[platz]

are non-exhaustive pattern matches in kotlin warnings, errors, or neither?

looks like from the docs:

"If `when` is used as an expression, the `else` branch is mandatory, unless the compiler can prove that all possible cases are covered with branch conditions."

So it is an error, I assume?

[kcorbitt]

With the caveat that I haven't tried it myself:

> What about pattern matching? This is where the word “sealed” above comes in handy; we can do an exhaustive pattern match without an else/otherwise branch

from https://medium.com/@octskyward/kotlin-fp-3bf63a17d64a#.8dbwa...

[commentzorro]

This looks to be single value matching only and no compile time exhaustion checks. This looks like really nice (i.e. sugary) alternative to switch but not quite pattern matching.

[udalov]

It's an error only when the 'when'-expression is used as a value, like assigned to a property, passed as an argument or returned from a function. Otherwise it's not an error. There's also a warning for matching enum values over an incomplete set of enum entries, but otherwise non-exhaustive 'when'-expression is green code.

You can basically consider 'when'-expressions to be syntax sugar over Java's if-else or ternary operator (?:) chains.

[platz]

> non-exhaustive pattern matches

So there are no non-exhaustive pattern matches, is what you are saying? i.e. they all must be exhaustive. Except for your enum example

[udalov]

No, as I mentioned, only those 'when'-expressions, the result of which is used as a value, must be exhaustive. Those which are not, we call them 'when'-statements, correspond to an if-else chain in C-like languages and thus are not required to be exhaustive.

More info in the official reference: https://kotlinlang.org/docs/reference/control-flow.html#when...