Scala Either : simplest way to get a property that exists on right and left

Question

I have a template using a valueObject that might be one of two flavours depending on where it is used in our app. So I am importing it as an Either:

valueObject: Either[ ObjectA, ObjectB ]

Both objects have an identically named property on them so I would like to retrieve it just by calling

valueObject.propertyA

Which doesn't work.

What is the most concise/ best way of doing this?

Do the two object share a common superclass / trait with propertyA defined? or do they just "happen" to have that common property? — Tzach Zohar, Jul 06 '16 at 11:05
They extend a common trait. The common property is however not defined on the trait - there is a third child that doesn't require it - but if that solves my problem elegantly I could include it there. — xeno.be, Jul 06 '16 at 11:09

score 10 · Accepted Answer · answered Jul 06 '16 at 11:12

Assuming the two objects have the same type (or a supertype / trait) that defines that property - you can use merge which returns left if it exists and right otherwise, with the lowest common type of both:

scala> class MyClass {
 | def propertyA = 1
 | }
defined class MyClass

scala> val e1: Either[MyClass, MyClass] = Left(new MyClass)
e1: Either[MyClass,MyClass] = Left(MyClass@1e51abf)

scala> val e2: Either[MyClass, MyClass] = Right(new MyClass)
e2: Either[MyClass,MyClass] = Right(MyClass@b4c6d0)

scala> e1.merge.propertyA
res0: Int = 1

scala> e2.merge.propertyA
res1: Int = 1

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

Using `fold`

Assuming the two objects do not share a common supertype that holds the property/method, then you have to resort to fold:

scala> case class A(a: Int)
defined class A

scala> case class B(a: Int)
defined class B

scala> def foldAB(eab: Either[A,B]): Int = eab.fold(_.a,_.a)
foldAB: (eab: Either[A,B])Int
   
scala> foldAB(Left(A(1)))
res1: Int = 1

scala> foldAB(Right(B(1)))
res2: Int = 1

Pattern matching

Another possibility is to use pattern matching:

scala> def matchAB(eab: Either[A,B]): Int = eab match { case Left(A(i)) => i; case Right(B(i)) => i}
matchAB: (eab: Either[A,B])Int

scala> matchAB(Left(A(1)))
res3: Int = 1

scala> matchAB(Right(B(1)))
res4: Int = 1

Scala Either : simplest way to get a property that exists on right and left

2 Answers2

Using fold

Pattern matching

Using `fold`