break-continue-in-inline-lambdas.md

Non-local break and continue

• Type: Design proposal
• Authors: Nikita Bobko, Pavel Mikhailovskii
• Contributors: Alexander Udalov, Roman Elizarov, Simon Ogorodnik
• Status: Experimental since 1.8.20, Beta in 2.1
• Discussion and feedback: KEEP-326
• YouTrack issues: KT-1436, KT-19748, KT-54906

Introduction

One of the key features of Kotlin inline functions is that lambdas passed as arguments to them can contain non-local return statements which jump over the lambda's boundaries and exit from the enclosing function. However, a similar non-local behavior is not yet implemented for break and continue statements.

var firstNumber: Int
for (file in files) {
    file.bufferedReader().use { reader ->
        firstNumber = reader.readLine()?.toInt() ?: continue
        //                                          ^^^ 'break' or 'continue' jumps across a function or a class boundary
        break
     // ^^^ 'break' or 'continue' jumps across a function or a class boundary
    }
}

Proposed change

Make it possible to use non-local (applied to a loop belonging to the enclosing function) break and continue statements within lambdas passed as arguments to an inline function. Similarly to non-local returns, non-local break and continue could only be used in lambdas passed to parameters without noinline or crossinline modifiers.

@outer for (department in departments) {
    for (employee in department.employees) {
        employee.apply { // We're inside an inline lambda now
            if (isRetired) continue // Unlabeled break/continue works with the closest enclosing for/while
            if (position == "Developer" && age > 70) {
                println("We have some senior developers in our organisation!")
                break@outer
            }
        }
    }
}

Motivation

Inline functions with trailing lambdas are designed in a way that allows using them as if they were first-class language constructs (e.g. synchronized or run, with, let scope functions from stdlib).

Since users can invoke return, break or continue inside of bodies of native language constructs (such as body of if or body of try-catch), it would be only logical to allow the same for lambdas passed as parameters to inline functions. At the moment, only non-local returns are supported. That's why we are proposing to add support for non-local break/continue as well.

So the motivation is the following:

1. Make inline lambdas truly first-class language constructs that are seamless to use.
2. Fix consistency with non-local return. It's inconsistent that currently we allow non-local return statements but don't allow non-local break and continue statements.

Design problems

This feature may interfere with another proposed feature, allowing to use break/continue not only inside loop statements, but also within lambda arguments of loop-like functions like forEach or takeWhile (the notion of a loop-like function would require a strict definition; we leave this question open for now).

for (i in 0 until 100) {
    (0 until 200).forEach {
        if (it == 42) break // Should it break from `for` or `forEach`?
        if (i == 10) continue // Where to continue?
    }
}

However, it seems that both features can be implemented without introducing ambiguities.

First of all, we should never allow unlabeled break/continue for loop-like functions because users wouldn't be able to understand whether it is applied to a certain function without checking its implementation.

foo { bar { break } } // which function is loop-like?

It leaves us only with the labeled syntax option for loop-like functions:

for (i in 0 until 100) {
    (0 until 200).forEach {
        if (it == 42) break@forEach // Obviously, it breaks `forEach`
        if (i == 10) continue // No label, so it applies to the 'for' loop
    }
}

A similar rule applies to return statements: an unlabeled return within a lambda is always non-local, a labeled one returns to the label. In other words, an unlabeled return goes to an innermost enclosing block that is clearly marked with fun hard keyword. In a similar way, an unlabeled break/continue goes to the innermost enclosing block that is clearly marked with for, do, or while hard keywords.

Tooling for puzzling code

Even if we don't implement support for break/continue for loop-like functions, some users may find constructions containing break or continue within lambda arguments of loop-like inline functions puzzling:

fun printUntilZero(producer: () -> List<Int>) {
    while(true) {
        val list = producer()
        list.forEach {
            if (it == 0) break // Does it exit from while or from forEach?
            println(it)
        }
    }
}

To reduce the risk of confusion, it was proposed to introduce an IDE inspection and a quickfix recommending to use labeled break and continue in lambdas passed to parameters without an EXACTLY_ONCE or AT_MOST_ONCE contract (potentially loop-like). After the inspection is applied:

fun printUntilZero(producer: () -> List<Int>) {
    myLoop@ while(true) {
        val list = producer()
        list.forEach {
            if (it == 0) break@myLoop
            println(it)
        }
    }
}

It will typically be used only with stdlib, and we must be sure it is not triggered on the usual scope functions. It is not bad if it is triggered on a user-defined function without a contract. It is better to be safe (by suggesting to use labelled break/continue) than end up with a more ambiguous code on a user-defined loop-like function.

The inspection shouldn't be triggered for let, run, etc., as they all have the corresponding contract. There seems to be no need for a similar inspection for non-local returns. We assume that they are less confusing.

Why an IDE warning instead of a compilation error

Since by allowing non-local break and continue, we introduce this new puzzler, one may wonder why not make it a compilation error instead of an IDE inspection. The problem is that the compiler can't reliably detect jumps over lambdas of loop-like functions because Kotlin doesn't have a concept of loop-like functions.

First of all, while it's totally fine the proposed IDE inspection would use a heuristic that is based on the AT_MOST_ONCE/EXACTLY_ONCE contract, compiler errors can't be based on a heuristic. Another objection is that contracts remain an experimental feature, and we shouldn't force people to use an experimental feature if they decide to use non-local break/continue with user-defined inline functions.

Instead, we could be more restrictive and make labels mandatory for non-local break and continue. But we can't do that because it would mean that we would need to forbid unlabeled non-local return for consistency. It would not only break a lot of existing code (e.g. 1212 occurrences in IntelliJ monorepo), but it would also break inline functions' first-class citizenship as language constructs. E.g. we value "return if not null" idiomatic Kotlin construct nullable?.let { return it }.

Issues with data-flow and control-flow analyses

The proposed change breaks control-flow analysis in K1 (see KT-54906). That issue in non-trivial, so that fixing in K1 doesn't look like a reasonable option at the moment. This means that we have to postpone finalization of the feature until K2 is ready.

So far, testing of the prototype against K2 hasn't revealed any CFA/DFA-related issues.