Class methods in modules

[Adrian]

Hi

This maybe a pretty obvious question - but I decided to ask anyway.

I am trying to class a method defined within an included module from a
class level method as below.

Is there a better way to do this? (Assumming that the class level
method cannot be changed to a instance method.)

class Report
include FormatHelper

self.generate_report
...
my_date = self.format_date(...)
...
end
end

module FormatHelper

self.format_date(date)
...
end
end

Any help welcome

Cheers
Adrian

 

[Florian Gilcher]

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Hi

This maybe a pretty obvious question - but I decided to ask anyway.

I am trying to class a method defined within an included module from a
class level method as below.

Is there a better way to do this? (Assumming that the class level
method cannot be changed to a instance method.)

class Report
include FormatHelper

self.generate_report
...
my_date = self.format_date(...)
...
end
end

module FormatHelper

self.format_date(date)
...
end
end

Any help welcome

Cheers
Adrian

Hi,

Class Methods of Modules are local to the module.

If you want to extend a Class by the instance Methods of a Module, you
have to possibilities:

Use "extend" instead of "include":

module A
def foo
"foo"
end
end

class C
extend A

def self.bar
foo
end
end

include the Methods within the "metaclass":

class D
class << self
include A
end

def self.bar
foo
end
end

If you wish to keep class an instance methods separated, it is
somewhat popular
to define them in different modules:

module FormatHelper
module ClassMethods
def foo
#####
end

#some more methods
end

module InstanceMethods
def foo
#####
end

#some more methods
end
end

class D
include FormatHelper::InstanceMethods
extend FormatHelper::ClassMethods
end

Have a lot of fun.
Florian Gilcher
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[Rick DeNatale]

If you wish to keep class an instance methods separated, it is somewhat
popular
to define them in different modules:

module FormatHelper
module ClassMethods
def foo
#####
end

#some more methods
end

module InstanceMethods
def foo
#####
end

#some more methods
end
end

class D
include FormatHelper::InstanceMethods
extend FormatHelper::ClassMethods
end

Or for a module, which adds both class and instance methods, the
popular way is something like this:

module FormatHelper

def self.included(class_or_module)
class_or_module.extend(ClassMethods)
end

module ClassMethods
... class methods go here
end

.. instance methods go here
end

Then when a class includes the module it gets both sets of methods.

 

[Christoph Schiessl]

Interesting - so class methods in modules don't make any sense at all?
Calling the modules class methods directly and special class methods
such as included(base) and extended(base) are the exception of course.

Would you agree with that statement (question to everyone)?

 

[Rick DeNatale]

Interesting - so class methods in modules don't make any sense at all?
Calling the modules class methods directly and special class methods such as
included(base) and extended(base) are the exception of course.

Would you agree with that statement (question to everyone)?

Well, since Modules aren't classes, they can't have class methods per
se. They can have module methods:

module Mod
def self.i_am_a_module_method
end
end

Modules are objects themselves so can have methods.

For an example of module methods in the standard library, look at the
Math module. For example to get the natural log of 10 you can write:

Math.log(10)

If you simply write:

log(10)

you'll get a method not found error. But you could also include Math
into a class or module and then any methods of that class or module,
could use the simpler call.

Ruby has a method in Module#module_function which takes one or more
method names of module instance methods and copies them as module
methods. The Math module effectively does just that to make it's
methods available either as methods of the Math object, or as instance
methods when the Math module is included.

As an interesting sidenote, although Class is a subclass of Module,
you can't use module_function in a class to copy instance methods to
class methods, the MRI implementation undefines the method in Class.