= Mixins =

== Introduction ==

Provide a non inheritance based capability for shared/common behaviour.

Basically, selected traits or behaviours are described in a special Type called a 'mixin' (analogous to an interface),[[BR]]
these can then be specified on a class and are injected into that class to provide those behaviours on that class.  


Mixin behaviours can be specified using object:
  * vars
  * properties
  * methods
They do not support constructors

(For more see the [http://cobra-language.com/forums/viewtopic.php?f=4&t=306 Forum discussion thread]).

== Compared to Extensions ==

 * Extensions enable adding methods to existing classes whereas mixins only augment types that you declare.
 * Mixins can add state (object variables declared with `var`) whereas extensions cannot.
 * Mixins can add properties whereas extensions cannot.
 * Mixin members will be found at run-time via reflection whereas extension members will not.
 * Mixins create new types. From a VirtualMachine perspective, they are seen as interfaces (since the popular VMs don't support mixins).
 * Mixins cannot be used from a [wiki:BinaryLibrary binary library]. The source must be included for the compiler.

In summary, mixins are more about implementing shared capabilities among the types you are declaring. [[BR]]
Extensions are about adding convenience methods to existing types (regardless of who declared them).

== Restrictions ==

As we gain more experience with using mixins, and as the implementation of mixins matures, some of the following restrictions are likely to be removed:

 * Mixins cannot declare initializers.
 * Mixins cannot inherit from other types.
 * Mixins cannot implement interfaces.
 * Mixins cannot add other mixins.
 * Mixins cannot declare generic parameters (see [http://cobra-language.com/forums/viewtopic.php?f=4&t=532 forum's thread]) nor being generic themselves (see [http://cobra-language.com/forums/viewtopic.php?f=4&t=533 forum's thread]).
 * Mixins cannot add invariants.
 * Two or more mixins added to a single type cannot have collision in their member names.

== Example ==
{{{
#!cobra

# The behaviour desired - a name property
mixin Named
    var _name as String?
    pro name as String
        get
	    return _name ? ''
	set
	    _name = value

   get capName
        return _name[:1].toUpper + _name[1:]
	
	
# the (otherwise empty) class with mixin added
class A adds Named	
    pass

def main is shared
    a = A()
    assert a inherits Named
    assert a.name == ''
    a.name = 'arosebyanyothername'
    assert a.name == 'arosebyanyothername'
    assert a.capName == 'Arosebyanyothername'
}}}

== See also ==

 * [http://cobra-language.com/trac/cobra/browser/cobra/trunk/Tests/150-mixins/ Test cases] in the Subversion repository
 * [http://cobra-language.com/forums/viewtopic.php?f=4&t=306 Announcement thread] on discussion forums
 * [http://cobra-language.com/forums/search.php?keywords=mixin "mixin" search] on discussion forums
 * [http://bloggingabout.net/blogs/erwyn/archive/2005/10/20/9925.aspx A blog post] by Erwyn Van Der Meer covers limitations experienced without mixins, and the various suboptimal workarounds.