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:

valdoubledOdds:PartialFunction[Int, Int]={caseiifi%2==1=>i*2}

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

doubledOdds.isDefinedAt(3)// truedoubledOdds.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:

valres=List(1,2,3).collect({caseiifi%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:

valincrementedEvens:PartialFunction[Int, Int]={caseiifi%2==0=>i+1}valres2=List(1,2,3).collect(doubledOdds.orElse(incrementedEvens))// List(2, 3, 6)