@DisallowLambdaCapture checker example

[raulraja]

The @DisallowLambdaCapture macro is an annotation that can be applied to functions. It checks that functions annotated with @DisallowLambdaCapture are not called inside a non-inline anonymous function that captures variables. If it finds such a call, it will report an error with a message specified in the @DisallowLambdaCapture annotation or a default message if the msg parameter is not specified.

To implement this macro, the DisallowLambdaCapture companion object extends the Meta.Checker.Expression interface and overrides the check method. If present, this method first retrieves the msg argument of the DisallowLambdaCapture annotation. It then filters the declarations in the current scope to find any non-inline anonymous functions and checks if any of these functions capture variables. If it finds a function that captures variables and the function being checked is annotated with DisallowLambdaCapture, it reports an error with the specified message.

The DisallowLambdaCapture macro can be helpful in cases where you want to ensure that certain functions are only called in a context where variable capture is not allowed. This can be useful for avoiding unintended side effects or enforcing a certain code style. For example, you might use this macro to disallow calling functions that modify the global state inside anonymous functions to ensure that the state changes are properly isolated and not leaked through the lambda capture.

package foo.bar

import arrow.meta.samples.DisallowLambdaCapture

interface Raise<in E> {
  @DisallowLambdaCapture("It's unsafe to capture `raise` inside non-inline anonymous functions") fun raise(e: E): Nothing
}

context(Raise<String>)
fun shouldNotCapture(): () -> Unit {
  return { <!UnsafeCaptureDetected!>raise("boom")<!> }
}

context(Raise<String>)
fun inlineCaptureOk(): Unit {
  listOf(1, 2, 3).map { raise("boom") }
}

context(Raise<String>)
fun leakedNotOk(): () -> Unit = {
  listOf(1, 2, 3).map { <!UnsafeCaptureDetected!>raise("boom")<!> }
}

context(Raise<String>)
fun ok(): Unit {
  raise("boom")
}

[nomisRev]

LGTM, but I am missing expertise to review all changes in this PR.

[nomisRev]

Would this not be more precise as msg: String??

[nomisRev]

Not sure where this is checked in DisallowLambdaCapture.kt? 🤔

[JavierSegoviaCordoba]

I will review it too, sorry I miss it!

[what-the-diff]

• Added a new annotation DisallowLambdaCapture
• Updated the meta checker to support checking for calls to annotated functions and not just declarations with annotations
• Added tests that demonstrate how this works in practice (see capture_test)