PartialFunction

trait PartialFunction[-A, +B] extends A => B

A partial function of type PartialFunction[A, B] is a unary function where the domain does not necessarily include all values of type A. The function isDefinedAt allows to test dynamically if a value is in the domain of the function.

Even if isDefinedAt returns true for an a: A, calling apply(a) may still throw an exception, so the following code is legal:

val f: PartialFunction[Int, Any] = { case x => x / 0 }   // ArithmeticException: / by zero

It is the responsibility of the caller to call isDefinedAt before calling apply, because if isDefinedAt is false, it is not guaranteed apply will throw an exception to indicate an error condition. If an exception is not thrown, evaluation may result in an arbitrary value.

The usual way to respect this contract is to call applyOrElse, which is expected to be more efficient than calling both isDefinedAt and apply.

The main distinction between PartialFunction and scala.Function1 is that the user of a PartialFunction may choose to do something different with input that is declared to be outside its domain. For example:

val sample = 1 to 10
def isEven(n: Int) = n % 2 == 0
val eveningNews: PartialFunction[Int, String] = {
  case x if isEven(x) => s"$x is even"
}

// The method collect is described as "filter + map"
// because it uses a PartialFunction to select elements
// to which the function is applied.
val evenNumbers = sample.collect(eveningNews)

val oddlyEnough: PartialFunction[Int, String] = {
  case x if !isEven(x) => s"$x is odd"
}

// The method orElse allows chaining another PartialFunction
// to handle input outside the declared domain.
val numbers = sample.map(eveningNews orElse oddlyEnough)

// same as
val numbers = sample.map(n => eveningNews.applyOrElse(n, oddlyEnough))

val half: PartialFunction[Int, Int] = {
  case x if isEven(x) => x / 2
}

// Calculating the domain of a composition can be expensive.
val oddByHalf = half.andThen(oddlyEnough)

// Invokes `half.apply` on even elements!
val oddBalls = sample.filter(oddByHalf.isDefinedAt)

// Better than filter(oddByHalf.isDefinedAt).map(oddByHalf)
val oddBalls = sample.collect(oddByHalf)

// Providing "default" values.
val oddsAndEnds = sample.map(n => oddByHalf.applyOrElse(n, (i: Int) => s"[$i]"))

Note:

Optional Functions, PartialFunctions and extractor objects can be converted to each other as shown in the following table.

How to convert ...	to a PartialFunction	to an optional Function	to an extractor
from a PartialFunction	Predef.identity	lift	Predef.identity
from optional Function	Function1.UnliftOps#unlift or Function.unlift	Predef.identity	Function1.UnliftOps#unlift
from an extractor	`{ case extractor(x) => x }`	`extractor.unapply _`	Predef.identity

Companion:

object

Source:

PartialFunction.scala

trait A => B

class Object

trait Matchable

class Any

trait MapOps[K, V, CC, C]

trait Map[K, V]

class AbstractMap[K, V]

class HashMap[K, V]

class IntMap[T]

class ListMap[K, V]

class LongMap[T]

class Map1[K, V]

class Map2[K, V]

class Map3[K, V]

class Map4[K, V]

class WithDefault[K, V]

class TreeMap[K, V]

class TreeSeqMap[K, V]

class VectorMap[K, V]

class AbstractMap[K, V]

class TrieMap[K, V]

class AnyRefMap[K, V]

class CollisionProofHashMap[K, V]

class HashMap[K, V]

class LinkedHashMap[K, V]

class ListMap[K, V]

class LongMap[V]

class WithDefault[K, V]

class OpenHashMap[Key, Value]

class TreeMap[K, V]

class SystemProperties

trait DefaultMap[K, V]

trait SeqMap[K, V]

trait SortedMap[K, V]

trait Map[K, V]

trait MultiMap[K, V]

trait MapFactoryDefaults[K, V, CC, WithFilterCC]

class WeakHashMap[K, V]

trait MapView[K, V]

class AbstractMapView[K, V]

class Filter[K, V]

class FilterKeys[K, V]

class Id[K, V]

class MapValues[K, V, W]

class TapEach[K, V, U]

trait SortedMapFactoryDefaults[K, V, CC, WithFilterCC, UnsortedCC]

trait SortedMapOps[K, V, CC, C]

trait StrictOptimizedSortedMapOps[K, V, CC, C]

trait SortedMapOps[K, V, CC, C]

trait StrictOptimizedMapOps[K, V, CC, C]

trait MapOps[K, V, CC, C]

trait Seq[A]

class AbstractSeq[A]

class ArraySeq[A]

class ofBoolean

class ofByte

class ofChar

class ofDouble

class ofFloat

class ofInt

class ofLong

class ofRef[T]

class ofShort

class ofUnit

class LazyList[A]

class List[A]

class ::[A]

object Nil.type

class NumericRange[T]

class Exclusive[T]

class Inclusive[T]

class Queue[A]

class Range

class Exclusive

class Inclusive

class Stream[A]

