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Idiosyncratic Ruby: Big Data Without End

 3 years ago
source link: https://idiosyncratic-ruby.com/59-big-data-without-end.html
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Big Data Without End

Ruby's big DATA constant does more than you might expect!

Everything after the __END__ keyword (at the beginning of the line) is not interpreted as Ruby, but can be retrieved with the big¹ DATA constant.² This is an example big-data.rb script:

p DATA.read
__END__
big data

Big DATA is a File object, which you can read. The example will output "big data". An example of real-world usage is inline templating within the sinatra web framework.³

¹ Do not confuse with the small Data class, which is a CRuby implementation detail
² Big DATA is not defined, if you have no __END__. Furthermore, it is not available if you did not execute the script directly, but loaded or required it.
³ Actually, not really

Is the Data Section Enough?

Wait a minute! Big DATA is a File object? What file exactly?

p DATA.path
p DATA.lineno
__END__
big data

The output will be "big-data.rb" and 3. The big DATA object points to the source file itself at a specific position! And look, we can alter this:

DATA.rewind
puts DATA.gets("\n__END__")[0..-9]
__END__
big data

It will now read the source code of itself:

DATA.rewind
puts DATA.gets("\n__END__")[0..-9]

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