[ruby-core:54003] [ruby-trunk - Feature #8191] Short-hand syntax for duck-typing

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


=begin
rosenfeld (Rodrigo Rosenfeld Rosas) wrote:
 > I don't believe this proposal, as I understand it, is about chicken
 > typing. It would be if a??.b was equivalent to a.respond_to?(:b) ? a.b :
 > nil. But, as I understand, the proposal is about a.nil? ? nil : a.b.

This really needs to be cleared up once and for all.  Are we proposing a chicken-typing syntax, or an andand syntax?  The OP never made it clear.

Assuming chicken-typing is Bad(tm), my proposal is one or either of the following (pending discussion):

1. conditional method invocation
  a.?b.?c.?d
  # (tmp2 = ((tmp1 = ((tmp0=a) && a.b) && tmp0.c) && tmp1.d)
  # (tmp2 = (tmp1 = (tmp0 = a.nil? nil : a.b).nil? nil : tmp0.c).nil? nil : tmp1.d)

This means that in (({foo.?bar.inspect})), the (({foo.bar})) call may work or not, but (({.inspect})) will always be called on the result, possibly resulting in (({"nil"})).  There remains a question of whether or not ((|false|)) is considered "valid."

Essentially (({.?})) becomes a pretty syntax for (({send_if})).

2. logic short-circuit
  a&&.b&&.c&&.d
  # (tmp0=a) && (tmp1=tmp0.b) && (tmp2=tmp1.c) && tmp2.d

This means that in (({foo&&.bar.inspect})) we'll either get the inspected form of (({foo.bar})), or ((|nil|)).

I believe both have their uses.

==== Incidentally

 > In CoffeeScript, something that helps a lot and I'd love to see in Ruby
 > is that it allows us to declare a variable in a post-if and use that
 > variable in the statement:
 > 
 > console.log a if a = b.length

This already works, as long as ((|a|)) is already defined in scope as a variable:

 irb(main):001:0> a = nil
 irb(main):002:0> b = 'abc'
 irb(main):003:0> puts a if a = b[1]
 b
 => nil
 irb(main):004:0> b = ''
 irb(main):005:0> puts a if a = b[1]
 => nil

=end

----------------------------------------
Feature #8191: Short-hand syntax for duck-typing
https://bugs.ruby-lang.org/issues/8191#change-38237

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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