FsCodec by @jet @moneylion @amjjd @bartelink @eiriktsarpalis and contributors

.NET API 79,872 bytes

EventCodec<Event, Format, Context>

public static class EventCodec<Event, Format, Context>

Documentation
Code

public static IEventCodec<Event, TargetFormat, Context> Map<TargetFormat>(IEventCodec<Event, Format, Context> native, Func<Format, TargetFormat> up, Func<TargetFormat, Format> down)