From: Sean O'Halpin <sean.ohalpin@...>
Date: 2010-09-07T09:07:00+09:00
Subject: Re: How to require Ruby 1.9 from a required file?

On Mon, Sep 6, 2010 at 9:40 PM, Sarah Allen <sarah@ultrasaurus.com> wrote:
> Is there any way to require Ruby 1.9 from a required file such that I
> can trigger an error message before the main file is parsed?
>
> Suppose I'm making a DSL that depends on the very nice Ruby 1.9 hash
> syntax.  Here's an example:
>
> app.rb
>
>    $LOAD_PATH << File.dirname(__FILE__)
>    require 'my_library'
>
>    make_pie apple:2, butter:1, flour:4
>    puts "bye"
>
> in my_library I want to report an error if we're not running with 1.9,
> so I did this:
>
>    if RUBY_VERSION < "1.9.0"
>      abort <<-end_message
>        This requires Ruby 1.9
>      end_message
>    end
>
>    def make_pie options
>      return unless options.respond_to? :key
>      puts "with " + options.keys.join(',') + " we can make a great pie"
>    end
>
> It works great as long as I don't have Ruby 1.9 syntax in the main file,
> but when I do, Ruby 1.8.7 will show that error first.
>
>    $ rvm use 1.9.2-head
>    Using ruby 1.9.2 head
>    $ ruby app.rb
>    with apple,butter,flour we can make a great pie
>    bye
>    $ rvm use 1.8.7
>    Using ruby 1.8.7 p249
>    $ ruby app.rb
>    app.rb:4: syntax error, unexpected tINTEGER, expecting
> tSTRING_CONTENT or tSTRING_DBEG or tSTRING_DVAR or tSTRING_END
>    make_pie apple:2, butter:1, flour:4
>                    ^
>
> I posted the files at:  http://github.com/ultrasaurus/ruby_version_check
>
> Sure this can be solved with documentation OR changing the syntax of my
> DSL to use 1.8.7, but I was hoping for a nicer solution for end users
> (who are expected to be kids).
>
> Thanks in advance,
>
> Sarah
>

You can't catch a SyntaxError in same file in which it occurs (if you
think about it - your rescue statement has to come after the place
where the syntax error has already occurred). You ~can~ catch a
SyntaxError in eval'd or loaded code. So, instead of requiring your
library, why not use a script loader like this:

file: recipe-dsl

  #!/usr/bin/env ruby
  def make_pie(params)
    p params
  end

  begin
    load ARGV[0]
  rescue SyntaxError => e
    if RUBY_VERSION < '1.9.0'
      puts "You need ruby 1.9.x to run this script"
    else
      raise
    end
  end

which you use to run the DSL scripts like so:

  $ recipe-dsl ~/apple-pie.recipe

where apple-pie.recipe contains just your DSL statements:

file: apple-pie.recipe

  make_pie apple: 2, butter: 1, flour: 4

If you try to run this under 1.8.x, you'll get:

  $ ./recipe-dsl apple-pie.recipe
  You need ruby 1.9.x to run this script

If you really do have a syntax error in your recipe script, that will
be reported under 1.9.x.

HTH

Regards,
Sean