How come I can write: ```val file = File("hello.tx...

# announcements

[glenkpeterson]

How come I can write:

val file = File("hello.txt")
return when {
    file.isFile -> Or.good(file)
    else        -> Or.bad(chapter.fileName)
}

But if I try to capture the when subject in a variable like this:

return when(val file = File("hello.txt")) {
    file.isFile -> Or.good(file)
    else        -> Or.bad(chapter.fileName)
}

I get, "Incompatible Types: Boolean and File" on the file.isFile part?

The end of this section makes it look like I should be able to do this: https://kotlinlang.org/docs/control-flow.html#when-expression

[diesieben07]

The first one is a when without subject, so all branches are boolean expressions.
The second one is a when with subject of type File so all branches are compared using == against that subject. The first branch is of type Boolean and comparing File and Boolean makes no sense.

[glenkpeterson]

But the example doesn't show == against the subject. It's got is Success or is HttpError.

[ephemient]

when-with-subject supports == comparison, `in`/`!in`, and `is`/`!is`.

[diesieben07]

Yes, you can use in and is as well, I forgot to mention that, sorry

[ephemient]

https://youtrack.jetbrains.com/issue/KT-28359 there's a discussion about how to add other types of checks, but that gets wrapped up with other deeper pattern matching questions so any resolution will be a while off

[ephemient]

for now, if you want to have boolean expressions as your cases, you need to use when without a subject

[glenkpeterson]

Thanks to the link to the YouTrack issue. I think I just found the when - in documentation, but if there's more/better docs for in somewhere, I'd love to know. https://kotlinlang.org/docs/basic-syntax.html#collections

[ephemient]

the getting started docs don't cover every aspect of the language, just enough to get started

[ephemient]

occurrences of in keyword in the language specification:
https://kotlinlang.org/spec/statements.html#for-loop-statements
https://kotlinlang.org/spec/expressions.html#type-checking-and-containment-checking-expressions
https://kotlinlang.org/spec/expressions.html#when-expressions

[Nir]

gotta say i find this case very very surprising