scala

# PartialFunction

### Related Docs: object PartialFunction | package scala

#### 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
Version

1.0, 16/07/2003

Linear Supertypes
(A) ⇒ B, AnyRef, Any
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Inherited
1. PartialFunction
2. Function1
3. AnyRef
4. Any
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2. by StringFormat
3. by Ensuring
4. by ArrowAssoc
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Visibility
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### 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. #### final def !=(arg0: Any): Boolean

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): String

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to any2stringadd[PartialFunction[A, B]] performed by method any2stringadd in scala.Predef.
Definition Classes
4. #### def ->[B](y: B): (PartialFunction[A, B], B)

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to ArrowAssoc[PartialFunction[A, B]] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc
Annotations
()
5. #### final def ==(arg0: Any): Boolean

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)`.

arg0

the object to compare against this object for equality.

returns

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

Definition Classes
AnyRef → Any
6. #### 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.

C

the result type of the transformation function.

k

the transformation function

returns

a partial function with the same domain as this partial function, which maps arguments `x` to `k(this(x))`.

Definition Classes
PartialFunctionFunction1
7. #### 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.

Since

2.10

8. #### 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`.

9. #### def clone(): AnyRef

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[java.lang]
Definition Classes
AnyRef
Annotations
( ... )
Note

not specified by SLS as a member of AnyRef

10. #### 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
()
11. #### def ensuring(cond: (PartialFunction[A, B]) ⇒ Boolean, msg: ⇒ Any): PartialFunction[A, B]

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to Ensuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
12. #### def ensuring(cond: (PartialFunction[A, B]) ⇒ Boolean): PartialFunction[A, B]

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to Ensuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
13. #### def ensuring(cond: Boolean, msg: ⇒ Any): PartialFunction[A, B]

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to Ensuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
14. #### def ensuring(cond: Boolean): PartialFunction[A, B]

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to Ensuring[PartialFunction[A, B]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
15. #### final def eq(arg0: AnyRef): Boolean

Tests whether the argument (`arg0`) is a reference to the receiver object (`this`).

Tests whether the argument (`arg0`) 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
16. #### def equals(arg0: Any): Boolean

The equality method for reference types.

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

See also `equals` in scala.Any.

returns

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

Definition Classes
AnyRef → Any
17. #### 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[java.lang]
Definition Classes
AnyRef
Annotations
( classOf[java.lang.Throwable] )
Note

not specified by SLS as a member of AnyRef

18. #### def formatted(fmtstr: String): 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 information
This member is added by an implicit conversion from PartialFunction[A, B] to StringFormat[PartialFunction[A, B]] performed by method StringFormat in scala.Predef.
Definition Classes
StringFormat
Annotations
()
19. #### final def getClass(): Class[_]

A representation that corresponds to the dynamic class of the receiver object.

A representation that corresponds to the dynamic class of the receiver object.

The nature of the representation is platform dependent.

returns

a representation that corresponds to the dynamic class of the receiver object.

Definition Classes
AnyRef → Any
Note

not specified by SLS as a member of AnyRef

20. #### 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
21. #### final def isInstanceOf[T0]: Boolean

Test whether the dynamic type of the receiver object is `T0`.

Test whether the dynamic type of the receiver object is `T0`.

Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression `1.isInstanceOf[String]` will return `false`, while the expression `List(1).isInstanceOf[List[String]]` will return `true`. 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 specified type.

returns

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

Definition Classes
Any
22. #### 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

23. #### final def ne(arg0: AnyRef): Boolean

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
24. #### 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
Note

not specified by SLS as a member of AnyRef

25. #### 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
Note

not specified by SLS as a member of AnyRef

26. #### 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.

27. #### 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.

Since

2.10

`applyOrElse`.

28. #### final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
29. #### 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
30. #### final def wait(): Unit

Definition Classes
AnyRef
Annotations
( ... )
31. #### final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
( ... )
32. #### final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
( ... )
33. #### def →[B](y: B): (PartialFunction[A, B], B)

Implicit information
This member is added by an implicit conversion from PartialFunction[A, B] to ArrowAssoc[PartialFunction[A, B]] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc