Extending classes from the pkl standard library

[emileakbarzadeh]

Hi there, great work on pkl!

One pain point I've come across is that pkl does not allow you to extend base classes (such as set or listing) to add properties or functions

A use case I have for this is to be able to remove items from a set by some key.

Example below of my attempted code

class SongSet extends Set<Song>{
    function removeSongsByUri(songsToRemove: Set<String>): SongSet = new Listing<Song>{
        for(song in this){
            when(!songsToRemove.any((songToRemove) -> song.uri == songToRemove)){
                song
            }
        }
    }.toSet()
}

//Usage
boogie = super.boogie.removeSongsByUri(Set("example_uri"))

Currently this results in an error because only open classes can extended.
Cannot extend non-open class Set.

Solutions

In Swift, for example, you can extend a closed class by using the extension keyword. Perhaps it could be possible to add this to the language, or is there something I'm missing here?

Thanks for your time!

[jjmaestro]

I think the issue is different, you can avoid that error by opening the Set class but then you get a different error:

class Song {
  uri: Uri
}

open class Set

class SongSet extends Set<Song> {
    function removeSongsByUri(songsToRemove: SongSet): SongSet =
      new Listing<Song> {
        for (song in this) {
            when (!songsToRemove.any((songToRemove) -> song.uri == songToRemove)) {
                song
            }
        }
    }.toSet()
}

songs1 = SongSet(
  new Song { uri = "http://foo.com/1" },
  new Song { uri = "http://foo.com/2" }
)

songs2 = songs1.removeSongsByUri(Set("http://foo.com/1"))

The error I get is:

$ pkl eval songs.pkl

–– Pkl Error ––
Expected 0 type argument(s) but got 1.

7 | class SongSet extends Set<Song> {
                          ^^^^^^^^^
at songs#SongSet (file:///tmp/songs.pkl, line 7)

106 | text = renderer.renderDocument(value)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
at pkl.base#Module.output.text (https://github.com/apple/pkl/blob/0.25.3/stdlib/base.pkl#L106)