class Cons[A]

object Empty.type

class Vector[A]

class WrappedString

class AbstractSeq[A]

class AbstractBuffer[A]

class ArrayBuffer[A]

class ArrayDeque[A]

class Queue[A]

class Stack[A]

class ListBuffer[A]

class UnrolledBuffer[T]

class ArraySeq[T]

class ofBoolean

class ofByte

class ofChar

class ofDouble

class ofFloat

class ofInt

class ofLong

class ofRef[T]

class ofShort

class ofUnit

class StringBuilder

trait IndexedSeq[A]

trait IndexedSeq[T]

trait IndexedBuffer[A]

trait LinearSeq[A]

trait Seq[A]

trait Buffer[A]

class Accumulator[A, CC, C]

class AnyAccumulator[A]

class DoubleAccumulator

class IntAccumulator

class LongAccumulator

class AbstractPartialFunction[T1, R]

PartialFunction[A, B]

Value members

Abstract methods

Checks if a value is contained in the function's domain.

Value parameters:

x: the value to test

Returns:

true, iff x is in the domain of this function, false otherwise.

Source:

PartialFunction.scala

Concrete methods

Composes this partial function with a transformation function that gets applied to results of this partial function.

If the runtime type of the function is a PartialFunction then the other andThen method is used (note its cautions).

Type parameters:

C: the result type of the transformation function.

Value parameters:

k: the transformation function

Returns:

a partial function with the domain of this partial function, possibly narrowed by the specified function, which maps arguments x to k(this(x)).

Definition Classes

Function1

Source:

PartialFunction.scala

Composes this partial function with another partial function that gets applied to results of this partial function.

Note that calling isDefinedAt on the resulting partial function may apply the first partial function and execute its side effect. For efficiency, it is recommended to call applyOrElse instead of isDefinedAt or apply.

Type parameters:

C: the result type of the transformation function.

Value parameters:

k: the transformation function

Returns:

a partial function with the domain of this partial function narrowed by other partial function, which maps arguments x to k(this(x)).

Source:

PartialFunction.scala

Applies this partial function to the given argument when it is contained in the function domain.

Applies this partial function to the given argument when it is contained in the function domain. Applies fallback function where this partial function is not defined.

Note that expression pf.applyOrElse(x, default) is equivalent to

if(pf isDefinedAt x) pf(x) else default(x)

except that applyOrElse method can be implemented more efficiently. For all partial function literals the compiler generates an applyOrElse implementation which avoids double evaluation of pattern matchers and guards. This makes applyOrElse the basis for the efficient implementation for many operations and scenarios, such as:

- combining partial functions into orElse/andThen chains does not lead to excessive apply/isDefinedAt evaluation - lift and unlift do not evaluate source functions twice on each invocation - runWith allows efficient imperative-style combining of partial functions with conditionally applied actions

For non-literal partial function classes with nontrivial isDefinedAt method it is recommended to override applyOrElse with custom implementation that avoids double isDefinedAt evaluation. This may result in better performance and more predictable behavior w.r.t. side effects.

Value parameters:

default: the fallback function
x: the function argument

Returns:

the result of this function or fallback function application.

Source:

PartialFunction.scala

Composes another partial function k with this partial function so that this partial function gets applied to results of k.

Type parameters:

R: the parameter type of the transformation function.

Value parameters:

k: the transformation function

Returns:

a partial function with the domain of other partial function narrowed by this partial function, which maps arguments x to this(k(x)).

Source:

PartialFunction.scala

Returns an extractor object with a unapplySeq method, which extracts each element of a sequence data.

Example:

val firstChar: String => Option[Char] = _.headOption
Seq("foo", "bar", "baz") match {
  case firstChar.unlift.elementWise(c0, c1, c2) =>
    println(s"$c0, $c1, $c2") // Output: f, b, b
}

Source:

PartialFunction.scala

Turns this partial function into a plain function returning an Option result.

Returns:: a function that takes an argument x to Some(this(x)) if this is defined for x, and to None otherwise.
See also:: Function.unlift
Source:: PartialFunction.scala

Composes this partial function with a fallback partial function which gets applied where this partial function is not defined.

Type parameters:

A1: the argument type of the fallback function
B1: the result type of the fallback function

Value parameters:

that: the fallback function

Returns:

a partial function which has as domain the union of the domains of this partial function and that. The resulting partial function takes x to this(x) where this is defined, and to that(x) where it is not.

Source:

PartialFunction.scala

Composes this partial function with an action function which gets applied to results of this partial function.

Composes this partial function with an action function which gets applied to results of this partial function. The action function is invoked only for its side effects; its result is ignored.

Note that expression pf.runWith(action)(x) is equivalent to

if(pf isDefinedAt x) { action(pf(x)); true } else false

except that runWith is implemented via applyOrElse and thus potentially more efficient. Using runWith avoids double evaluation of pattern matchers and guards for partial function literals.

Value parameters: