E4211

Compiler diagnostic name: control_in_list_comprehension.

Control flow is not allowed inside a list comprehension.

A list comprehension builds a new collection from the elements produced by its clauses. Control operations such as break, continue, return, raise, and async control do not have a surrounding loop or function boundary inside the comprehension body that can consume them.

Erroneous example

pub fn values() -> Array[Int] {
  label~: for ;; {
    let xs = [for _ in 0..<3 => { break label~ }]
    return xs
  }
  []
}

The break appears inside the list comprehension body, so MoonBit reports an error.

Suggestion

Move the control flow outside the comprehension, or rewrite the code as an explicit loop.

pub fn values() -> Array[Int] {
  [for value in 0..<3 => value]
}