A Writable is a source of bytes that can be written to an OutputStream.

Essentially a push-based version of java.io.InputStream, that allows an implementation to guarantee that cleanup logic runs after the bytes are written.

Writable is also much easier to implement than java.io.InputStream: any code that previously wrote output to an ByteArrayOutputStream or StringBuilder can trivially satisfy the Writable interface. That makes Writable very convenient to use for allowing zero-friction zero-overhead streaming data exchange between different libraries.

Writable comes with implicit constructors from Array[Byte], String and InputStream, and is itself a tiny interface with minimal functionality. Libraries using Writable are expected to extend it to provide additional methods or additional implicit constructors that make sense in their context.