named-arguments-in-their-own-position.md

Named arguments in their own position

• Type: Design proposal
• Author: Denis Zharkov
• Contributors: Mikhail Zarechenskiy
• Status: Submitted
• Prototype: Implemented in 1.3.60
• Discussion: KEEP-193
• Tracker: KT-7745

Summary

Currently, named arguments have a specific limitation: when a function is called with both positional and named arguments, all the positional arguments should be placed before the first named one. For example, the call f(1, y = 2) is allowed, but f(x = 1, 2) is not.

But in some cases, such limitation might be inconvenient.

Motivation

Documenting literal arguments

The major known use-case when one may want to mix named and positioned arguments is documentation: sometimes it's useful to specify in the source explicitly to what parameter exactly this argument matches. Especially, it's important when the argument is appeared as some plain literal:

someFunction(true, someOtherArgumentThatAppearsObviousOnCallSite)
someFunction(runInTheSameThread=true, someOtherArgumentThatAppearsObviousOnCallSite)

The second call version seems to be much more clear than the first one: it becomes obvious on the call-site that computation would be run in the same thread.

Rules and semantics

• Proposed semantics is the following: if there was a successfully resolved call in 1.3 it's allowed to add a name to any positioned non-vararg argument.
• In other words, if there were positioned non-vararg arguments a_1, ..., a_k in a successfully resolved call f(a_1, .., a_k, ...) then it's allowed to specify names for any subset of those a_1, ..., a_k arguments.

Examples

Simple

fun foo(
    p1: Int,
    p2: String,
    p3: Double
) {}

fun main() {
    foo(p1 = 1, "2", 3.0) // OK
    foo(1, p2 = "2", 3.0) // OK
    foo(1, "2", p3 = 3.0) // OK

    foo(p1 = 1, p2 = "2", 3.0) // OK

    foo(p2 = "2", p1 = 1, 3.0) // Error: p1 and p2 are not on their position in the list
    foo(1, p3 = 2.0, "") // Error, p3 is not on its position
}

Varargs

fun foo1(
    vararg p1: Int,
    p2: String,
    p3: Double
) {}

fun main() {
    foo1(1, 2, p2 = "3", 4.0) // Is not allowed because first arguments match to a vararg parameter
    foo1(p2 = "3", 4.0) // Is not allowed because there's implicit empty vararg argument in the beginning
}

Remaining questions/issues

• None