist in Object Pascal, due to static name lookup checking. That is to say, the call a.b() causes the compiler to verify that the symbol b exists in the namespace associated with expression a, and refers to a function.
To avoid the possibility of an abstract method call, one common solution is to leave an empty implementation for the virtual method instead of making it abstract. This is what an abstract method call looks like:
<lang pascal> type
Tanimal = class public procedure bark(); virtual; abstract; end;
implementation
var
animal: Tanimal;
initialization
animal := Tanimal.Create; animal.bark(); // abstract method call exception at runtime here animal.Free;
end. </lang>
Objective-C
-forwardInvocation: is usually used to "forward" the message on to another object to handle.
<lang objc>#include <Foundation/Foundation.h>
// The methods need to be declared somewhere @interface Dummy : NSObject - (void)grill; - (void)ding:(NSString *)s; @end
@interface Example : NSObject - (void)foo; - (void)bar; @end
@implementation Example - (void)foo {
NSLog(@"this is foo");
}
- (void)bar {
NSLog(@"this is bar");
}
- (void)forwardInvocation:(NSInvocation *)inv {
NSLog(@"tried to handle unknown method %@", NSStringFromSelector([inv selector]));
NSUInteger n = [[inv methodSignature] numberOfArguments];
for (NSUInteger i = 0; i < n-2; i++) { // First two arguments are the object and selector.
id __unsafe_unretained arg; // We assume that all arguments are objects.
// getArguments: is type-agnostic and does not perform memory management,
// therefore we must pass it a pointer to an unretained type
[inv getArgument:&arg atIndex:i+2];
NSLog(@"argument #%lu: %@", i, arg);
}
}
// forwardInvocation: does not work without methodSignatureForSelector: // The runtime uses the signature returned here to construct the invocation. - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
int numArgs = [[NSStringFromSelector(aSelector) componentsSeparatedByString:@":"] count] - 1;
// we assume that all arguments are objects
// The type encoding is "v@:@@@...", where "v" is the return type, void
// "@" is the receiver, object type; ":" is the selector of the current method;
// and each "@" after corresponds to an object argument
return [NSMethodSignature signatureWithObjCTypes:
[[@"v@:" stringByPaddingToLength:numArgs+3 withString:@"@" startingAtIndex:0] UTF8String]];
} @end
int main() {
@autoreleasepool {
id example = [[Example alloc] init];
[example foo]; // prints "this is foo"
[example bar]; // prints "this is bar"
[example grill]; // prints "tried to handle unknown method grill"
[example ding:@"dong"]; // prints "tried to handle unknown method ding:"
// prints "argument #0: dong"
} return 0;
}</lang>
Oforth
In Oforth, a method is an object of Method class. This object is not related to a particular class or hierarchy : each class can provide an implementation for a particular method.
Method call is resolved at runtime : oforth looks at the top of the stack (that will become the method receiver) and searchs for a valid implementation of the method called (into receiver class or its parents). If no implementation is found for this method, doesNotUnderstand method is called instead.
<lang Oforth>1 first [1:interpreter] ExRuntime : 1 does not understand method <#first></lang>
<lang Oforth>1 "first" asMethod perform [1:interpreter] ExRuntime : 1 does not understand method <#first></lang>
Oforth has not concept such as "unknow method" : if a method exists, it can be called (even if the object on top of stack does not understand it). If not, no call is possible.
<lang Oforth>1 "unknow_method" asMethod perform [1:interpreter] ExRuntime : null does not understand method <#perform></lang>
doesNotUnderstand can be redefined for a particular class :
<lang Oforth>Object Class new: MyClass MyClass method: doesNotUnderstand(m)
"Well, sorry, I don't understand " print m println ;
MyClass new first Well, sorry, I don't understand #first</lang>
ooRexx
To respond to unknown method calls, classes can implement an unknown method. This method is passed the name of the method and an array of the arguments that were used on the call.
<lang ooRexx>u = .unknown~new
u~foo(1, 2, 3)
- class unknown
- method unknown
use arg name, args
say "Unknown method" name "invoked with arguments:" args~tostring('l',', ')</lang>
Output:
Unknown method FOO invoked with arguments: 1, 2, 3
Oz
To respond to unknown method calls, classes can implement the otherwise method. As its sole argument, this method gets the received message, i.e. a record with the name of the unknown method as its label and the arguments as the record features.
