From: "wardrop (Tom Wardrop)" <tom@...>
Date: 2013-04-04T09:28:14+09:00
Subject: [ruby-core:53975] [ruby-trunk - Feature #8191] Short-hand syntax for duck-typing


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


=begin
I think whatever the solution, it needs to be generic. Amending the language is a pretty big deal. Whatever we implement needs to have many potential applications. The nice thing about the double question-mark is it is generic. It's a "access method or name only if it exists, otherwise nil". It can be used for tentative method chaining, safely accessing potentially non-existant local variables, dealing with inconsistant API's, etc. That's what I like about it. I wouldn't want a solution that only addressed the method chaining problem for example.

Personally, the double-question mark syntax is still the best proposal. The question marks do pretty well to indicate the uncertainty of the operation, where ase using ((|&|)) is potentially confusing because of logical && and bitwise &.
=end

----------------------------------------
Feature #8191: Short-hand syntax for duck-typing
https://bugs.ruby-lang.org/issues/8191#change-38193

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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