From: David Masover <ninja@...>
Date: 2009-12-04T07:54:16+09:00
Subject: Re: Variable references in yaml files

On Thursday 03 December 2009 03:08:03 pm dara wrote:
> The contents of sampl.yml are:
> :first: 'John'
> :second: 'Jane'
> :third: 'Jack'
> :fourth: "#{fourth_member}"

In other words, you have a local variable fourth_member which you want to 
appear there?

> Can anyone
> either:
> 1) Show me a way to do this (much preferred ;) ).
> 2) Confirm this can't be done (would at least save me wasting further
> time on it).

I can do both.

I doubt very much that Yaml itself supports this, and it would be very 
dangerous and scary if it does.

However, you could easily do something like this yourself. The quick-and-
dirty, dangerous way would probably go something like this:

hash.each_pair do |key, value|
  hash[key] = eval "\"#{value}\""
end

It should be obvious why this is dangerous, though -- that yaml file can now 
contain arbitrary code, probably not what you want. But you get the idea -- 
it's easy enough to build some kind of template system. Here's a safer way -- 
first, don't use a local variable, use a hash of variables you want to make 
available to the script:

yaml_variables = {:fourth_member => 'Joan'}

Then it's a simple matter of substitution:

hash.each_value do |string|
  yaml_variables.each_pair do |key, value|
    string.gsub! "\#{#{key}}", value
  end
end

Note that you're not constrained to the #{} syntax. You can make up your own.

This still has some flaws -- for example, if you have something like this:

yaml_variables = {:foo => '#{bar}', :bar => 'baz'}

Depending what order you iterate through the yaml_variables hash, you might 
get either the string '#{bar}' (probably what you wanted), or the string 
'baz', replacing any occurrence of '#{foo}'.

There are other problems -- I'm assuming your yaml file is a single, flat hash 
-- but I'm sure you can come up with something better.

Also, it might be helpful to know what you're actually doing with this. It's 
quite possible you don't need anything nearly this complex, and Yaml does have 
some substitution of its own that might be handy -- though that's within a 
file, it doesn't pull values out of your script. Also, if you're just trying 
to define a default value, leave the value nil in the Yaml file, and merge it 
into a hash of default values.