Kotlin lambdas and type inference

Question

I implemented the following enum in Kotlin

enum class PlaylistAction(val playlistFilter:(Playlist) -> Boolean) {
    PLAY ({ it.playListOwner.Id == "xxx" }),
    SAVE({true})
}

I would like to use it to filter a List like this:

var test = playlists.filter { playlistActionType.playlistFilter}

where playlistActionType is of type PlaylistAction and playlists is List<Playlist>

But i am getting the following error:

Error:(122, 34) Type mismatch: inferred type is (Playlist) -> (Playlist) -> Boolean but (Playlist) -> Boolean was expected

Why is the inferred type (Playlist) -> (Playlist) -> Boolean and not (Playlist) -> Boolean?

Grzegorz Piwowarek Grzegorz Piwowarek · Accepted Answer · 2016-11-16T21:29:15

By putting your predicate in {}, you are actually creating another lambda that returns your predicate as a result and this is why you end up with: (Playlist) -> (Playlist) -> Boolean instead of (Playlist) -> Boolean.

Using normal brackets should do the trick:

var test = playlists.filter(playlistActionType.playlistFilter)

Kotlin lambdas and type inference

2 Answers