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]"))
- 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 _
- Alphabetic
- By Inheritance
- PartialFunction
- Function1
- AnyRef
- Any
- by UnliftOps
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
- Hide All
- Show All
- Public
- Protected
Abstract Value Members
- 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
- 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, iffxis in the domain of this function,falseotherwise.
Concrete Value Members
- final def !=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
- returns
trueif !(this == that), false otherwise.
- Definition Classes
- AnyRef → Any
- final def ##: Int
Equivalent to
x.hashCodeexcept for boxed numeric types andnull.Equivalent to
x.hashCodeexcept for boxed numeric types andnull. 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. Fornullreturns a hashcode wherenull.hashCodethrows aNullPointerException.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
- def +(other: String): 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
- any2stringadd
- 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
- @inline()
- final def ==(arg0: Any): Boolean
The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).- returns
trueif the receiver object is equivalent to the argument;falseotherwise.
- Definition Classes
- AnyRef → Any
- 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
xtok(this(x)).
- 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
PartialFunctionthen the otherandThenmethod 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
xtok(this(x)).
- Definition Classes
- PartialFunction → Function1
- 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 toif(pf isDefinedAt x) pf(x) else default(x)
except that
applyOrElsemethod can be implemented more efficiently. For all partial function literals the compiler generates anapplyOrElseimplementation which avoids double evaluation of pattern matchers and guards. This makesapplyOrElsethe basis for the efficient implementation for many operations and scenarios, such as:- combining partial functions into
orElse/andThenchains does not lead to excessiveapply/isDefinedAtevaluation liftandunliftdo not evaluate source functions twice on each invocationrunWithallows efficient imperative-style combining of partial functions with conditionally applied actions
For non-literal partial function classes with nontrivial
isDefinedAtmethod it is recommended to overrideapplyOrElsewith custom implementation that avoids doubleisDefinedAtevaluation. 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.
- combining partial functions into
- 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 aClassCastExceptionat runtime, while the expressionList(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
the receiver object.
- Definition Classes
- Any
- Exceptions thrown
ClassCastExceptionif the receiver object is not an instance of the erasure of typeT0.
- def clone(): AnyRef
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the
clonemethod is platform dependent.- returns
a copy of the receiver object.
- def compose[R](k: PartialFunction[R, A]): PartialFunction[R, B]
Composes another partial function
kwith this partial function so that this partial function gets applied to results ofk.Composes another partial function
kwith this partial function so that this partial function gets applied to results ofk.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
xtothis(k(x)).
- 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
gcan be applied- g
a function A => T1
- returns
a new function
fsuch thatf(x) == apply(g(x))
- Definition Classes
- Function1
- Annotations
- @unspecialized()
- def elementWise: ElementWiseExtractor[A, B]
Returns an extractor object with a
unapplySeqmethod, which extracts each element of a sequence data.Returns an extractor object with a
unapplySeqmethod, which extracts each element of a sequence data.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 }
Example: - 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
- 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
- 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
- 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
- final def eq(arg0: AnyRef): Boolean
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
eqmethod implements an equivalence relation on non-null instances ofAnyRef, and has three additional properties:- It is consistent: for any non-null instances
xandyof typeAnyRef, multiple invocations ofx.eq(y)consistently returnstrueor consistently returnsfalse. - For any non-null instance
xof typeAnyRef,x.eq(null)andnull.eq(x)returnsfalse. null.eq(null)returnstrue.
When overriding the
equalsorhashCodemethods, 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
trueif the argument is a reference to the receiver object;falseotherwise.
- Definition Classes
- AnyRef
- It is consistent: for any non-null instances
- def equals(arg0: AnyRef): Boolean
The equality method for reference types.
- 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
finalizemethod is invoked, as well as the interaction betweenfinalizeand non-local returns and exceptions, are all platform dependent.- Attributes
- protected[lang]
- Definition Classes
- AnyRef
- Annotations
- @throws(classOf[java.lang.Throwable])
- Note
not specified by SLS as a member of AnyRef
- final def getClass(): Class[_ <: AnyRef]
Returns the runtime class representation of the object.
- def hashCode(): Int
The hashCode method for reference types.
