[ruby-core:25192] [Feature #2013] [PATCH] a = *b calls b.*@

Feature #2013: [PATCH] a = *b calls b.*@
http://redmine.ruby-lang.org/issues/show/2013

Author: Jeremy Evans
Status: Open, Priority: Normal

This makes the * operator operate more similarly to the + and - operators.  The binary versions of + and - call methods named + and -, while the unary versions call methods named +@ and -@.  The binary * operator calls the method named *, but the unary * operator calls the method to_a currently, and used to call to_ary in 1.8.6 (and at some point may have called to_splat).

I think this makes for more consistent behavior, and hopefully it isn't just a foolish consistency.  I brought up this idea as a question in Shugo Maeda's presentation at RubyKaigi 2009, and discussed it with Matz at Lone Star Ruby Conf 2009, and he thought the idea had merit.

Here's a basic example how this would look:

  class MultiplePersonality
    def *@
      [self, [self, self]]
    end
  end

  mp = MultiplePersonality.new
  p1, mp2 = *mp

This comes with a patch that appears to work from my simple testing, but this is my first time working with the internal ruby code, so I apologize in advance if it doesn't do things correctly.

The patch modifies parse.y so the above is no longer a syntax error.  It adds a USTAR token to the parser to represent that *@ token.  It modifies the splatting to call *@ instead of to_a.  Also, for backwards compatibility, it adds *@ to BasicObject, and has it call to_a if it responds to to_a.  This allows code that defines to_a and expects that the unary * operator will call to_a to still work.  

This patch is mostly for consistency, but it also allows the programmer to make to_a return one thing, and the unary * operator return something else.  I can think of the following use case:

  # Represents an abstract set of rows in the database
  class Dataset
    def to_a
      retrieve_database_rows
    end
    def *@
      [self]
    end
  end
  dataset = Dataset.new

  # Explicitly asking for an array means I want
  # an array of database rows represented by the
  # dataset.
  rows = dataset.to_a

  # If the dataset had any rows, I want to debug print
  # each row separately.  However, if it did not have
  # any rows, I want to debug print the dataset itself.
  p(*(rows.empty? ? dataset : rows))

I'd like to thank Eleanor McHugh for helping me find the key part of ruby that needed to be modified to support this (in vm_insnhelper.c).


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