From: Gavin Sinclair <gsinclair@...>
Date: 2002-08-05T00:20:41+09:00
Subject: Implicit local variables for do...end blocks

In the thread "Should this work?", I proposed that :collect should be able
to operate without an explicit block parameter.  For example:

["john", "mary"].map { |name| name.capitalize }
  CAN BE EXPRESSED AS
["john", "mary"].map { $_.capitalize }   # -> ["john", "mary"]


[1, 2, 3, 4].map { |i| i ** 2 }          # -> [1, 4, 9, 16]
  CAN BE EXPRESSED AS
[1, 2, 3, 4].map { $_ ** 2 }             # -> [1, 4, 9, 16]


Guy Decoux pointed out that the blocks are independent of the method calling
them, so the above could only be done by allowing implicit assignment of the
*local* variable $_ in *every* "do ... end" block.

Is this agreeable?

I would usually continue to name my parameters explicitly, just to make it
clear what's being operated on (useful in a dynaically typed language!).
However, the motivation for my proposal is very simple cases like the
following:

   input = ARGV.map { |s| s.to_i }

And, of course, this whole discussion was prompted by such constructs as:

   [x, y, z].each { |arr| arr << nil }
   [x, z, z].map  { |arr| arr == [nil] }

The above three examples would be more concisely written as:

   input = ARGV.map { $_.to_i }

   [x, y, z].each { $_ << nil }

   [x, y, z].map { $_ == [nil] }

Whether this terseness is useful is for discussion.

Cheers,
Gavin

--
Gavin Sinclair                                       Software Engineer
Sydney, Australia                            Soyabean Software Pty Ltd