- final def isInstanceOf[T0]: Boolean
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
T0is, the test is done in one of the below ways:T0is a non-parameterized class type, e.g.BigDecimal: this method returnstrueif the value of the receiver object is aBigDecimalor a subtype ofBigDecimal.T0is a parameterized class type, e.g.List[Int]: this method returnstrueif the value of the receiver object is someList[X]for anyX. For example,List(1, 2, 3).isInstanceOf[List[String]]will return true.T0is some singleton typex.typeor literalx: this method returnsthis.eq(x). For example,x.isInstanceOf[1]is equivalent tox.eq(1)T0is an intersectionX with YorX & Y: this method is equivalent tox.isInstanceOf[X] && x.isInstanceOf[Y]T0is a unionX | Y: this method is equivalent tox.isInstanceOf[X] || x.isInstanceOf[Y]T0is a type parameter or an abstract type member: this method is equivalent toisInstanceOf[U]whereUisT0's upper bound,AnyifT0is unbounded. For example,x.isInstanceOf[A]whereAis an unbounded type parameter will return true for any value ofx.
This is exactly equivalent to the type pattern
_: T0- returns
trueif the receiver object is an instance of erasure of typeT0;falseotherwise.
- Definition Classes
- Any
- Note
due to the unexpectedness of
List(1, 2, 3).isInstanceOf[List[String]]returning true andx.isInstanceOf[A]whereAis a type parameter or abstract member returning true, these forms issue a warning.
- def lift: (A) => Option[B]
Turns this partial function into a plain function returning an
Optionresult.Turns this partial function into a plain function returning an
Optionresult.- returns
a function that takes an argument
xtoSome(this(x))ifthisis defined forx, and toNoneotherwise.
- See also
Function.unlift
- final def ne(arg0: AnyRef): Boolean
Equivalent to
!(this eq that).Equivalent to
!(this eq that).- returns
trueif the argument is not a reference to the receiver object;falseotherwise.
- Definition Classes
- AnyRef
- 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
- @native()
- Note
not specified by SLS as a member of AnyRef
- 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
- @native()
- Note
not specified by SLS as a member of AnyRef
- 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 takesxtothis(x)wherethisis defined, and tothat(x)where it is not.
- 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 toif(pf isDefinedAt x) { action(pf(x)); true } else false
except that
runWithis implemented viaapplyOrElseand thus potentially more efficient. UsingrunWithavoids double evaluation of pattern matchers and guards for partial function literals.- action
the action function
- returns
a function which maps arguments
xtoisDefinedAt(x). The resulting function runsaction(this(x))wherethisis defined.
- See also
applyOrElse.
- final def synchronized[T0](arg0: => T0): T0
Executes the code in
bodywith an exclusive lock onthis.Executes the code in
bodywith an exclusive lock onthis.- returns
the result of
body
- Definition Classes
- AnyRef
- def toString(): String
Creates a String representation of this object.
- def unapply(a: A): Option[B]
Tries to extract a
Bfrom anAin a pattern matching expression. - 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
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") }
Example: - final def wait(): Unit
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.
- Definition Classes
- AnyRef
- Annotations
- @throws(classOf[java.lang.InterruptedException])
- Note
not specified by SLS as a member of AnyRef
- final def wait(arg0: Long, arg1: Int): Unit
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-
- Definition Classes
- AnyRef
- Annotations
- @throws(classOf[java.lang.InterruptedException])
- Note
not specified by SLS as a member of AnyRef
- final def wait(arg0: Long): Unit
See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.
Deprecated Value Members
- def formatted(fmtstr: String): String
Returns string formatted according to given
formatstring.Returns string formatted according to given
formatstring. Format strings are as forString.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 @inline()
- Deprecated
(Since version 2.12.16) Use
formatString.format(value)instead ofvalue.formatted(formatString), or use thef""string interpolator. In Java 15 and later,formattedresolves to the new method in String which has reversed parameters.
- 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
- 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.
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int,Float,ArrayorOptionwhich are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collectionand its sub-packages contain Scala's collections frameworkscala.collection.immutable- Immutable, sequential data-structures such asVector,List,Range,HashMaporHashSetscala.collection.mutable- Mutable, sequential data-structures such asArrayBuffer,StringBuilder,HashMaporHashSetscala.collection.concurrent- Mutable, concurrent data-structures such asTrieMapscala.concurrent- Primitives for concurrent programming such asFuturesandPromisesscala.io- Input and output operationsscala.math- Basic math functions and additional numeric types likeBigIntandBigDecimalscala.sys- Interaction with other processes and the operating systemscala.util.matching- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect- Scala's reflection API (scala-reflect.jar)scala.xml- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel- Parallel collections (scala-parallel-collections.jar)scala.util.parsing- Parser combinators (scala-parser-combinators.jar)scala.swing- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predefobject are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
Listis an alias forscala.collection.immutable.List.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
Stringis an alias forjava.lang.String.