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GLP-00004: All methods are virtual
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:author: Yusuke Tsutsumi
:created_date: 2014-05-24
:updated_date: 2014-05-24
:approval: review
:approvers: yusuke
:status: unimplemented

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Proposed Implementation
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Some languages require a specific keyword to make methods
'virtual'. Virtual methods are methods who, regardless of casting,
will call their actual class's method instead.

This GLP proposes to make all methods virtual. If someone wants to
call a specific class's method on an object, that can be done with::

    apply(BaseClass.method, object)(*args)


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Implementation in other languages
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Other languages deal with 'virtual' methods with:

* C++: an explicit keyword 'virtual' on a method
* Java: methods are virtual, with the exception of the keywords 'static' and 'final'
* Python: methods are virtual by nature of the class model


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Further Discussion
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I think that explicitely allowing certain methods as virtual and
others as non-virtual (as c++ does) can be confusing, and not
particularly benifical. Virtual methods can come in handy with heavy
reliance on casting, but I think from a consumer of an api or class,
it requires special attention to understand what's going on when there
is a mix of virtual/non-virtual methods being called.

Using explicitly different syntax to denote that distinction, however,
is a better decision.