Question

I’m relatively new to scala and made some really simple programs succesfully.
However, now that I’am trying some real world problem resolution, things are getting a little bit harder…

I want to read some files into ‘Configuration’ objects, using various ‘FileTypeReader’ subtypes that can ‘accept’ certain files (one for each FileTypeReader subtype) and return an Option[Configuration] if it can extract a configuration from it.

I’m trying to avoid the imperative style and wrote, for exemple, something like this (using scala-io, scaladoc for Path here http://jesseeichar.github.com/scala-io-doc/0.3.0/api/index.html#scalax.file.Path ) :

(...)
trait FileTypeReader {
   import scalax.file.Path
   def accept(aPath : Path) : Option[Configuration]
}
var readers : List[FileTypeReader] = ...// list of concrete readers
var configurations = for (
          nextPath <- Path(someFolder).children();
          reader <- readers
      ) yield reader.accept(nextPath);
(...)

Of course, that does not work, for-comprehensions return a collection of the first generator type (here, some IterablePathSet).

Since I tried many variant and feel like running in circle, I beg for you advices on that matter to solve my – trivial ? – problem with elegance ! 🙂

Many thanks in advance,

sni.

Answer 1

If I understand correctly, your problem is that you have a Set[Path] and want to yield a List[Option[Configuration]]. As written, configurations will be a Set[Option[Configuration]]. To change this to a List, use the toList method i.e.

val configurations = (for {
    nextPath <- Path(someFolder).children
    reader   <- readers
  } yield reader.accept(nextPath) ).toList

or, change the type of the generator itself:

val configurations = for {
    nextPath <- Path(someFolder).children.toList
    reader   <- readers
  } yield reader.accept(nextPath)

You probably actually want to get a List[Configuration], which you can do elegantly since Option is a monad:

val configurations = for {
    nextPath <- Path(someFolder).children.toList
    reader   <- readers
    conf     <- reader.accept(nextPath)
  } yield conf