On 9/29/2015 12:35 PM, Sven R. Kunze wrote:
On 29.09.2015 02:38, Terry Reedy wrote:
On 9/28/2015 5:48 PM, Luciano Ramalho wrote:
Glyph tweeted yesterday that everyone should watch the "Nothing is Something" 35' talk by Sandi Metz at RailsConf 2015. It's great and, in a way, relevant to this discussion.
I understood Metz as advocation avoidig the nil (None) problem by giving every class an 'active nothing' that has the methods of the class. We do that for most builtin classes -- 0, (), {}, etc. She also used the identity function with a particular signature in various roles.
I might stress here that nobody said there's a single "active nothing".
Ruby's nil and Python's None are passibe nothings. Any operation other than those inherited from Object raise an exception.
There are far more "special case objects" (as Robert C. Martin calls it) than 0, (), {}, etc.
Metz's point is that there is potentially one for most classes than one might write. Some people have wondered why Python does not come with a builtin identity function. The answer has been that one is not needed much and and it is easy to create one. Metz's answer is that they are very useful for generalizing classes. But she also at least implied that they should be specific to each situation. Certainly in Python, if code were to check signature, and even type annotation, then a matching id function would be needed.
I fear, however, the stdlib cannot account for every special case object possible.
Right. It is not possible to create a null instance of a class that does not yet exist.
Without None available in the first place,
The problem of a general object is that it is general. It should either be a ghost that does nothing, as with None, or a borg than does everything, as with the Bottom of some languages.
users would be forced to create their domain-specific special case objects.
Metz recomends doing this voluntarily ;-) perhaps after an initial prototype.
None being available though, people need to be taught to avoid it, which btw. she did a really good job of.
I think None works really well as the always-returned value for functions that are really procedures. The problem comes with returning something or None, versus something or raise, or something or null of the class of something. -- Terry Jan Reedy