akka stream - How do I create a Flow with a different input and output types for use inside of a graph? -

i making custom sink building graph on inside. here broad simplification of code demonstrate question:

def mysink: sink[int, unit] = sink() { implicit builder =>      val entrance = builder.add(flow[int].buffer(500, overflowstrategy.backpressure))     val tostring = builder.add(flow[int, string, unit].map(_.tostring))     val printsink = builder.add(sink.foreach(elem => println(elem)))      builder.addedge(entrance.out, tostring.in)     builder.addedge(tostring.out, printsink.in)      entrance.in } 

the problem having while valid create flow same input/output types single type argument , no value argument like: flow[int] (which on documentation) not valid supply 2 type parameters , 0 value parameters.

according reference documentation flow object apply method looking defined as

def apply[i, o]()(block: (builder[unit]) ⇒ (inlet[i], outlet[o])): flow[i, o, unit] 

and says

creates flow passing flowgraph.builder given create function.

the create function expected return pair of inlet , outlet correspond created flows input , output ports.

it seems need deal level of graph builders when trying make think simple flow. there easier , more concise way create flow changes type of it's input , output doesn't require messing it's inside ports? if right way approach problem, solution like?

bonus: why easy make flow doesn't change type of input it's output?

if want specify both input , output type of flow, indeed need use apply method found in documentation. using it, though, done pretty same did.

flow[string, message]() { implicit b =>   import flowgraph.implicits._    val reversestring = b.add(flow[string].map[string] { msg => msg.reverse })   val mapstringtomsg = b.add(flow[string].map[message]( x => textmessage.strict(x)))    // connect graph   reversestring ~> mapstringtomsg    // expose ports   (reversestring.inlet, mapstringtomsg.outlet) } 

instead of returning inlet, return tuple, inlet , outlet. flow can used (for instance inside builder, or directly runwith) specific source or sink.