From: Navindra Umanee <navindra@...>
Date: 2005-07-16T17:23:52+09:00
Subject: delaying string evaluation

Hi,

I would like to partially expand a string and then further expand it
later on when more information is available.  I am currently using
string interpolation with a reeval method suggested by Bill Kelly.

class String
  def reeval(b = TOPLEVEL_BINDING)
    eval("<<END_REEVAL\n" + self + "\nEND_REEVAL\n", b).chomp!
  end
end

This lets me do stuff like this:

expanded = "one"
delayed = "two"

s = "#{expanded} \#{delayed}"       # => "one #{delayed}"
s = s.reeval(binding)               # => "one two"

As a more complex example, this is a string I wish to partially expand:

options = [ 10,  20,  30,  40,  50 ]

combobox =<<"END"
...
#{options.map {|n|
%{<option value="#{n}"\#{thread_threshold == n ?  ' SELECTED' : ''}>#{n} comments</option>}
}.join("\n")
}
...
END

The idea is to create the combobox and then later insert the SELECTED
part, when I know the value of thread_threshold.  Unfortunately, the
above partial expansion is:

<option value="10"#{thread_threshold == n ?  ' SELECTED' : ''}>10 comments</option>
<option value="20"#{thread_threshold == n ?  ' SELECTED' : ''}>20 comments</option>
<option value="30"#{thread_threshold == n ?  ' SELECTED' : ''}>30 comments</option>
<option value="40"#{thread_threshold == n ?  ' SELECTED' : ''}>40 comments</option>
<option value="50"#{thread_threshold == n ?  ' SELECTED' : ''}>50 comments</option>

This would have been great, except that I have all these values for
'n' that have been left out of the expressions that I want to evaluate
afterwards.  That is, I want the partial expansion to be this:

<option value="10"#{thread_threshold == 10 ?  ' SELECTED' : ''}>10 comments</option>
<option value="20"#{thread_threshold == 20 ?  ' SELECTED' : ''}>20 comments</option>
<option value="30"#{thread_threshold == 30 ?  ' SELECTED' : ''}>30 comments</option>
<option value="40"#{thread_threshold == 40 ?  ' SELECTED' : ''}>40 comments</option>
<option value="50"#{thread_threshold == 50 ?  ' SELECTED' : ''}>50 comments</option>

and later when I reeval the string:

thread_threshold = 50
combobox.reeval(binding)

the result should be:

<option value="10">10 comments</option>
<option value="20">20 comments</option>
<option value="30">30 comments</option>
<option value="40">40 comments</option>
<option value="50" SELECTED>50 comments</option>

Anyone have any ideas for how I can get a correct partial expansion?

I have a lambda/closure feeling about how this should work, but I
can't quite see the solution in the context of strings.

Thanks,
Navin.