t

scala

PartialFunction

Companion object PartialFunction

traitPartialFunction[-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]"))
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

from optional Function

from an extractor

{ case extractor(x) => x }

extractor.unapply _

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
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)

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. final def !=(arg0: Any)

Test two objects for inequality.

Test two objects for inequality.

returns

true if !(this == that), false otherwise.

Definition Classes
AnyRef → Any
2. final def ##: Int

Equivalent to x.hashCode except for boxed numeric types and null.

Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

returns

a hash value consistent with ==

Definition Classes
AnyRef → Any
3. def +(other: String)
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toany2stringadd[PartialFunction[A, B]] performed by method any2stringadd in scala.Predef.
Definition Classes
4. def ->[B](y: B): (PartialFunction[A, B], B)
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toArrowAssoc[PartialFunction[A, B]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if B is a subclass of Option[Nothing] (B <: Option[Nothing]).
Definition Classes
ArrowAssoc
Annotations
()
5. final def ==(arg0: Any)

The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

returns

true if the receiver object is equivalent to the argument; false otherwise.

Definition Classes
AnyRef → Any
6. 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. For efficiency, it is recommended to call applyOrElse instead of isDefinedAt or apply.

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)).

7. 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
8. 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.

9. final def asInstanceOf[T0]: T0

Cast the receiver object to be of type T0.

Cast the receiver object to be of type T0.

Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

returns

Definition Classes
Any
Exceptions thrown

ClassCastException if the receiver object is not an instance of the erasure of type T0.

10. def clone()

Create a copy of the receiver object.

Create a copy of the receiver object.

The default implementation of the clone method is platform dependent.

returns

a copy of the receiver object.

Attributes
protected[lang]
Definition Classes
AnyRef
Annotations
(classOf[java.lang.CloneNotSupportedException]) ()
Note

not specified by SLS as a member of AnyRef

11. 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. For efficiency, it is recommended to call applyOrElse instead of isDefinedAt or apply.

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)).

12. 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
()
13. 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
}
14. def ensuring(cond: (PartialFunction[A, B]) => Boolean, msg: => Any): PartialFunction[A, B]
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toEnsuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
15. def ensuring(cond: (PartialFunction[A, B]) => Boolean): PartialFunction[A, B]
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toEnsuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
16. def ensuring(cond: Boolean, msg: => Any): PartialFunction[A, B]
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toEnsuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
17. def ensuring(cond: Boolean): PartialFunction[A, B]
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toEnsuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
18. final def eq(arg0: AnyRef)

Tests whether the argument (that) is a reference to the receiver object (this).

Tests whether the argument (that) is a reference to the receiver object (this).

The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

• It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
• For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
• null.eq(null) returns true.

When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

returns

true if the argument is a reference to the receiver object; false otherwise.

Definition Classes
AnyRef
19. def equals(arg0: AnyRef)

The equality method for reference types.

The equality method for reference types. Default implementation delegates to eq.

returns

true if the receiver object is equivalent to the argument; false otherwise.

Definition Classes
AnyRef → Any
20. def finalize(): Unit

Called by the garbage collector on the receiver object when there are no more references to the object.

Called by the garbage collector on the receiver object when there are no more references to the object.

The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

Attributes
protected[lang]
Definition Classes
AnyRef
Annotations
(classOf[java.lang.Throwable])
Note

not specified by SLS as a member of AnyRef

21. final def getClass(): Class[_ <: AnyRef]

Returns the runtime class representation of the object.

Returns the runtime class representation of the object.

returns

a class object corresponding to the runtime type of the receiver.

Definition Classes
AnyRef → Any
Annotations
()
22. def hashCode(): Int

The hashCode method for reference types.

The hashCode method for reference types. See hashCode in scala.Any.

returns

the hash code value for this object.

Definition Classes
AnyRef → Any
Annotations
()
23. final def isInstanceOf[T0]

Test whether the dynamic type of the receiver object has the same erasure as T0.

Test whether the dynamic type of the receiver object has the same erasure as T0.

