Enforce multiline lambda parameters to be on the first line

[t3hk0d3]

Expected Rule behavior

Currently it is possible to have lambdas to be formatted like following:

foo.bar { 
    param -> 
        param.doSomething()
         // ...
}

However this is incorrect behaviour according to Kotlin Code Conventions:

https://kotlinlang.org/docs/coding-conventions.html#lambdas

When declaring parameter names in a multiline lambda, put the names on the first line, followed by the arrow and the newline:

appendCommaSeparated(properties) { prop ->
    val propertyValue = prop.get(obj)  // ...
}

Additional information

• Current version of ktlint: 0.49.1
• Styleguide section: https://kotlinlang.org/docs/coding-conventions.html#lambdas

[paul-dingemans]

Currently it is possible to have lambdas to be formatted like following:

foo.bar { 
    param -> 
        param.doSomething()
         // ...
}

Code above is not accepted by ktlint. With code style intellij_idea or android_studio, this code is formatted as:

fun foo2() {
    foo1.bar {
            param ->
        param.doSomething()
    }
}

which indeed deviates from the kotlin coding conventions.

The idea behind this deviation is that code sample like below:

fun foo3() {
    foo1.bar {
        param1,
        param2 ->
            param1.doSomething()
            param2.doSomething()
    }
}

are nicely formatted as:

fun foo3() {
    foo1.bar {
            param1,
            param2 ->
        param1.doSomething()
        param2.doSomething()
    }
}

which is more consist with indentation of parameters in a function call:

fun foo(
    param1: Any,
    param2: Any
)  {
    // do something
}

Note that with code style ktlint_official there is additional formatting to change code below:

fun foo4() {
    foo1.bar { param1,
        param2 ->
            param1.doSomething()
            param2.doSomething()
    }
}

to

fun foo4() {
    foo1.bar {
            param1,
            param2,
        ->
        param1.doSomething()
        param2.doSomething()
    }
}

[paul-dingemans]

I have misread the original post and focused too much on the indentation only.

The original post identifies that code:

foo.bar { 
    param -> 
        param.doSomething()
         // ...
}

should be formatted as:

foo.bar {  param -> 
    param.doSomething()
    // ...
}

to comply with:

When declaring parameter names in a multiline lambda, put the names on the first line, followed by the arrow and the newline:

For this a new rule is needed. This rule should also take care of:

If the parameter list is too long to fit on a line, put the arrow on a separate line:

foo {
    context: Context,
    environment: Env
    ->
    context.configureEnv(environment)
. }

[paul-dingemans]

The Kotlin Coding conventions are not entirely comsistent regard indenting of a multiline parameter list.

https://kotlinlang.org/docs/coding-conventions.html#lambdas

foo {
   context: Context,
   environment: Env
   ->
   context.configureEnv(environment)
}

https://kotlinlang.org/docs/coding-conventions.html#parameters-in-lambdas

fun main() {
    val x = {
            x: Comparable<Number>,
            y: Iterable<Number>, // trailing comma
        ->
        println("1")
    }
    println(x)
}

[t3hk0d3]

Thanks for a rapid fix!