Type Inference with `Array.filter` and nullable fields.

type DatabaseResult = {
  thread: {
    id: number
  }
  message: {
    id: number
    thread_id: number
  } | null
  attachment: {
    id: number
    message_id: number
  } | null
}[]

const toThread = (rows: DatabaseResult) =>
  Effect.gen(function*() {
    pipe(
      rows,
      Array.filter((row) => Predicate.isNotNull(row.message)),
      Array.groupBy((row) => String(row.message.id)) // 'row.message' is possible 'null'!
    )
  })