/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2002-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala
/** 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 }
* }}}
*
* 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)
* }}}
*
*
* @author Martin Odersky
* @version 1.0, 16/07/2003
*/
trait PartialFunction[-A, +B] extends (A => B) {
/** Checks if a value is contained in the function's domain.
*
* @param x the value to test
* @return `'''true'''`, iff `x` is in the domain of this function, `'''false'''` otherwise.
*/
def isDefinedAt(x: A): Boolean
//protected def missingCase[A1 <: A, B1 >: B]: PartialFunction[A1, B1] = PartialFunction.empty
protected def missingCase(x: A): B = throw new MatchError(x)
/** Composes this partial function with a fallback partial function which
* gets applied where this partial function is not defined.
*
* @param that the fallback function
* @tparam A1 the argument type of the fallback function
* @tparam B1 the result type of the fallback function
* @return 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.
*/
def orElse[A1 <: A, B1 >: B](that: PartialFunction[A1, B1]) : PartialFunction[A1, B1] =
new runtime.AbstractPartialFunction[A1, B1] {
def _isDefinedAt(x: A1): Boolean =
PartialFunction.this.isDefinedAt(x) || that.isDefinedAt(x)
def apply(x: A1): B1 =
if (PartialFunction.this.isDefinedAt(x)) PartialFunction.this.apply(x)
else that.apply(x)
}
def orElseFast[A1 <: A, B1 >: B](that: PartialFunction[A1, B1]) : PartialFunction[A1, B1] =
orElse(that)
/** Composes this partial function with a transformation function that
* gets applied to results of this partial function.
* @param k the transformation function
* @tparam C the result type of the transformation function.
* @return a partial function with the same domain as this partial function, which maps
* arguments `x` to `k(this(x))`.
*/
override def andThen[C](k: B => C) : PartialFunction[A, C] = new runtime.AbstractPartialFunction[A, C] {
def _isDefinedAt(x: A): Boolean = PartialFunction.this.isDefinedAt(x)
def apply(x: A): C = k(PartialFunction.this.apply(x))
}
/** Turns this partial function into an plain function returning an `Option` result.
* @see Function.unlift
* @return a function that takes an argument `x` to `Some(this(x))` if `this`
* is defined for `x`, and to `None` otherwise.
*/
def lift: A => Option[B] = new (A => Option[B]) {
def apply(x: A): Option[B] = if (isDefinedAt(x)) Some(PartialFunction.this.apply(x)) else None
}
}
/** A few handy operations which leverage the extra bit of information
* available in partial functions. Examples:
* {{{
* import PartialFunction._
*
* def strangeConditional(other: Any): Boolean = cond(other) {
* case x: String if x == "abc" || x == "def" => true
* case x: Int => true
* }
* def onlyInt(v: Any): Option[Int] = condOpt(v) { case x: Int => x }
* }}}
*
* @author Paul Phillips
* @since 2.8
*/
object PartialFunction {
private[this] final val empty_pf: PartialFunction[Any, Nothing] = new runtime.AbstractPartialFunction[Any, Nothing] {
def _isDefinedAt(x: Any) = false
override def isDefinedAt(x: Any) = false
def apply(x: Any): Nothing = throw new MatchError(x)
override def orElse[A1, B1](that: PartialFunction[A1, B1]): PartialFunction[A1, B1] = that
override def orElseFast[A1, B1](that: PartialFunction[A1, B1]): PartialFunction[A1, B1] = that
override def lift = (x: Any) => None
}
def empty[A, B] : PartialFunction[A, B] = empty_pf
/** Creates a Boolean test based on a value and a partial function.
* It behaves like a 'match' statement with an implied 'case _ => false'
* following the supplied cases.
*
* @param x the value to test
* @param pf the partial function
* @return true, iff `x` is in the domain of `pf` and `pf(x) == true`.
*/
def cond[T](x: T)(pf: PartialFunction[T, Boolean]): Boolean =
(pf isDefinedAt x) && pf(x)
/** Transforms a PartialFunction[T, U] `pf` into Function1[T, Option[U]] `f`
* whose result is `Some(x)` if the argument is in `pf`'s domain and `None`
* otherwise, and applies it to the value `x`. In effect, it is a
* `'''match'''` statement which wraps all case results in `Some(_)` and
* adds `'''case''' _ => None` to the end.
*
* @param x the value to test
* @param pf the PartialFunction[T, U]
* @return `Some(pf(x))` if `pf isDefinedAt x`, `None` otherwise.
*/
def condOpt[T,U](x: T)(pf: PartialFunction[T, U]): Option[U] =
if (pf isDefinedAt x) Some(pf(x)) else None
}
