Ruby: eval with string interpolation

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独厮守ぢ
独厮守ぢ 2021-02-04 12:09

I don\'t understand, why eval works like this:

\"123 #{456.to_s} 789\" # => \"123 456 789\"
eval(\'123 #{456.to_s} 789\') # => 123


        
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  • 2021-02-04 12:42

    You wanted:

    eval('"123 #{456.to_s} 789"')
    

    . . . hopefully you can see why?

    The code passed to the interpretter from eval is exactly as if you had written it (into irb, or as part of a .rb file), so if you want an eval to output a string value, the string you evaluate must include the quotes that make the expression inside it a String.

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  • 2021-02-04 13:01

    What's happening, is eval is evaluating the string as source code. When you use double quotes, the string is interpolated

    eval '"123 #{456.to_s} 789"'
    # => "123 456 789"
    

    However when you use single quotes, there is no interpolation, hence the # starts a comment, and you get

    123 #{456.to_s} 789
    # => 123
    

    The string interpolation happens before the eval call because it is the parameter to the method.

    Also note the 456.to_s is unnecessary, you can just do #{456}.

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