playdate-kit/Sources/PlaydateKit/Sound/Source/Aliases/SoundCallbackSource.swift

extension Sound.CallbackSource {
    /// Fills the sample buffers and returns `true` if output was
    /// produced. `right` is empty for mono sources.
    public typealias Callback = (_ left: inout MutableSpan<Int16>,
                                 _ right: inout MutableSpan<Int16>) -> Bool
}