From: Bill Kelly <billk@...>
Date: 2002-12-27T20:46:35+09:00
Subject: Re: viral arguments

Hi,

From: "Tom Sawyer" <transami@transami.net>
>
> the idea is that rather then pass simple argumments, one could pass a little 
> piece of code that would execute in the context of the method.
> 
> here's a foo example (vproc is like proc but viral):
> 
> def foo(x)
> i = 1
> puts "#{x + i}"
> end
> 
> puts foo(4) --> 5
> puts foo(vproc {i + 1}) --> 3
> 
> any thoughts? can ruby achieve something like this already?

I did something that sounds a little like that the other day
when making a web site for my girlfriend ( http://zip.to/beadyourselfup :)

#
# Adding a new instance method to String class which "re evaluates"
# the string in the context of the Kernel::binding object we pass in.
# (Note that the here-document approach this uses tacks on an extra
# \n to the resulting string.... but that was fine for my purposes.)
#
class String
  def reeval(b = TOPLEVEL_BINDING)
    eval(%Q{<<"END_REEVAL"\n} + self + "\nEND_REEVAL\n", b)
  end
end

class BeadShot

  # ...

  def gen_fullsize_html(prev_shot, next_shot)
    #
    # Here, a string containing the HTML template for the
    # page we're generating is "reevaluated" in the current
    # context - so it has access (via interpolation #{ }) to
    # the current BeadShot instance's methods, and even the
    # local variables in scope at the moment (prev_shot,
    # next_shot).
    #
    html = $bead_fullsize_page.reeval(binding)
  end

  def item_number
    @file_num + $item_number_offset
  end

  # ... etc.

end

#
# Here's an excerpt from the string reevaluated by the code
# above.  Note that I wanted interpolation to occur both when
# the string was defined - to pull in other components of the
# page like $header_html - and later when re-evaluated; so
# the interpolation that's deferred for "reeval" is escaped
# like \#{ } .
#

$bead_fullsize_page = <<"END_HTML"
#$header_html
<body>
<table>
  <tr align="right">
    <td><font class="item">item # \#{ item_number }</font></td>
  </tr>
  <tr align="center">
    <td> \#{ gen_fullsize_inline_img(1) } </td>
  </tr>
  <tr align="center">
    <td> \#{ prev_shot ? prev_shot.gen_link("previous", 1) : "" }
         \#{ (prev_shot && next_shot)? $sep_bar : "" }
         \#{ next_shot ? next_shot.gen_link("next", 1)     : "" }
    </td>
  </tr>
  <tr> <td> &nbsp; </td> </tr>
  <tr align="center">
    <td> <a href="../index.html">... back to home page ...</a> </td>
  </tr>
</table>
</body>
</html>
END_HTML


An example of the resulting HTML can be seen at the bottom of:
http://www.foosenblat.com/beadyourselfup/bead_html/bead00057.html


[Flames about anything stupid I may have done in the HTML that 
aren't Ruby-related should probably be sent to me off-list. <grin> ]


Anyway I wondered if this might be similar to what you had in
mind with "viral arguments". ?


Regards,

Bill