From: tho_mica_l <micathom@...>
Date: 2008-01-07T01:34:58+09:00
Subject: Re: Programmatically creating a local variable using a symol

> Why do you want to do this with a local variable?

First, I'd like to know a little bit more about ruby. And I would
have
found such a thing useful once or twice in the past.

Secondly (the background for my question), I'd like to implement some
ocaml'ish match command (for the moment, as some kind of exercise
only).
I'm not sure yet how this could best be done but I thought it would
be
nice if it could look somewhat like this:

    val = Foo(1, 2)

    case val
    when Foo(:x, :y)
        a = x * y
    when Bar(:x)
        a = x
    end

But this form would require the variables to be set from Foo#=== which
I
don't think is possible anyway.

What I managed to do is this:

    class Matchtype
      class << self
          def match(other, &block)
              if other.class == self
                  block.call(*other.value)
                  true
              else
                  false
              end
          end
      end

      attr_reader :value

      def initialize(*value)
          @value = value
      end

    end

Sample use:

    class Foo < Matchtype
    end

    class Bar < Matchtype
    end

    x = Foo.new(1, 2)
    y = Bar.new(1)

    if Foo.match(x) {|a, b| p "Foo", a + b}
    elsif Bar.match(x) {|a| p "Bar", a}
    end

    if Foo.match(y) {|a, b| p "Foo", a + b}
    elsif Bar.match(y) {|a| p "Bar", a}
    end

But this is quite useless the way it is because the value of the
block
cannot be used and because it's ugly and ...

One could of course do it this way which would come close:

    class Boo < Array
    end

    class Baa < Array
    end

    z = Boo[1, 2]
    case z
    when Baa
        p "Nope"
    when Boo
        a, b = z
        p "Boo", a + b
    end

It would be nice being able to eliminate the "a, b = z" line though.

Ideally, one would also be able to do something like this:

    z = Boo[1, :a]
    case z
    when Boo(0, '')
        p "Nope"
    when Boo(1, :a)
        p "Boo", a
    end

Any ideas are welcome.

Regards,
Thomas.