How to make Ruby partially parse the source code?

Question

I am trying to write a script in Ruby which interactively generate some input data for a program. The idea is use QtRuby when it exists, otherwise console is used. What I tried is to

begin
  require "Qt4"
rescue LoadError => load_err
  puts "Qt not found, using console"
end

class ConsoleDefine
  # console code
end

class QtDefine < Qt::Widget
  # GUI code
end

but the interpreter refused my code when Qt4 does not exist. is there a way to deal it similar to C++, like:

#ifdef QT4
class qt4gui 
{
    // some code
};
#else
class qt4gui
{
    // dummy
};
#endif // Qt4

Source files are just source files; did you try wrapping it in something that tries to evaluate something from the package in question? — Dave Newton
– Dave Newton, Commented Feb 11, 2013 at 19:11
The class keyword in Ruby is just an expression (this differs from languages like Java/C# where it is a declaration). As such they can be nested in other statements/expressions: if defined? Qt; class QtDefine < Qt::Widget; ..; end; end .. now, is this "good"? Well, that's what the real answers are for :D — user166390
– user166390, Commented Feb 11, 2013 at 19:12

Reactormonk · Accepted Answer · 2013-02-11 19:15:16Z

4

Use require to your advantage:

begin
  require "Qt4"
  require "my_lib/qt4"
rescue LoadError => load_err
  puts "Qt not found, using console"
  require "my_lib/console"
end

Create the two files:

# my_lib/console.rb
class ConsoleDefine
  # console code
end

# my_lib/qt4.rb
class QtDefine < Qt::Widget
  # GUI code
end

answered Feb 11, 2013 at 19:15

Reactormonk

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Comments

Necto · Accepted Answer · 2013-02-11 19:15:51Z

1

As @pst said, you don't need a preprocessor in Ruby, since it is dynamic. So:

begin
  require "Qt4"
   class QtDefine < Qt::Widget
     # GUI code
   end
rescue LoadError => load_err
  puts "Qt not found, using console"
   class ConsoleDefine
     # console code
   end
end

answered Feb 11, 2013 at 19:15

Necto

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2 Comments

Reactormonk Over a year ago

That was my first idea, but now you have two different systems in your main file cluttered in some barely-visible rescue.

Necto Over a year ago

That what @xis19 wanted. In C preprocessor, the file looks the same.

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