(scalaz) to optionally invoke a chained method

Question

I have:

val having: Option[String] = ... val averageStream = having match { case Some(expr) => stream[TimeMovingAverage].filter("name = '#name'").where(expr) case None => stream[TimeMovingAverage].filter("name = '#name'") }

Is there some scalaz magic that would eliminate the duplication?

I could do something like

val averageStream1 = stream[TimeMovingAverage].filter("name = '#name'") val averageStream2 = having.map(averageStream1.where(_))

But that seems a bit awkward. I guess I am looking for something like

val averageStream = stream[TimeMovingAverage].filter("name = '#name'") |?| having.cata(_.where(_), _) // Clearly some ambiguities with the _'s here

Debilski · Accepted Answer · 2011-12-08 17:15:26Z

stream[TimeMovingAverage].filter("name = '#name'") |> { str => having.cata(str.where(_), str) }

or even

stream[TimeMovingAverage].filter("name = '#name'") |> ( having.cata(expr => _.where(expr), identity) : T => T )

T => T must be changed to the type of stream[TimeMovingAverage] which makes it less practical unless you find the following specialisation of |> in scalaz or define it yourself:

implicit def WrapIt[T](t: T) = new { def |~> (arg: T => T) = arg(t) def |?> (arg: Option[T => T]): T = arg.getOrElse(identity (_:T)).apply(t) } stream[TimeMovingAverage].filter("name = '#name'") |~> having.cata(expr => _.where(expr), identity) stream[TimeMovingAverage].filter("name = '#name'") |?> having.map(expr => _.where(expr))

Not sure, if one should really come up with something like this, though.

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