From: Robert Klemme <bob.news@...>
Date: 2004-05-01T20:54:01+09:00
Subject: Re: def [](v) xx; return yy; end # returned value is ignored !?


"Jean-Hugues ROBERT" <jean_hugues_robert@yahoo.com> schrieb im Newsbeitrag
news:6.0.1.1.0.20040430224610.01cde668@pop.mail.yahoo.com...
> At 05:40 01/05/2004 +0900, you wrote:
>
> >On Apr 30, 2004, at 11:58 AM, Jean-Hugues ROBERT wrote:
> >
> >>At 01:49 01/05/2004 +0900, you wrote:
> >>>"Jean-Hugues ROBERT" <jean_hugues_robert@yahoo.com> schrieb im
Newsbeitrag
> >>>news:6.0.1.1.0.20040430083010.01ce4ab8@pop.mail.yahoo.com...
> >>> > Hi,
> >>> >
> >>> > For some class I am redefining operator []=(x). Works great.
> >>> >
> >>> > However when I use it, p xx[] = zz prints zz, not the value
> >>> > returned by my method. It makes some sense yet I would have
> >>> > thought that the responsibility for that should belong to
> >>> > my method. Am I missing something ?
> >>>
> >>>Yes, it's defined that way (see below).
> >>
> >>Humm, things are the way they are. When I get annoyed by my
> >>kinds questions I end up answering "that's because it's been
> >>that way for billion years". I am there quoting the E.T. answer
> >>in movie Contact by Robert Zemeckis.
> >
> >It wasn't always that way. Calling "Foo.bar = value" used to respect
> >#bar='s return value. Later, it was decided by matz that consistancy was
> >best, and the behavior was changed to be consistant with the behavior of
> >the normal assignment operator.
> >
> >One reason for this is that people would tend to expect this behavior,
and
> >perhaps not think about return values when writing the functions. So the
> >following code might not do what is expected.
> >
> >class Foo
> >   def bar=(val)
> >     @bar = val.to_i
> >   end
> >end
> >
> >f = Foo.new
> >g = f.bar = h = "23"
> >
> >In this case, you would probably expect g, h and f.bar to have been
> >assigned the same way. If the return value of #bar= is respected,
however,
> >h and g would have different values.
> >
> >cheers,
> >--Mark
>
> That makes sense. Yet, I am missing the xxx= where I do control the value
> assigned to the lvalue.

Well, you can get close by using the same approach that OpenStruct uses: you
need an instance to host all variables and then you can fully control the
behavior of assignment.  A simple example:

class Scope
  def method_missing(sym, *args)
    str = sym.to_s

    case str
      when /^(.+)=$/
        define_member($1)
      else
        raise ArgumentError unless args.empty?
        define_member(str)
    end

    send(sym, *args)
  end

  def define_member(name)
    eval "def self.#{name}=(x);@#{name}=x;end"
    eval "def self.#{name}();@#{name};end"
  end
end


irb(main):058:0> env = Scope.new
=> #<Scope:0x101725e8>
irb(main):059:0> env.x
=> nil
irb(main):060:0> env.x=100
=> 100
irb(main):061:0> env.y="foo"
=> "foo"
irb(main):062:0> env.y
=> "foo"
irb(main):063:0> env
=> #<Scope:0x101725e8 @x=100, @y="foo">

Now you can modify those methods that are defined as accessors in
define_member() in any way you like and thus control the value assigned.
It's not the full integration into the language that you are looking for,
but it might come close enough.

The problem is that to achieve what you want the language would need to be
radically changed and I don't think it's a good idea to allow for global
redefinition of =.  That could wreak too much havoc on existing code.

Btw, by adding:

class Scope
  def with(&b)
    instance_eval &b
  end
end

You can use variables that you have defined already quite naturally:

irb(main):034:0> env.x=10
=> 10
irb(main):035:0> env.with { p x; x = 20; p x; p self }#
10
20
#<Scope:0x10182e60 @x=10>
=> nil

> I am not alone. At this point there are multiple known cases where it
would
> be needed for transparency:
>    - Reference, a reference is an indirect mean of access to a lvalue's
> value that is dereferenced when the value is needed and that also makes it
> possible to assign a value to the lvalue at any time.
>    - Lazy, a Lazy is a value that is computed when it is needed, not
before.

You don't necessarily need references for that.  Typically it is used as a
member and you can do it this way:

class Foo
  def lazy=(x);@lazy=x;end
  def lazy
    if @lazy.nil?
       @lazy = compute_lazy()
    end
    @lazy
  end
end

You can even automate that by providing blocks that do the initialization
calculation.

>    - Future, a Future is the returned value of an asynchronous method call
> that is waited for when it is needed, not before.
>    - LogicVariable, a LogicVariable is a variable that can be free in
> addition to being bound to some value as regular variables are.

IMHO there are lots of ways to do what you want in Ruby, only the exact way
you want is not possible.  Sometimes it's better to step back and open one's
mind for other solutions.  As I suggested before, a Hash can pretty much do
what you want - it just doesn't give you the exact syntactic sugar you want.

env = Hash.new do |h,key|
  # do whatever is needed to calculate the value of key
end

env[ :x ] = true
env[ :y ] = false
env[ :x ] && env[ :y ]
....

Or, with the example above:

env.x=true
env.y=false
env.x && env.y
....

Regards

    robert