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Comment by onion2k | Hacker News Reader

if you are a Ruby programmer

cyberferret explicitly said it's a good idea to avoid shorthand methods because new programmers don't understand them, and you're making the argument that they're OK because experienced programmers will understand them. The essence of your argument is that if you've used the language enough to know what the or-equals statement means then you'll know what it means. I'm not convinced that's a good reason.

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For that specific example though, it's so common in the colloquial Ruby language that all normal Ruby programmers should learn and use it. If you're just taking about readability, the time saved skimming during your career is worth the time spent learning the idiom once in your life.

Cyberferrit said new programmers to the project, not specifically new to the language or programming itself.

Working on emulation software I commonly run across snippets such as `res = !!iRegisterVal` which results in some C newbies scratching their heads as to why a useless double negation would be there. But for more seasoned developers its intent can pretty easily recognized...

Why is !! useful? Asking as a ruby programmer.

It is a shorthand method of casting to a boolean value - C didn't have a true bool type until "recently" (but did have the concept of truthiness, where true is `is not 0`), so many projects and libraries use `#define TRUE 1`, and #define FALSE 0` or some similar variant.

So rather than write `foo ? TRUE : FALSE` or being more explicit `(foo != 0) ? 1 : 0`, you can just do `!!foo`.

It's a quick cast to a boolean.

Because Ruby doesn't have a to_boolean method. !! is used to convert "truthy" or "falsy" values [0] into true boolean true or false values.

For example,

    def user_signed_in?
      !!current_user
    end

This returns "true" or "false", instead of a (truthy) User object or a (falsy) nil.

[0] https://gist.github.com/jfarmer/2647362

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