From: "duerst (Martin Dürst)" <duerst@...>
Date: 2013-03-31T17:56:09+09:00
Subject: [ruby-core:53851] [ruby-trunk - Feature #8191] Short-hand syntax for duck-typing


Issue #8191 has been updated by duerst (Martin D��rst).


You call something like

    obj.respond_to? :empty ? obj.empty : nil

"achieving duck typing", but that's not how duck typing works. If you mix all kinds of unrelated stuff in your program and then have to check whether they implement a message, you should probably think better about how to organize this stuff. On the other hand, if you have objects that might be empty in one way or another, but currently are not, then you should add an empty? (including the question mark) method to the respective class. In the extreme, that could go as far as doing:

    class Object
      def empty()  nil;  end
    end

So in my book, if you have lots of statements like the above in your code, you either haven't understood duck typing yet, or you are not using it well. Of course, we would have to look at actual code samples to find out what should be changed.
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Feature #8191: Short-hand syntax for duck-typing
https://bugs.ruby-lang.org/issues/8191#change-38061

Author: wardrop (Tom Wardrop)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


=begin
As a duck-typed language, Ruby doesn't provide any succinct way of safely calling a potentially non-existant method. I often find myself doing (({obj.respond_to? :empty ? obj.empty : nil})), or if I'm feeling lazy, (({obj.empty? rescue nil})). Surely we can provide a less repetitive way of achieving duck-typing, e.g. I don't care what object you are, but if you (the object) can't tell me whether you're empty, I'm going to assume some value, or do something else instead.

I'm not sure what the best way to implement this is. The easiest would be to just define a new conditional send method:

    obj.send_if(:empty?, *args) { nil }

    obj.try(:empty?, *args) { nil }

But that's really not much of an improvement; it's ugly. Preferably, it'd be nice to build it into the language given how fundamental duck-typing is to Ruby. One potential syntax is:

   obj.empty? otherwise nil

The ((|otherwise|)) keyword would be like a logical or, but instead of short-circuiting on true, it short-circuits on some other condition. That condition can be one of two things. It can either wait for a NoMethodError (like an implicit (({rescue NoMethodError}))), proceeding to the next expression if one is raised, or it can do a pre-test using (({respond_to?})). Each option has its pro's and con's.

The implicit rescue allows you to include expressions, e.g. 

    obj.empty? otherwise obj.length == 0 otherwise true

Going with the implicit (({respond_to?})) implementation probably wouldn't allow that. You'd instead need to limit it just to method calls, which is not as useful. The only problem with implicitly rescuing NoMethodError's though, is that you'd need to ensure the NoMethodError was raised within the target object, and not some dependancy, as you could potentially swallow valid exceptions.

The benefit of this over current methods of duck-typing, is that you're not testing a condition, then running an action, you're instead doing both at the same time making it much more DRY.

One other potential syntax however is a double question mark, or question mark prefix. This could act as an implicit (({respond_to?})) pre-condition, returning nil if the method doesn't exist.

    obj.empty??? || obj.length?? == 0 || nil

    obj.?empty? || obj.?length == 0 || nil

I'm not completely satisfied with either syntax, so at this point I'm merely hoping to start a discussion. 

Thoughts?
=end


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