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That is a terrible example of a reason to panic. If the programmer believes the list to be exhaustive and it isn’t, then the programmer is wrong. So raise an exception and let the caller decide what to do with it. But don’t punish the caller by forcing it to crash just because you made a mistake as a programmer.

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(in go, "raise an exception" means "return an error.")

error values are meant to represent expected, unsuccessful conditions—internal properties of the program—during the program's execution. for example, looking up the address for a domain name is an operation that can be expected to fail at times, and an error is appropriate here.

errors in go are not meant to represent properties external to the program, such as mistakes by the package author or documented incorrect usage of the package by the package user. in go these are panics.

perhaps other languages use exceptions for both but that conflates.

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