small stdlib proposals to improve smart-casting co...

# stdlib

[Leon K]

small stdlib proposals to improve smart-casting concerning nullability:
we need a way to do if(foo != null) for multiple values and collection contents.

Proposal part one
add a ifNotNull function that takes vararg arguments of type Any?.
given

val a: Int? = 1
val b: String? = null
val c: Boolean? = false

this would allow to replace this:

if (a != null && b != null && c != null) {
    println("$a, $b, $c")
}

with this:

ifNotNull(a, b, c) {
    println("$a, $b, $c")
}

which imo is a lot more readable, and also easier to type.

(small note/question: would these checks - if the function was just implemented in terms
of contracts and the if-statement - short-circuit, like they do with the
if-statement?)

proposal part two
higher-order-functions on collections should do smart-casting. (I guess this should be doable via contracts?)
given

fun useList(nums: List<Int>) = println(nums)
val foo: List<Int?> = listOf(12, 13, null, 14)

this would allow to do something like this:

if (foo.all { it != null } ) {
	useList(foo)
}

currently, afaik, this check is not possible with compiler-guarantees, as there
is no Collection<T>.hasNoNulls() check.

there are some simmilar functions, but nothing that actually does this.
there is
- Iterable<T?>.requireNoNulls() - the most simmilar, but this throws
exceptions and isn't as general as making all and any-calls smart-cast
- Iterable<T?>.filterNotNull() - could be currently used in combination with
the all-check to "simulate" a smart-casting all, but as it just filters out the
null-values, this could to easily be broken if someone isn't careful when
modifiying the preceding if-statement.

[karelpeeters]

You can't really implement this in the stdlib yet, the closest you could get is having lots of overloads for all possible arguments counts:

fun notNull(a: Any?, b: Any?, c: Any?): Boolean {
    contract { returns(true) implies (a != null && b != null && c != null) }
    return a != null && b != null && c != null
}

[Leon K]

i'm not too confident with my knowledge about the contract-API, but couldn't you do it via vararg, and then do the checks via .all or something, and then specify in the contract that returns(true) implies (args is Array<T>) (where args is vararg args: T?? or why would this not work?

[nkiesel]

the real problem with `ifNotNull`(IMHO better named `noneNull`is that it evaluates all parameters first. Thus, noneNull(a, b, b.c) will not work. You would have to use `noneNull(a, b?.c)`instead but in my experience (I tried that) this leads to more problems than it solves

[Leon K]

thats a valid concern,... so this would need to be implemented on a language level to allow for short-circuiting and such, correct?