Default arguments

[Søren Hauberg]

Hi,
  I'd really like Octave to have some way of having default function 
arguments. In C++ you can do

  void fn(int arg = 7) {}

I think R supports the same syntax. I don't think matlab has something 
similar, so if Octave adds support for default it should be something 
simple that won't break whatever syntax matlab decides to add in the 
future that'll handle default arguments.

  So I suggest that Octave just adds some functions to make it easy to 
work with default arguments, instead of changing the syntax of the 
language. The C++ example above would then be written something like this

  function fn(arg)
    arg = default_value("arg", 7);
  endfunction

The function default_value should then expand (like a macro in C/C++) 
into this code

    if (!exist(arg, "var")
      arg = val;
    endif

One thing that's missing from this idea, is a way to tell the function 
to use the default argument. As an example, think of a function that 
takes three arguments

  function fn2(arg1, arg2, arg3)
    ...
  endfunction

How does the user tell fn2 that he wants to set the value of arg1 and 
arg3, but use the default value for arg2? I suggest adding a function 
called "default" that returns an object of type "default". That way the 
user can write

  fn2(arg1, default, arg3)

I don't know how to handle varargin, but I don't think there is much one 
can do...


Søren