howto method reference

[oravard]

Hi,

I recently discovered Cobra and I found it very interesting although I am not very
familiar with .NET
I am trying to do some tests and I want to pass to a method a reference
to another method.
The objective is to do smething like that :

class MyClass
def method1(m)
...
if method1 <> nil
method1()
...
MyClass().method1(ref ob.other_method)

I know this latter example does not work since m is declared as dynamic
and there is a conversion problem.
I tried to use the MethodInfo class without sucess.

Can you help me for this general purpose ?

Thanks.

[Charles]

Yes. See the Sorter Example posted recently in this forum. Notice the use of "sig" rather than "def" to declare a "method signature". The name you use in that method signature becomes a type name. You'll then see that type name used subsequently in the declaration of an argument in order to accept a method reference.

Also, this is mentioned briefly in the Cobra 0.7.4 release notes.

Regarding your example, I would like to eventually make that work in Cobra, but for now passing methods around has to be strongly typed. Many other things can be dynamic however.

[oravard]

Thanks, it works.

There is another problem in my implementation. See the folowing code.
Is it a mistake from me or a compilator bug ? See the compilation error too.

Code: Select all

class MyClass
       sig BindMethod

       var _command as BindMethod

       def init(command as BindMethod)
             _command = ref command       #!! if I comment this line, there is no compilation error

class AnotherClass
       def method1
             print 'method 1'

       def main is shared
             p = AnotherClass()
             ob = MyClass(ref p.method1)

Here is the compilation error :
error: COBRA INTERNAL ERROR / UnknownMemberException / obj=Param(11344, "command" (ID), command, BindMethod), name='EnglishName', type=Param
Compilation failed - 1 error, 0 warnings

Thanks.

[Charles]

Is it a mistake from me or a compilator bug ? See the compilation error too.

Turns out to be both. Change "_command = ref command" to "_command = command". You need the "ref" to refer to dotted methods without invoking them. Once you have a value of the signature type, no "ref" is needed.

Think of it this way: "ref obj.foo" creates a pointer to the "foo" method. So you don't need "ref someVar" as someVar is already the pointer.

And there is a bug in the Cobra error message code which was trying to say "Only methods can be referenced, not variables." I'll fix.

[oravard]

Fine !!

I have a few more question (that should be another thread) :

- can we define default values for method arguments ?
- can we use keyword arguments when calling methods ?
about this lattern, I saw a compiler message that says that will
be possible in the future.

Thanks.

[Charles]

Currently, neither default values nor keyword arguments are implemented. I want to at least have keyword arguments for various benefits including making calls more readable, making the argument order flexible and passing key/value pairs.

But implementing these features requires a bit of extra thought because Cobra also has method overloading and it interfaces with C#. So everything has to fit together.

Note that overloading allows one to simulate simple default values:

class X

    def foo(i as int) as int
        return .foo(i, 1)

    def foo(i as int, j as int) as int
        return i * j

    def main is shared
        obj = X()
        print obj.foo(2)
        print obj.foo(2, 7)

[oravard]

Yes. That what I used today. Thanks.