From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2001-10-28T13:39:23+09:00
Subject: [ruby-talk:23665] Re: How to get a String to interpolate itself?

In article <3BDB73A2.7B1C14FF@path.berkeley.edu>,
 <vjoel@PATH.Berkeley.EDU> wrote:
>Phil Tomson wrote:
>> 
>> I'm trying to read lines from a file that might contain something like:
>> 
>> Here is a line with a variable to interpolate: #{var}
>> 
>> Now when I read it in like:
>> 
>>   var = 99
>>   File.foreach("junk.rbt") {|line|
>>     puts line
>>   }
>> 
>> of course I get the same line in the file with the #{var} in it.
>> I'd like to get:
>> Here is a line with a variable to interpolate: 99
>> 
>> I suspect there must be a way to do this since using "" causes
>> interpolation, how can I get an existing String to interpolate itself?
>> 
>> Phil
>
>How's this?
>
> var = 3
> str = 'This is the value of var: #{var}'
> puts eval "\"#{str}\""
> # ==> This is the value of var: 3
>
> # also:
> puts eval '"' + str + '"'
> # ==> This is the value of var: 3
>

It works, but it seems a bit kludgy.  Certainly not very OO.  It seems to 
me that we need a String#interpolate method.
str.interpolate => "This is the value of var: 3"

Speaking of string interpolation and double quotes....
When you do:
"this is a string".type => String

Which means that double quotes are kind of constructor for strings - with 
the added functionality of interpolating Ruby expressions which are between 
'#{' and '}'.  Where is this functionality of double quotes defined (as in 
which class is it defined in?)?  Is it possible that String#interpolate 
could somehow call this already-existing functionality instead of using 
the eval trick shown above?  Are double quotes actually a method on a 
class or something built-in at a deeper level?

Phil