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Perl, the defined-or operator.

Like ||, but // tests definedness instead of truthiness.

The equivalent in several other languages is ??, which is uglier imho, but it's just syntax. https://en.wikipedia.org/wiki/Null_coalescing_operator

I use // and ?? often as it's conceptually clean, fast and concise. When available, it arises naturally in logic like caches, memoizations and defaults. //= is pretty useful too, and I'm surprised it's not more widely available.




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