Packages

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 _ => 1/0 }

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 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
val isEven: PartialFunction[Int, String] = {
  case x if x % 2 == 0 => x+" is even"
}

// the method collect can use isDefinedAt to select which members to collect
val evenNumbers = sample collect isEven

val isOdd: PartialFunction[Int, String] = {
  case x if x % 2 == 1 => x+" is odd"
}

// the method orElse allows chaining another partial function to handle
// input outside the declared domain
val numbers = sample map (isEven orElse isOdd)
Self Type
PartialFunction[A, B]
Source
PartialFunction.scala
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

 

Linear Supertypes
(A) => B, AnyRef, Any
Type Hierarchy
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. PartialFunction
  2. Function1
  3. AnyRef
  4. Any
Implicitly
  1. by UnliftOps
  2. by any2stringadd
  3. by StringFormat
  4. by Ensuring
  5. by ArrowAssoc
  1. Hide All
  2. Show All
Visibility
  1. Public
  2. Protected

Abstract Value Members

  1. abstract def apply(v1: A): B

    Apply the body of this function to the argument.

    Apply the body of this function to the argument.

    returns

    the result of function application.

    Definition Classes
    Function1
  2. abstract def isDefinedAt(x: A): Boolean

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

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

    x

    the value to test

    returns

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

Concrete Value Members

  1. def andThen[C](k: PartialFunction[B, C]): PartialFunction[A, C]

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

    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. It is highly recommended to call applyOrElse instead of isDefinedAt / apply for efficiency.

    C

    the result type of the transformation function.

    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)).

  2. def andThen[C](k: (B) => C): PartialFunction[A, C]

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

    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).

    C

    the result type of the transformation function.

    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
    PartialFunctionFunction1
  3. def applyOrElse[A1 <: A, B1 >: B](x: A1, default: (A1) => B1): B1

    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.

    x

    the function argument

    default

    the fallback function

    returns

    the result of this function or fallback function application.

  4. def compose[R](k: PartialFunction[R, A]): PartialFunction[R, B]

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

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

    Note that calling isDefinedAt on the resulting partial function may apply the first partial function and execute its side effect. It is highly recommended to call applyOrElse instead of isDefinedAt / apply for efficiency.

    R

    the parameter type of the transformation function.

    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)).

  5. def compose[A](g: (A) => A): (A) => B

    Composes two instances of Function1 in a new Function1, with this function applied last.

    Composes two instances of Function1 in a new Function1, with this function applied last.

    A

    the type to which function g can be applied

    g

    a function A => T1

    returns

    a new function f such that f(x) == apply(g(x))

    Definition Classes
    Function1
    Annotations
    @unspecialized()
  6. def elementWise: ElementWiseExtractor[A, B]

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

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

    Example:
    1. 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
      }
  7. def lift: (A) => Option[B]

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

    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

  8. def orElse[A1 <: A, B1 >: B](that: PartialFunction[A1, B1]): PartialFunction[A1, B1]

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

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

    A1

    the argument type of the fallback function

    B1

    the result type of the fallback function

    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.

  9. def runWith[U](action: (B) => U): (A) => Boolean

    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.

    action

    the action function

    returns

    a function which maps arguments x to isDefinedAt(x). The resulting function runs action(this(x)) where this is defined.

    See also

    applyOrElse.

  10. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    Function1 → AnyRef → Any
  11. def unapply(a: A): Option[B]

    Tries to extract a B from an A in a pattern matching expression.

  12. def unlift: PartialFunction[A, B]

    Converts an optional function to a partial function.

    Converts an optional function to a partial function.

    Implicit
    This member is added by an implicit conversion from PartialFunction[A, B] toUnliftOps[A, B] performed by method UnliftOps in scala.Function1.This conversion will take place only if B is a subclass of Option[B] (B <: Option[B]).
    Definition Classes
    UnliftOps
    Example:
    1. Unlike Function.unlift, this UnliftOps.unlift method can be used in extractors.

      val of: Int => Option[String] = { i =>
        if (i == 2) {
          Some("matched by an optional function")
        } else {
          None
        }
      }
      
      util.Random.nextInt(4) match {
        case of.unlift(m) => // Convert an optional function to a pattern
          println(m)
        case _ =>
          println("Not matched")
      }