[swift-users] Why does Array subscript fail at runtime?

H. Kofi Gumbs h.kofigumbs at gmail.com
Wed Jul 20 23:07:29 CDT 2016

Hello Swift community,

Here's a philosophy I've struggled with since I started learning Swift. In
general, it seems that failable function calls return `Optional`s; however,
`Array`s violate this rule by failing fast at runtime. I understand that
subscripts can't throw, so the only way to fail fast is to do so at
runtime. I also realize that there are many implementations of the
`array[safe: index]` that I could choose to use instead. However, I do not
understand why the default behavior is still to fail at runtime. Especially
given how they are often introduced as a beginner-friendly data structure.
Am I missing some language philosophy or major decision?

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