<lang oz>declare
class Example
meth init skip end
meth foo {System.showInfo foo} end
meth bar {System.showInfo bar} end
meth otherwise(Msg)
{System.showInfo "Unknown method "#{Label Msg}}
if {Width Msg} > 0 then
{System.printInfo "Arguments: "}
{System.show {Record.toListInd Msg}}
end
end
end
Object = {New Example init}
in
{Object foo}
{Object bar}
{Object grill}
{Object ding(dong)}</lang>
Output:
foo bar Unknown method grill Unknown method ding Arguments: [1#dong]
Perl
<lang perl>package Example; sub new {
bless {}
} sub foo {
print "this is foo\n";
} sub bar {
print "this is bar\n";
} sub AUTOLOAD {
my $name = $Example::AUTOLOAD;
my ($self, @args) = @_;
print "tried to handle unknown method $name\n";
if (@args) {
print "it had arguments: @args\n";
}
} sub DESTROY {} # dummy method to prevent AUTOLOAD from
# being triggered when an Example is
# destroyed
package main; my $example = Example->new;
$example->foo; # prints "this is foo" $example->bar; # prints "this is bar" $example->grill; # prints "tried to handle unknown method Example::grill" $example->ding("dong"); # prints "tried to handle unknown method Example::ding"
# and "it had arguments: dong"</lang>
Perl 6
<lang perl6>class Farragut {
method FALLBACK ($name, *@rest) {
say "{self.WHAT.perl}: $name.tc() the @rest[], full speed ahead!";
}
}
class Sparrow is Farragut { }
Farragut.damn: 'torpedoes'; Sparrow.hoist: <Jolly Roger mateys>;</lang>
- Output:
Farragut: Damn the torpedoes, full speed ahead! Sparrow: Hoist the Jolly Roger mateys, full speed ahead!
FALLBACK will be called for any method that is not defined. Since any class inherits from Any, there will be plenty of already defined methods. Those which are not defined can also be used as L-Values by the magic of is rw.
<lang perl6>class L-Value {
our $.value = 10;
method FALLBACK($name, |c) is rw { $.value }
}
my $l = L-Value.new; say $l.any-odd-name; # 10 $l.some-other-name = 42; say $l.i-dont-know; # 42</lang>
PHP
<lang php><?php class Example {
function foo() {
echo "this is foo\n";
}
function bar() {
echo "this is bar\n";
}
function __call($name, $args) {
echo "tried to handle unknown method $name\n";
if ($args)
echo "it had arguments: ", implode(', ', $args), "\n";
}
}
$example = new Example();
$example->foo(); // prints "this is foo" $example->bar(); // prints "this is bar" $example->grill(); // prints "tried to handle unknown method grill" $example->ding("dong"); // prints "tried to handle unknown method ding"
// prints "it had arguments: dong
?></lang>
PicoLisp
The function 'try' is used to send a message to an object for which it is not known whether it inherits a method for that message or not. As opposed to the syntacically equivalent 'send' function, 'try' does not give an error, but returns NIL. We might redefine 'send' to get an effect analog to CLOS. <lang PicoLisp>(redef send (Msg Obj . @)
(or
(pass try Msg Obj)
(pass 'no-applicable-method> Obj Msg) ) )
(de no-applicable-method> (This Msg)
(pack "No method for " Msg " on " This) )
(class +A)
(dm do-something> ()
(pack "Do something to " This) )</lang>
Test: <lang PicoLisp>: (object 'A '(+A)) -> A
- (object 'B '(+B))
-> B
- (list (send 'do-something> 'A) (send 'do-something> 'B))
-> ("Do something to A" "No method for do-something> on B")</lang>
Pike
Pike allows to overload the -> operator used to access object members:
<lang Pike>class CatchAll
{
mixed `->(string name)
{
return lambda(int arg){ write("you are calling %s(%d);\n", name, arg); };
}
}
> CatchAll()->hello(5); you are calling hello(5); > CatchAll()->something(99); you are calling something(99);</lang>
Python
Python objects can implement a __getattr__() method to handle accesses of unknown attributes (methods are just attributes that are callable; so this function handles both methods and non-method fields). Here we assume that if you access an unknown attribute, you want a method, so we return a function that can be called.
<lang python>class Example(object):
def foo(self):
print("this is foo")
def bar(self):
print("this is bar")
def __getattr__(self, name):
def method(*args):
print("tried to handle unknown method " + name)
if args:
print("it had arguments: " + str(args))
return method
example = Example()
example.foo() # prints “this is foo” example.bar() # prints “this is bar” example.grill() # prints “tried to handle unknown method grill” example.ding("dong") # prints “tried to handle unknown method ding”
# prints “it had arguments: ('dong',)”</lang>
Racket
Racket's usual object system can't deal with unknown methods, but we can capture the relevant exception and deal with it: <lang racket>
- lang racket
(require racket/class)
(define-syntax-rule (send~ obj method x ...)
