Explore Kotlin contracts for Boolean check functions

[Jolanrensen]

It's odd that after a

if (myCol.isColumnGroup())

check you still need an explicit cast to ColumnGroup<*> to be able to call columnsCount() on myCol. The check should tell the compiler that it's actually a column group, similar to what an myCol is ColumnGroup<*> would do.

This is actually possible with Kotlin contracts:

@OptIn(ExperimentalContracts::class)
public fun AnyCol.isColumnGroup(): Boolean {
    contract {
        returns(true) implies (this@isColumnGroup is ColumnGroup<*>)
    }
    return kind() == ColumnKind.Group
}

makes

if (myCol.isColumnGroup()) {
    myCol.columnsCount() // compile
} else {
    myCol.columnsCount() // not compile
}

Unfortunately contracts aren't all-powerful (yet):

@OptIn(ExperimentalContracts::class)
public inline fun <reified T> AnyCol.isType(): Boolean {
    contract {
        returns(true) implies (this@isType is DataColumn<T>) //  !! Cannot check for instance of erased type: DataColumn<T> :(
    }
    return type() == typeOf<T>()
}