[ruby-core:53982] [ruby-trunk - Feature #8191] Short-hand syntax for duck-typing

Issue #8191 has been updated by phluid61 (Matthew Kerwin).


=begin
wardrop (Tom Wardrop) wrote:
 > Personally, the double question mark syntax is still the best proposal.
 > The question marks do pretty well to indicate the uncertainty of the
 > operation, [...]

I agree, but it seems a little bit teenage-girl to me (omg?? really???).  I prefer (({obj.?method})) because:
(1) not too much punctuation (compare (({foo.?empty?})) vs (({foo.empty???})), or (({bar.?upcase!})) vs (({bar.upcase!??})))
(2) it makes (({.?})) a special operator, which is like a questionable version of (({.}))
(3) putting the question-mark before the method name gives it the same syntactic (and semantic) order as (({obj && obj.method})), rather than (({obj.method rescue nil})) (i.e. it suggests a proactive check, rather than a reactive recovery)

I still have an issue with the semantics of chaining.  Should (({a.?b.?c})) or however you want to write it be parsed like chained ternary operations or like chained method calls? And what about side-effects?  I.e. should it be equivalent to
 a.respond_to? :b ? (a.b.respond_to :c ? a.b.c : nil) : nil
or
 a.respond_to? :b ? ((tmp = a.b).respond_to? c : tmp.c : nil) : nil
(being a side-effect-limiting version of the above), or
 (tmp = a.respond_to? :b ? a.b : nil).respond_to? :c ? tmp.c : nil

I've always imagined the third, but some other comments in this thread suggest people have other ideas.  It's the only one that behaves like the send_if method in the original proposal.
=end

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Feature #8191: Short-hand syntax for duck-typing
https://bugs.ruby-lang.org/issues/8191#change-38203

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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