New constructors for Writer?

[davegurnell]

I'd like to propose adding direct support for constructing a Writer from a log, and ensuring that there are syntactically similar equivalents for creating from a log, a result, and a log and a result together.

There's currently support for creating Writers using the pure syntax and the Writer.apply method. However, any other constructors require you to know about WriterT.

One possibility would be to add a tell syntax like the one in Scalaz:

scala> import scalaz._, Scalaz._

scala> Vector("Log message 1", "Log message 2").tell
// res1: Writer[Vector[String],Unit] = 
//   WriterT((Vector(Log message 1, Log message 2),()))

This would give us pure and tell to create from a result or a log, and Writer.apply to create from both values together.

One could also add an equivalent of Scalaz' set syntax, although IMO the name would have to change:

scala> "Result".set(Vector("Log message 1", "Log message 2"))
// res2: Writer[Vector[String],String] =
//   WriterT((Vector(Log message 1, Log message 2),Result))

Another possibility would be adding Writer.tell and Writer.put / Writer.value to mirror the constructors on WriterT.

I realise this is duplicating functionality that's already there with WriterT, but I think there's a cleaner learning curve if people can learn about Reader, Writer, and State before they jump into monad transformers.

Keen to know peoples' thoughts!

[davegurnell]

Concretely, I propose adding two bits of syntax:

logValue.tell

resultValue.writer(logValue)

The former is a direct port from Scalaz. The latter is like the set syntax un Scalaz, but renamed for clarity. Combined with pure, this would provide comprehensive syntax for creating Writers.

The syntax would go in cats.syntax.writer, be included in cats.syntax.all, and have tests in WriterSpec.