ValidReader

MapReader.scala

import scalaz._
import Scalaz._

object MapReader extends App {

  type ValidReader[S,X] = (S) => Validation[NonEmptyList[String],X]
  type MapReader[X] = ValidReader[Map[String,String],X]
  
  def read[A: Manifest]( key: String )( f: String => A ): MapReader[A] =
    map => {
      if( ! map.contains(key) ) {
	("Missing key: " +key).fail
      } else {
	try {
	  f( map(key) ).success
	} catch {
	  case _ => {
	    val value = map(key)
	    val klass = implicitly[Manifest[A]].erasure
	    ("Cannot convert '" + value + "' to " + klass).fail
	  }
	}
      }
    }.liftFailNel

  def readInt( k: String ): MapReader[Int] = read[Int](k)( _.toInt )
  def readString( k: String ): MapReader[String] = read[String](k)( s => s )
    
  val name = readString( "name" )
  val age = readInt( "age" )

  val data = Map( "name" -> "Paul", "age" -> "8" )

  println( name(data) )
  println( age(data) )

  case class Boy( name: String, age: Int )

  val boy = ( name |@| age ) {
    (n,a) => ( n |@| a ) { Boy(_,_) }
  }

  println( boy( data ) )

}