;; note: this is a naive macro, a real one should avoid evaluating `obj' and
;; the `xs' more than once
(with-handlers ([(λ(e) (and (exn:fail:object? e)
;; only do this if there *is* an `unknown-method'
(memq 'unknown-method (interface->method-names
(object-interface o)))))
(λ(e) (send obj unknown-method 'method x ...))])
(send obj method x ...)))
(define foo%
(class object%
(define/public (foo x)
(printf "foo: ~s\n" x))
(define/public (unknown-method name . xs)
(printf "Unknown method ~s: ~s\n" name xs))
(super-new)))
(define o (new foo%)) (send~ o foo 1) ; => foo: 1 (send~ o whatever 1) ; Unknown method whatever: (1) </lang>
Alternatively, we can use Swindle for a CLOS-like object system, and do something similar to the Common Lisp solution: <lang racket>
- lang swindle
(defgeneric (foo x)) (defmethod (no-applicable-method [m (singleton foo)] xs)
(echo "No method in" m "for" :w xs))
(defmethod (foo [x <integer>]) (echo "It's an integer"))
(foo 1)
- => It's an integer
(foo "one")
- => No method in #<generic
- foo> for "one"
</lang>
Ruby
<lang ruby>class Example
def foo
puts "this is foo"
end
def bar
puts "this is bar"
end
def method_missing(name, *args, &block)
puts "tried to handle unknown method %s" % name # name is a symbol
unless args.empty?
puts "it had arguments: %p" % [args]
end
end
end
example = Example.new
example.foo # prints “this is foo” example.bar # prints “this is bar” example.grill # prints “tried to handle unknown method grill” example.ding("dong") # prints “tried to handle unknown method ding”
# prints “it had arguments: ["dong"]”</lang>
Scala
As of scala 2.9, scalac must receive the -Xexperimental optional for Dynamic to receive this treatment. <lang scala>class DynamicTest extends Dynamic {
def foo()=println("this is foo")
def bar()=println("this is bar")
def applyDynamic(name: String)(args: Any*)={
println("tried to handle unknown method "+name)
if(!args.isEmpty)
println(" it had arguments: "+args.mkString(","))
}
}
object DynamicTest {
def main(args: Array[String]): Unit = {
val d=new DynamicTest()
d.foo()
d.bar()
d.grill()
d.ding("dong")
}
} </lang> Output:
this is foo this is bar tried to handle unknown method grill tried to handle unknown method ding it had arguments: dong
Sidef
The special AUTOLOAD method gets called when a method isn't defined in the current class: <lang ruby>class Example {
method foo {
say "this is foo"
}
method bar {
say "this is bar"
}
method AUTOLOAD(_, name, *args) {
say ("tried to handle unknown method %s" % name);
if (args.len > 0) {
say ("it had arguments: %s" % args.join(', '));
}
}
}
var example = Example.new;
example.foo; # prints “this is foo” example.bar; # prints “this is bar” example.grill; # prints “tried to handle unknown method grill” example.ding("dong"); # prints “tried to handle unknown method ding”
# prints “it had arguments: dong”</lang>
Slate
Here is an example of unknown methods being used to call shell commands (this is already defined in the base image):
<lang slate>define: #shell &builder: [lobby newSubSpace].
_@shell didNotUnderstand: message at: position "Form a command string and execute it." [
position > 0
ifTrue: [resend]
ifFalse:
[([| :command |
message selector isUnarySelector ifTrue: [command ; message selector. message optionals pairsDo: [| :key :value | command ; ' -' ; (key as: String) allButFirst allButLast ; ' ' ; (value as: String)]]. message selector isKeywordSelector ifTrue: [| keywords args | keywords: ((message selector as: String) splitWith: $:). command ; keywords first. keywords size = 1 ifTrue: "Read a string or array of arguments." [args: message arguments second. (args is: String) ifTrue: [command ; ' ' ; args] ifFalse: [args do: [| :arg | command ; ' ' ; arg]]]]] writingAs: String) ifNil: [resend] ifNotNilDo: [| :cmd | [Platform run: cmd]]] ].</lang>
Here is an example of it being used:
<lang slate>slate[1]> shell ls: '*.image'. kernel.new.little.64.1244260494374694.image slate2.image net.image slate.image True slate[2]></lang>
Smalltalk
<lang smalltalk>Object subclass: CatchThemAll [
foo [ 'foo received' displayNl ]
bar [ 'bar received' displayNl ]
doesNotUnderstand: aMessage [
('message "' , (aMessage selector asString) , '"') displayNl.
