Partial Functions

A partial function is a function that may not be defined for all values of its argument type. In Scala, partial functions are unary functions implementing the PartialFunction[A, B] trait, where A is the argument type and B the result type.

To define a partial function, use a case identical to those used in match expressions:

val doubledOdds: PartialFunction[Int, Int] = {
  case i if i % 2 == 1 => i * 2
}

To check if a partial function is defined for an argument, use the isDefinedAt method:

doubledOdds.isDefinedAt(3)  // true
doubledOdds.isDefinedAt(4)  // false

Trying to apply a partial function to an argument not belonging to its domain results in MatchError:

doubledOdds(4)  // Exception in thread "main" scala.MatchError: 4

Using partial functions

A partial function can be passed as an argument to a method:

val res = List(1, 2, 3).collect({ case i if i % 2 == 1 => i * 2 }) // List(2, 6)

You can define a default value for arguments not in domain with applyOrElse:

doubledOdds.applyOrElse(4, _ + 1)  // 5

Two partial function can be composed with orElse, the second function will be applied for arguments where the first one is not defined:

val incrementedEvens: PartialFunction[Int, Int] = {
  case i if i % 2 == 0 => i + 1
}

val res2 = List(1, 2, 3).collect(doubledOdds.orElse(incrementedEvens)) // List(2, 3, 6)