From: dara <dara.lillis@...>
Date: 2009-12-04T08:41:30+09:00
Subject: Re: Variable references in yaml files

Actually, thinking just a little further, your suggestion of merging
hashes is probably the way to go. Similar to below, but instead of the
convoluted substitution hash, I can just merge a hash of on-the-fly
expected results with the hard-coded ones.

So the solution doesn't really scratch my itch of hoping to keep all
the expected results in one place, but I think I straightened out my
thinking and learned something in the process. Thanks!

On Dec 3, 3:54 pm, David Masover <ni...@slaphack.com> wrote:
> 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.