"Parametric class" on inheritance

Ruby
#1

I found in “Ruby Cookbook” (recipe 8.8) a syntactically
interesting construct, which I would like to understand.
Here is the excerpt from the coding example:

require ‘delegate’
class OrdinalNumber < DelegateClass(Fixnum)
…
end

I am a bit puzzled about the way the parent class is
written: DelegateClass(Fixnum). This looks like a
function call, only that DelegateClass is not a
function (since it starts with an upper-case letter).
How does it work out that DelegateClass(Fixnum) evaluates
to something of type “Class”?

Ronald

#2

On Jan 24, 2008, at 10:23 AM, Ronald F. wrote:

written: DelegateClass(Fixnum). This looks like a
function call, only that DelegateClass is not a
function (since it starts with an upper-case letter).
How does it work out that DelegateClass(Fixnum) evaluates
to something of type “Class”?

It is a method call actually. You are allowed to create method that
begin with a capital letter, though it’s best only used for special
cases like this. Usually a capitalized method does some kind of
conversion, like what we see here or Kernel#Array, Kernel#String, etc.

James Edward G. II

#3

On Jan 24, 2008 11:23 AM, Ronald F. [email protected] wrote:

written: DelegateClass(Fixnum). This looks like a
function call, only that DelegateClass is not a
function (since it starts with an upper-case letter).
How does it work out that DelegateClass(Fixnum) evaluates
to something of type “Class”?

When in doubt check it out in irb:

require ‘delegate’
=> true

DelegateClass(Fixnum)
=> #Class:0x35dd9c

So DelegateClass is indeed a function that returns a class and the <
syntax does allow expressions on the right hand side.

Brian.

#4

James G. wrote:

On Jan 24, 2008, at 10:23 AM, Ronald F. wrote:

How does it work out that DelegateClass(Fixnum) evaluates
to something of type “Class”?

It is a method call actually. You are allowed to create method that
begin with a capital letter, though it’s best only used for special
cases like this. Usually a capitalized method does some kind of
conversion, like what we see here or Kernel#Array, Kernel#String, etc.

Thanks a lot (also to Brian) for the helpful explanation!

Ronald