Constructor cannot be instantiated to expected type; p @ Person

Question

I was trying the deep case matching construct from here: http://ofps.oreilly.com/titles/9780596155957/RoundingOutTheEssentials.html and the code is as follows match-deep.scala:

class Role
case object Manager extends Role
case object Developer extends Role

case class Person(name:String, age: Int, role: Role)

val alice = new Person("Alice", 25, Developer)
val bob = new Person("Bob", 32, Manager)
val charlie = new Person("Charlie", 32, Developer)

for( person <- List(alice, bob, charlie) ) {
  person match {
    case (id, p @ Person(_, _, Manager)) => println("%s is overpaid".format(p))
    case (id, p @ Person(_, _, _)) => println("%s is underpaid".format(p))
  }
}

I am getting following errors:

match-deep.scala:13: error: constructor cannot be instantiated to expected type;
 found   : (T1, T2)
 required: this.Person
    case (id, p @ Person(_, _, Manager)) => println("%s is overpaid".format(p))
         ^
match-deep.scala:13: error: not found: value p
    case (id, p @ Person(_, _, Manager)) => println("%s is overpaid".format(p))
                                                                            ^
match-deep.scala:14: error: constructor cannot be instantiated to expected type;
 found   : (T1, T2)
 required: this.Person
    case (id, p @ Person(_, _, _)) => println("%s is underpaid".format(p))
         ^
match-deep.scala:14: error: not found: value p
    case (id, p @ Person(_, _, _)) => println("%s is underpaid".format(p))

What am I doing wrong here?

Any idea what is the purpose of adding 'id' to the case statement ? Perhaps it has something to do with old Scala syntax? — tuxdna, Feb 25 '13 at 07:47

Eastsun · Accepted Answer · 2013-02-25T08:08:53.527

The error information is clear

for( person <- List(alice, bob, charlie) ) {
  person match {
    case p @ Person(_, _, Manager) => println("%s is overpaid".format(p.toString))
    case p @ Person(_, _, _) => println("%s is underpaid".format(p.toString))
  }
}

Here is a short way to do the same thing:

for(p @ Person(_, _, role) <- List(alice, bob, charlie) ) {
  if(role == Manager) println("%s is overpaid".format(p.toString))
  else println("%s is underpaid".format(p.toString))
}

EDIT I am not sure what the real mean of id you wanted, I suppose it is the index of the person in the list. Here we go:

scala> for((p@Person(_,_,role), id) <- List(alice, bob, charlie).zipWithIndex ) {
     |   if(role == Manager) printf("%dth person is overpaid\n", id)
     |   else printf("Something else\n")
     | }
Something else
1th person is overpaid
Something else

Hello Eastsun! Thanks for your answer :) Please also check my comment in the question posted above? — tuxdna, Feb 25 '13 at 07:48

korefn · Answer 2 · 2013-02-21T11:14:57.313

The error is because your matching on person not a tuple of (id,person)

person match{
  case p @ Person(_, _, Manager)) => println("%s is overpaid".format(p)
  case p : Person => println("%s is underpaid".format(p.toString))
}

This should be enough.

EDIT

You could adapt this to use foreach easily

List(alice, bob, charlie) foreach {
    case p@Person(_,_,Manager)=>println("%s is overpaid".format(p);
    case p:Person =>println("%s is underpaid".format(p.toString)) 
}

Constructor cannot be instantiated to expected type; p @ Person

2 Answers2