scala.PartialFunction
See thePartialFunction companion objectA 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]"))
Optional Functions, PartialFunctions and extractor objects can be converted to each other as shown in the following table.
trait
MapOps[
K,
V,
CC,
C]
trait
MapOps[
K,
V,
CC,
C]
trait
MapOps[
K,
V,
CC,
C]
Show allChecks if a value is contained in the function's domain.
Checks if a value is contained in the function's domain.
Value parametersthe value to test
true, iff x is in the domain of this function, false otherwise.
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).
the result type of the transformation function.
the transformation function
a partial function with the domain of this partial function, possibly narrowed by the specified function, which maps arguments x to k(this(x)).
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.
Type parametersthe result type of the transformation function.
the transformation function
a partial function with the domain of this partial function narrowed by other partial function, which maps arguments x to k(this(x)).
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.
the fallback function
the function argument
the result of this function or fallback function application.
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.
Type parametersthe parameter type of the transformation function.
the transformation function
a partial function with the domain of other partial function narrowed by this partial function, which maps arguments x to this(k(x)).
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.
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
}
Turns this partial function into a plain function returning an Option result.
Turns this partial function into a plain function returning an Option result.
a function that takes an argument x to Some(this(x)) if this is defined for x, and to None otherwise.
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.
Type parametersthe argument type of the fallback function
the result type of the fallback function
the fallback function
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.
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.
the action function
a function which maps arguments x to isDefinedAt(x). The resulting function runs action(this(x)) where this is defined.
Tries to extract a B from an A in a pattern matching expression.
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.
the type to which function g can be applied
a function A => T1
a new function f such that f(x) == apply(g(x))
Returns a string representation of the object.
Returns a string representation of the object.
The default representation is platform dependent.
Attributesa string representation of the object.
Apply the body of this function to the argument.
Apply the body of this function to the argument.
Attributesthe result of function application.
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