Depending on what T0 is, the test is done in one of the below ways:

• T0 is a non-parameterized class type, e.g. BigDecimal: this method returns true if the value of the receiver object is a BigDecimal or a subtype of BigDecimal.
• T0 is a parameterized class type, e.g. List[Int]: this method returns true if the value of the receiver object is some List[X] for any X. For example, List(1, 2, 3).isInstanceOf[List[String]] will return true.
• T0 is some singleton type x.type or literal x: this method returns this.eq(x). For example, x.isInstanceOf is equivalent to x.eq(1)
• T0 is an intersection X with Y or X & Y: this method is equivalent to x.isInstanceOf[X] && x.isInstanceOf[Y]
• T0 is a union X | Y: this method is equivalent to x.isInstanceOf[X] || x.isInstanceOf[Y]
• T0 is a type parameter or an abstract type member: this method is equivalent to isInstanceOf[U] where U is T0's upper bound, Any if T0 is unbounded. For example, x.isInstanceOf[A] where A is an unbounded type parameter will return true for any value of x.

This is exactly equivalent to the type pattern _: T0

returns

true if the receiver object is an instance of erasure of type T0; false otherwise.

Definition Classes
Any
Note

due to the unexpectedness of List(1, 2, 3).isInstanceOf[List[String]] returning true and x.isInstanceOf[A] where A is a type parameter or abstract member returning true, these forms issue a warning.

24. 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.

Function.unlift

25. final def ne(arg0: AnyRef)

Equivalent to !(this eq that).

Equivalent to !(this eq that).

returns

true if the argument is not a reference to the receiver object; false otherwise.

Definition Classes
AnyRef
26. final def notify(): Unit

Wakes up a single thread that is waiting on the receiver object's monitor.

Wakes up a single thread that is waiting on the receiver object's monitor.

Definition Classes
AnyRef
Annotations
()
Note

not specified by SLS as a member of AnyRef

27. final def notifyAll(): Unit

Wakes up all threads that are waiting on the receiver object's monitor.

Wakes up all threads that are waiting on the receiver object's monitor.

Definition Classes
AnyRef
Annotations
()
Note

not specified by SLS as a member of AnyRef

28. 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.

29. 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.

applyOrElse.

30. final def synchronized[T0](arg0: => T0): T0

Executes the code in body with an exclusive lock on this.

Executes the code in body with an exclusive lock on this.

returns

the result of body

Definition Classes
AnyRef
31. def toString()

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
32. def unapply(a: A): Option[B]

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

33. 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")
}
34. final def wait(): Unit
Definition Classes
AnyRef
Annotations
(classOf[java.lang.InterruptedException])
Note

not specified by SLS as a member of AnyRef

35. final def wait(arg0: Long, arg1: Int): Unit
Definition Classes
AnyRef
Annotations
(classOf[java.lang.InterruptedException])
Note

not specified by SLS as a member of AnyRef

36. final def wait(arg0: Long): Unit
Definition Classes
AnyRef
Annotations
(classOf[java.lang.InterruptedException]) ()
Note

not specified by SLS as a member of AnyRef

Deprecated Value Members

1. def formatted(fmtstr: String)

Returns string formatted according to given format string.

Returns string formatted according to given format string. Format strings are as for String.format (@see java.lang.String.format).

Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toStringFormat[PartialFunction[A, B]] performed by method StringFormat in scala.Predef.
Definition Classes
StringFormat
Annotations
()
Deprecated

(Since version 2.12.16) Use formatString.format(value) instead of value.formatted(formatString), or use the f"" string interpolator. In Java 15 and later, formatted resolves to the new method in String which has reversed parameters.

2. def [B](y: B): (PartialFunction[A, B], B)
Implicit
This member is added by an implicit conversion from PartialFunction[A, B] toArrowAssoc[PartialFunction[A, B]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if B is a subclass of Option[Nothing] (B <: Option[Nothing]).
Definition Classes
ArrowAssoc
Annotations
Deprecated

(Since version 2.13.0) Use -> instead. If you still wish to display it as one character, consider using a font with programming ligatures such as Fira Code.