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comp.lang.ruby

Curious include behavior

Tom Werner

9/28/2007 6:49:00 PM

This caused me a small headache the other day. By including a module, an
instance method suddenly becomes accessible as a module method. Seems
unintuitive, I'm curious to understand the reasoning behind it.

module Foo
module Bar
def hello
'hello'
end
end
end

Foo::Bar.hello # => -:9: undefined method `hello' for Foo::Bar:Module (NoMethodError)

include Foo::Bar

Foo::Bar.hello # => "hello"


--
Tom Preston-Werner

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7 Answers

David A. Black

9/28/2007 7:09:00 PM

0

7stud 7stud

9/29/2007 3:26:00 AM

0

David A. Black wrote:
>
> At the top level, the include will include the module in Object:
>

Where is the module before it is included?
--
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David A. Black

9/29/2007 7:07:00 AM

0

7stud 7stud

9/29/2007 8:48:00 AM

0

David A. Black wrote:
> Hi --
>
> On Sat, 29 Sep 2007, 7stud -- wrote:
>
>> David A. Black wrote:
>>>
>>> At the top level, the include will include the module in Object:
>>>
>>
>> Where is the module before it is included?
>
> I'm not sure what you mean. Can you elaborate?
>
>
> David

I would think the name Foo would be visible just like the name of a
method is visible, e.g.:

def greet
puts "hello"
end

According to pickaxe2 p. 346, greet is inserted as a private method of
Object, and hence can be called in any context without a receiver, i.e.
without 'some_obj." in front of the method name.

In the op's example, there is a module named Foo. Is Foo just a free
floating object? Or, is it an attribute of some omnipresent object?
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David A. Black

9/29/2007 11:13:00 AM

0

7stud 7stud

9/30/2007 3:10:00 AM

0

David A. Black wrote:
>
> Constants you define at the top level are owned by Object:
>

Is an include always necessary to be able to call a module's instance
methods?
--
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Robert Dober

9/30/2007 4:24:00 PM

0

On 9/30/07, 7stud -- <dolgun@excite.com> wrote:
> David A. Black wrote:
> >
> > Constants you define at the top level are owned by Object:
> >
>
> Is an include always necessary to be able to call a module's instance
> methods?

Yes I believe so, that is include or extend

irb(main):001:0> module M
irb(main):002:1> def a; 42 end
irb(main):003:1> end
=> nil
irb(main):009:0> m = M.instance_method("a")
=> #<UnboundMethod: M#a>
irb(main):010:0> m.bind("a").call
TypeError: bind argument must be an instance of M
from (irb):10:in `bind'
from (irb):10
from :0
irb(main):012:0> C = Class::new{ include M }
###################^^^^^^^
=> C
irb(main):013:0> m.bind(C.new).call
=> 42
irb(main):014:0> a="42"
=> "42"
irb(main):015:0> a.extend M
###########^^^^^^
=> "42"
irb(main):016:0> m.bind(a).call
=> 42

HTH
Robert

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