From: "wardrop (Tom Wardrop)" <tom@...>
Date: 2013-04-08T14:32:39+09:00
Subject: [ruby-core:54096] [ruby-trunk - Feature #8191] Short-hand syntax for duck-typing


Issue #8191 has been updated by wardrop (Tom Wardrop).


I guess we've determined that Ruby doesn't provide any fool proof means of detecting whether an object will or should respond to a method; any attempt to do so currently results in unreliable and potentially hazardous chicken-typing. This request has therefore shifted focus to logical method chaining which at least addresses part of the problem.

I've raised issue #8237 to as an official proposal for logical method chaining.

I still feel that there should be some means to safely call a potentially non-existant method on an object. As we know, objects can be extended as singletons, so really, there needs to be a way to know whether some object has some method. The problem is that you can't know until you execute the method. An implicit catch of a NoMethodError is the only means you can tell whether a method call was successful or not.

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Feature #8191: Short-hand syntax for duck-typing
https://bugs.ruby-lang.org/issues/8191#change-38348

Author: wardrop (Tom Wardrop)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
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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