CODEWITHSUNDEEP
scala
Manel
Manel

Reputation: 1654

fpinscala book : Error Handling with Option Type

In the book "Functional programming in Scala", Chapter 4 page 55 :

We have the code below :

case class Employee(name: String, department: String)

def lookupByName(name: String): Option[Employee] = ???

// Use of "map"
lookupByName("Joe").map(_.departement) 

// Use of "flatMap"
lookupByName("Joe").flatMap(_.manager)

For "Map" exemple side note say we have two results cases :

  1. Joe's dept if Joe is an employee
  2. None if Joe is not an employee

But for me response 1 is false. The function is not returning the department (= a string) but an Some(department) (= an Option).

Am i right or wrong ?

For "flatMap" exemple side note say we have two results cases :

  1. Some(manager) if Joe has a manager
  2. None if Joe is not an employee or doesn't have a manager

Problem 1 : But for me both responses are false because "manager" is not part of Employee's type so the program is not even compiling.

Problem 2 : Event if i add "manager: String" as part of the Employee type i can't even write lookupByName("Joe").flatMap(_.manager) in Intellij cause i get a "Type mismatch". I must write lookupByName("Joe").flatMap(x => Some(x.manager))

Problem 3 : When i write the function as lookupByName("Joe").flatMap(x => Some(x.manager)) to make my program compile then map and flatMap here have exactly the same results

What am i not understanding here ? What am i doing wrong ?

The resulting here is that i don't get the difference between map and flatMap from those exemples.

Upvotes: 3

Views: 239

Answers (1)

ZhekaKozlov
ZhekaKozlov

Reputation: 39536

Map

You are absolutely right. The return type of lookupByName("Joe").map(_.department) is Option[String] (not String). So, indeed the first case returns Some(<department>).

FlatMap

You are right as well. It seems that the authors forgot to declare a third field in the Employee class. The correct class declaration should be:

case class Employee(name: String, department: String, manager: Option[Employee])

Upvotes: 1

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