Flatten Scala Try

Question

Is there a simple way to flatten a collection of try\'s to give either a success of the try values, or just the failure? For example:

def map(l:List[Int]) =          


        

Answer 1

Have a look on the liftweb Box monad. With the help of the tryo constructor function, it gives you exactly the abstraction you are looking for.

With tryo you can lift a function into a Box. The box then either contains the result from the function or it contains an error. You can then access the box with the usual monadic helper functions (flatMap, filter, etc.), without bothering if the box contains an error or the result form the function.

Example:

import net.liftweb.util.Helpers.tryo

List("1", "2", "not_a_number") map (x => tryo(x.toInt)) map (_ map (_ + 1 ))

Results to

List[net.liftweb.common.Box[Int]] = 
  List(
    Full(2), 
    Full(3), 
    Failure(For input string: "not_a_number",Full(java.lang.NumberFormatException: For input string: "not_a_number"),Empty)
  )

You can skip the erroneous values with flatMap

List("1", "2", "not_a_number") map (x => tryo(x.toInt)) flatMap (_ map (_ + 1 ))

Results

List[Int] = List(2, 3)

There are multiple other helper methods, e.g. for combining boxes (while chaining error messages). You can find a good overview here: Box Cheat Sheet for Lift

You can use it on its own, no need to use the whole lift framework. For the examples above I used the follwing sbt script:

scalaVersion := "2.9.1"

libraryDependencies += "net.liftweb" %% "lift-common" % "2.5-RC2"

libraryDependencies += "net.liftweb" %% "lift-util" % "2.5-RC2"