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
- Since
1.0
- Alphabetic
- By Inheritance
- PartialFunction
- Function1
- AnyRef
- Any
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
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- Public
- All
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
, iffx
is in the domain of this function,false
otherwise.
Concrete Value Members
-
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
tok(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
applyOrElse
method can be implemented more efficiently. For all partial function literals the compiler generates anapplyOrElse
implementation which avoids double evaluation of pattern matchers and guards. This makesapplyOrElse
the basis for the efficient implementation for many operations and scenarios, such as:- combining partial functions into
orElse
/andThen
chains does not lead to excessiveapply
/isDefinedAt
evaluation lift
andunlift
do not evaluate source functions twice on each invocationrunWith
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 overrideapplyOrElse
with custom implementation that avoids doubleisDefinedAt
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
- combining partial functions into
-
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 thatf(x) == apply(g(x))
- Definition Classes
- Function1
- Annotations
- @unspecialized()
-
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
toSome(this(x))
ifthis
is defined forx
, and toNone
otherwise.
- See also
Function.unlift
-
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 takesx
tothis(x)
wherethis
is 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
runWith
is implemented viaapplyOrElse
and thus potentially more efficient. UsingrunWith
avoids double evaluation of pattern matchers and guards for partial function literals.- action
the action function
- returns
a function which maps arguments
x
toisDefinedAt(x)
. The resulting function runsaction(this(x))
wherethis
is defined.
- Since
2.10
- See also
applyOrElse
.
-
def
toString(): String
Creates a String representation of this object.
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.collection.parallel.immutable
- Immutable, parallel data-structures such asParVector
,ParRange
,ParHashMap
orParHashSet
scala.collection.parallel.mutable
- Mutable, parallel data-structures such asParArray
,ParHashMap
,ParTrieMap
orParHashSet
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.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.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.