Code Will Eat Itself

CODE WILL EAT ITSELF
@tomstuart / Scottish Ruby Conference / 2013-05-13

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A Ruby program run from a ﬁlesystem happens to be able
to read its own source code:
program = File.read(__FILE__)
But what about Ruby programs that you type straight into IRB?
What about compiled languages (Java, C)?
What about JavaScript in a browser?

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Kleene’s recursion theorem roughly :
Any computer program can calculate its own source code.
This means we can modify any Ruby program to read its own source,
without touching the ﬁlesystem.

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x = 1
y = 2
puts x + y

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program = '...'
x = 1
y = 2
puts x + y

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program = 'program = \'...\''
x = 1
y = 2
puts x + y

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program = 'program = \'program = \\\'program = \\\\\\\'...\\\\\\\'\\\'\''
x = 1
y = 2
puts x + y

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>> puts 'Single quotes look like \'this\'.'
Single quotes look like 'this'.
=> nil
>> puts %q{Curly brackets look like {this}.}
Curly brackets look like {this}.
=> nil
>> puts %q{An unbalanced curly bracket like this} is a problem.}
SyntaxError: syntax error, unexpected tIDENTIFIER, expecting end-of-input

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program = %q{program = %q{program = %q{program = %q{...}}}}}
x = 1
y = 2
puts x + y

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program = %q{program = %q{program = %q{program = %q{program = %q{program =
%q{program = %q{program = %q{...}}}}}}}}}}
x = 1
y = 2
puts x + y

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%q{program = %q{program = %q{program = %q{program = %q{program = %q{program =
%q{program = %q{program = %q{...}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
x = 1
y = 2
puts x + y

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%q{program = %q{program = %q{program = %q{program = %q{...}}}}}}}}}}}}}}}}}}}
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
x = 1
y = 2
puts x + y

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%q{program = %q{program = %q{...}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}
x = 1
y = 2
puts x + y

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Our mistake is to try to put the program literally inside itself.
A. Assign the current program’s source to program
B. Do whatever other work the program is supposed to do
i.e. the code we originally started with
We're trying to write the program in two parts:

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program = %q{program = %q{program = %q{program = %q{...}}}}}
x = 1
y = 2
puts x + y
A
B

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We can avoid the problem by computing the source
dynamically from data we provide.
Write a three-part program:
A. Assign a string literal to a variable call it data)
B. Use that string to compute the whole program’s source
and assign it to program
C. Do whatever other work the program is supposed to do
i.e. the code we originally started with

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data = %q{...}
program = ...
x = 1
y = 2
puts x + y
A
B
C

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A. Store the literal source code of parts B and C in data
B. Compute part A's source from data, then concatenate the
sources of A, B and C to get program
C. Do whatever other work the program is supposed to do i.e.
the code we originally started with
Here's how to actually do it:

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data = %q{...}
program = ...
x = 1
y = 2
puts x + y

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data = %q{...}
program = ...
x = 1
y = 2
puts x + y
We know most of the source of parts B and C already, so we can partially
complete the value of data.

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data = %q{
program = ...
x = 1
y = 2
puts x + y
}
program = ...
x = 1
y = 2
puts x + y
We also know that the source of part A is just the string 'data = %q{...}'
with the value of data ﬁlling the gap between the curly braces, so we can
partially complete the value of program too.

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data = %q{
program = ...
x = 1
y = 2
puts x + y
}
program = "data = %q{#{data}}" + ...
x = 1
y = 2
puts x + y
Now all that’s missing from program is the source code of parts B and C,
which is exactly what data contains, so we can append the value of data to
program to ﬁnish it off.

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data = %q{
program = ...
x = 1
y = 2
puts x + y
}
program = "data = %q{#{data}}" + data
x = 1
y = 2
puts x + y
Finally, we can go back and fix up the value of data to reflect what part B
actually looks like.

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data = %q{
x = 1
y = 2
puts x + y
}
x = 1
y = 2
puts x + y
That's it!

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puts program

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data = %q{...}
program = ...
puts program

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data = %q{
program = ...
puts program
}
program = ...
puts program

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data = %q{
puts program
}
puts program

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>> data = %q{
puts program
}
=> "\nprogram = \"data = %q{\#{data}}\" + data\nputs program\n"
>> program = "data = %q{#{data}}" + data
=> "data = %q{\nprogram = \"data = %q{\#{data}}\" + data\nputs program\n}
\nprogram = \"data = %q{\#{data}}\" + data\nputs program\n"
>> puts program
data = %q{
puts program
}
puts program
=> nil

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data = %q{
puts program
}
puts program
..is a quine.
The program...

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This technique works for any program.

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THE END
@tomstuart / [email protected] / computationbook.com

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Code Will Eat Itself

Code Will Eat Itself

Tom Stuart

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