Hi --

On Mon, 13 Feb 2006, the_crazy88 wrote:

> I've searched a bit about the subject, but I don't understand it. All I
> want is to simply pass a function of one instance to another. In Python
> this works, but in ruby not.
>
> Python code would be:
> def x(a,b):
>  return a+b
>
> def y(a):
>  return a(3,4)
>
> y(x)
> # Result is 7
> But the equivalent ruby code doesn't work:

That's because it's not equivalent :-)

> def a(x,y)
>  return x+y
> end
>
> def x(b)
>  return b(3,4)
> end
>
> x(a)
> #Gives the following error in irb:
>
> NoMethodError: undefined method `b' for main:Object
>        from (irb):40:in `x'
>        from (irb):42
>        from :0

It shouldn't; it should complain that you've called a with no
arguments instead of two.

When you do this:

   x(a)

Ruby evaluates the expression 'a', and that's a method call -- so it
tries to call the method a, but that method takes two arguments and
you haven't provided any.

You can pass method names around, as strings or symbols, and then
"send" the method name to an object (or self, by default):

   def a(x,y)
     x + y
   end

   def x(b)
     send(b,3,4)
   end

   puts x(:a)  # 7

There are also some more elaborate mechanisms involving Method
objects, but the above might achieve what you need.


David

-- 
David A. Black (dblack / wobblini.net)
Ruby Power and Light (http://www.rubypowerandlight.com)

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