[ACCEPTED]-Instance methods in modules-instance
Think about what
aux is. What object will respond 19 to
aux? It's an instance method, which means 18 that instances of classes that include ModName 17 will respond to it. The ModName module itself 16 is not an instance of such a class. This 15 would also not work if you had defined ModName 14 as a class — you can't call an instance 13 method without an instance.
Modules are very 12 much like classes that can be mixed into 11 other classes to add behavior. When a class 10 mixes in a module, all of the module's instance 9 methods become instance methods of the class. It's 8 a way of implementing multiple inheritance.
They 7 also serve as substitutes for namespaces, since 6 each module defines a namespace. But that's 5 somewhat unrelated. (Incidentally, classes 4 also have their own namespaces, but making 3 it a class implies that you'll create instances 2 of it, so they're conceptually wrong for 1 that purpose.)
You can do it this way:
module ModName def aux puts 'aux' end module_function :aux end
and then just call 1 it:
You can also say
module ModName extend self def aux puts 'aux' end end
Then you can include the 2 module normally, but also call methods via 1 ModName.
the module definition evaluates to 'this 34 is...', why?
In Ruby, everything is an expression, there 33 are no statements or declarations. Which 32 means that everyhing evaluates to a value. (However, this 31 doesn't necessarily mean that it evaluates 30 to a useful value. For example, the
puts method always 29 evaluates to
nil, just like a
def method definition 28 expression (except in Rubinius, where the 27
def expression evaluates to a
CompiledMethod object for the 26 method being defined).)
So, if everything 25 evaluates to a value, what should a module 24 definition expression evaluate to? Well, there 23 are a couple of candidates: it could evaluate 22 to
nil, just like a method definition expression. Or 21 it could evaluate to the module object itself, after 20 all, this is what we are defining, right? But 19 actually, Matz chose a third option: module 18 (and class) definition expressions actually 17 evaluate to whatever the last expression 16 inside the module definition body evaluates 15 to. (This allows you to easily simulate 14 the other two possibilities by just putting 13
self as the last expression inside a module 12 definition body.)
In your case, the last 11 expression inside the module definition 10 body is an assignment. But, an assignment? What 9 the heck does that return? Isn't that a statement? No, not 8 in Ruby. Everything is an expression, and assignments 7 are no exception: assignment expressions 6 evaluate to whatever the right hand side 5 evaluates to. Here, the right hand side 4 of the assignment expression is a string 3 literal, which evaluates to a string object.
So, the 2 entire module definition expression evaluates 1 to the string
'this is a const in module'.
And a little more...
module Y def Y.a puts "a" end def Y.b c end def self.c puts "b -- c" end end
call (without '.new'):
Y.a #=> "a" Y.b #=> "b -- c" Y.c #=> "b -- c"
class Foo include ModName end Foo.new.aux # output: aux
More Related questions