This case is not considered exhaustive and results...

# language-evolution

[Klitos Kyriacou]

This case is not considered exhaustive and results in compilation error:

when (Pair(bool1, bool2) {
    Pair(false, false) -> {...}
    Pair(false, true) -> {...}
    Pair(true, false) -> {...}
    Pair(true, true) -> {...}
}

Is it just because it's too difficult for the compiler to figure out that it is in fact exhaustive?

[dmitriy.novozhilov]

Yes, it's too difficult
For compiler Pair is just a regular class, like anyone else

[Casey Brooks]

What you’re thinking of is Pattern Matching, which is not supported yet, though there’s been discussions on it for a while (see https://youtrack.jetbrains.com/issue/KT-186). It’s a very complicated language feature, and the Kotlin team has never (to my knowledge) committed to actually implementing it, though they haven’t said “no” to it either. Kind of in a limbo where we may get it eventually, but don’t hold your breath waiting for it.

The thing I typically do in this kind of case is just throw an else -> error("will never happen") line in the when, knowing that the equality checks will be exhaustive even if the compiler cannot figure it out

[mbonnin]

There was a nice article sometime ago that was diving into that and how difficult it was. Can't find it anymore but maybe someone reading this will?