BTW motivation for this patch is to reduce the following example:

val emptySv1: State[Game,Cards] = for {
  stack ← State.gets[Game,Cards](_.stack)
  _     ← stackL := Cards()
} yield stack

... to this much shorter one:

val emptySv2: State[Game,Cards] = stackL.assigno(Cards())

AFAIK from looking at the core/src/main/scala/scalaz/Lens.scala source there is nothing like this yet.

@tonymorris @pthariensflame Could you have a look at that please?

I'm fine with modo and assigno. The Haskell lens package uses < prefixes for operations that return the old state, so modo would be <%= and assigno would be <:=.

Also, I'd rather tighten up the implementations as:

def modo(f: B1 => B2): IndexedState[A1, A2, B1] = 
  IndexedState(a => {
    val c = run(a)
    val o = c.pos
    (c put f(o), o)
  })

def assigno(b: => B2): IndexedState[A1, A2, B1] =
  modo(_ => b)

Thanks for the comments, @pthariensflame. The commits above apply the suggested changes.

You seem to have left the type parameter on modo and its synonym. They are provably unnecessary. :)

Merged as 620787c. Thanks!