(aMessage arguments) do: [ :a |
'argument: ' display. a printNl.
]
]
]
|a| a := CatchThemAll new. a foo. a bar. a weCanDoIt. a theyCanToo: 'eat' and: 'walk'.</lang>
There are two ways to catch unimplemented messages:
- on the receiver side, by redefining the "doesNotUnderstand:" method in the receiver class hierarchy, as shown above.
- on the sender side, by catching the MessageNotUnderstood exception, as follows:
<lang smalltalk>[
bla := someObject fooBar. foo := bla.
] on: MessageNotUnderstood do:[:ex |
ex return: 'fortyTwo'
]</lang> this will leave 'fortyTwo' on bla AND foo (because the handler proceeds). This sender-side handling is useful if you don't want to or if you are not allowed to change the receiver's class hierarchy (which is not a technical, but solely a political/conventional limitation, because in Smalltalk no class is closed, and extensions can be added to any class simply by loading or dynamically adding methods - even to the base system or third party packages).
Of course, this handler now catches any other unimplemented messages as well, thus if foobar was implemented, but itself sends another bad message, we'd catch that as well. We can check for this in the handler, by checking for which message got us there: <lang smalltalk>[
bla := someObject fooBar. foo := bla.
] on: MessageNotUnderstood do:[:ex |
((ex message selector == #fooBar) and: [ ex message receiver == someObject])
ifTrue:[
ex return: 'fortyTwo'
] ifFalse:[
ex reject
]
]</lang> the reject will re-reaise the exception, and lead us to an outer handler, or the debugger, if there is none.
There is a utility method for exactly the above (it catches only #fooBar to the original receiver): <lang smalltalk>anObject perform:#fooBar ifNotUnderstood:[ ...some alternative code and return value... ].</lang>
SuperCollider
<lang SuperCollider>Ingorabilis {
tell { "I told you so".postln; }
find { "I found nothing".postln }
doesNotUnderstand { |selector ... args| "Method selector '%' not understood by %\n".postf(selector, this.class); "Giving you some good arguments in the following".postln; args.do { |x| x.postln }; "And now I delegate the method to my respected superclass".postln; super.doesNotUnderstand(selector, args) }
} </lang> Usage:
<lang SuperCollider> i = Ingorabilis.new; i.tell; // prints "I told you so" i.find; // prints ""I found nothing" i.think(1, 3, 4, 7); </lang>
The latter answers: <lang SuperCollider>Method selector 'think' not understood by Ingorabilis Giving you some good arguments in the following 1 3 4 7 And now I delegate the method to my respected superclass ERROR: Message 'think' not understood. RECEIVER: Instance of Ingorabilis { (0x1817b1398, gc=D4, fmt=00, flg=00, set=00)
instance variables [0]
} <...></lang> Catch the method call: <lang SuperCollider> i = Ingorabilis.new try { i.think } { "We are not delegating to super, because I don't want it".postln }; </lang>
Tcl
or
<lang tcl>package require TclOO
- First create a simple, conventional class and object
oo::class create Example {
method foo {} {
puts "this is foo"
}
method bar {} {
puts "this is bar"
}
} Example create example
- Modify the object to have a custom ‘unknown method’ interceptor
oo::objdefine example {
method unknown {name args} {
puts "tried to handle unknown method \"$name\""
if {[llength $args]} {
puts "it had arguments: $args"
}
}
}
- Show off what we can now do...
example foo; # prints “this is foo” example bar; # prints “this is bar” example grill; # prints “tried to handle unknown method "grill"” example ding dong; # prints “tried to handle unknown method "ding"”
# prints “it had arguments: dong”</lang>
UNIX Shell
bash doesn't have objects with methods, but it can handle unknown commands: <lang Bash>function handle_error {
status=$?
# 127 is: command not found if $status -ne 127 ; then return fi
lastcmd=$(history | tail -1 | sed 's/^ *[0-9]* *//')
read cmd args <<< "$lastcmd"
echo "you tried to call $cmd"
}
- Trap errors.
trap 'handle_error' ERR</lang> Sample usage:
$ foo bash: foo: command not found you tried to call foo
zkl
If something can not be resolved (in a class) the function __notFound is called. It can redirect to another object. <lang zkl>class C{ fcn __notFound(name){println(name," not in ",self); bar}
fcn bar{vm.arglist.println("***")}
}</lang>
C.foo //-->"foo not in Class(C)", returns Fcn(bar) C.foo(1,2,3) //-->"foo not in Class(C)", "L(1,2,3)***"
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