How do I sort a stream in descending order in EK9?

EK9 has two sort forms in stream pipelines:

1. 'sort' — uses the natural ordering (ascending via <=> operator)
2. 'sort by comparatorFunction' — uses a custom comparator function

ASCENDING (natural order):

  cat scores | sort | head 3 | collect as List of Integer

DESCENDING (custom comparator):

  cat scores | sort by descending | head 3 | collect as List of Integer

The comparator function must take two parameters of the same type and return Integer:

  descending() as pure
    -> left as Integer, right as Integer
    <- rtn as Integer: right <=> left

Note: right <=> left reverses the order (compare right to left instead of left to right).

For ascending, left <=> right. For descending, right <=> left.

You can also use 'tail N' with ascending sort to get the N largest:

  cat scores | sort | tail 3 | collect as List of Integer

See Q979 for sort by examples. See Q120 for collection sorting. See Q235 for complete stream operations.

Example

defines module qa.streams.sortdescending

  defines function

    //Descending comparator: reverse the <=> order
    descending() as pure
      ->
        left as Integer
        right as Integer
      <- rtn as Integer: right <=> left

  defines program

    SortDemo()
      stdout <- Stdout()

      scores <- [85, 42, 91, 67, 73, 55, 88, 96, 61, 79]
      stdout.println(`All: ${scores}`)

      //Top 3 largest: sort descending with comparator, take head
      topThree <- cat scores
        | sort by descending
        | head 3
        | collect as List of Integer
      stdout.println(`Top 3: ${topThree}`)

      //Bottom 3 smallest: sort ascending (natural), take head
      bottomThree <- cat scores
        | sort
        | head 3
        | collect as List of Integer
      stdout.println(`Bottom 3: ${bottomThree}`)

Other ways to ask this

Can I reverse sort a stream pipeline?
How do I get the top N largest items from a list?
How does sort by work with a comparator function?

Coming from another language?

Java: stream().sorted(Comparator.reverseOrder()). Python: sorted(items, reverse=True). Rust: sort().rev(). EK9: sort by with reversed comparator (right <=> left).

Keywords: head, stream, reverse, pipeline, descending, sort, comparator