Issue #8237 has been updated by phluid61 (Matthew Kerwin).


phluid61 (Matthew Kerwin) wrote:
> I find myself drifting more to the -1 side for this feature.
> 
> 
> All that said, I quite like the idea of a magic variable that holds "the value of the last evaluated expression", in lexical order.  Rather than _ I'll use $% in some examples:
> 
>   # Rodrigo will hate this one, but I don't care:
>   a && $%.b && $%.c && $%.d
>   
>   foo = 1
>   bar = ->{ foo + 1 }
>   baz = ->{ $% + 1 }
>   foo = 99
>   bar[] # => 100
>   baz[] # => 2
> 
> However I'm not entirely convinced of its widespread usefulness.

Apologies for replying to myself, but I've just convinced myself against this whole proposal.  In the above code I originally had the baz lambda before the bar, but decided to swap them at the last minute.  Only now has it occurred to me that this will break the code, because $% in baz will actually refer to the 'bar' lambda object.  No matter what syntax is used, I'm convinced now that it will always be better to just create a local (explicitly named) variable.

  (x=a) && (x=x.b) && (x=x.c) && x.d
  !(x=a).nil? && !(x=x.b).nil? && !(x=x.c).nil? && x.d
  type.tap{|x| return x if x =~ /^[a-z]/ }

These all work even if #a, #b, #c, #d, and #type have side-effects.

Sorry for the noise.
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Feature #8237: Logical method chaining via inferred receiver
https://bugs.ruby-lang.org/issues/8237#change-38865

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


=begin
This is a feature suggestion that was raised while discussing issue #8191. The feature suggestion is to introduce some form of logical method chaining to address this reasonably common pattern:

    user && user.profile && user.profile.website && user.profile.website.thumbnail

It would be reasonably trivial to shorten this to:

    user && .profile && .website && .thumbnail

The implementation I propose would be for Ruby to allow an inferred receiver; the dot prefix would be the syntax for this. The inferred receiver would resolve to the result of the last expression in the current scope. For illustrative purposes, the following would work under this proposal:

    "some string"
    puts .upcase #=> SOME STRING

Another example:

    puts .upcase if obj.success_message || obj.error_message

    # Instead of...

    message = (obj.success_message || obj.error_message)
    puts message.upcase if message

This can also potentially provide an alternative option in syntactically awkward scenario's, such as dealing with the return value of an if statement or a catch block, avoiding the need for temporary variable assignment:

    catch :halt do
      # Do something
    end

    if .nil?
       log.info "Request was halted" 
       response.body = "Sorry, but your request could not be completed"
    end

The logical chaining scenario is the main use case however. I just wanted to demonstrate how the proposed implementation could also be used in other creative ways.

=end


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