Partial Functions(偏函数)

1.Definition

Partial functions are partial in the sense that they aren’t defined for all possible inputs, only those inputs that match at least one of the specified case clauses.

Only case clauses can be specified in a partial function and the entire function must be enclosed in curly braces. In contrast, “regular” function literals can be wrapped in parentheses or curly braces.

If the function is called with an input that doesn’t match one of the case clauses, a MatchError is thrown at runtime.

code example

package com.brown/**  * Created by BrownWong on 2016/9/29.  */object Hello {  def main(args: Array[String]): Unit = {    val pf: PartialFunction[Any, String] = {case s:String => "Yes"}    println(pf("HAHA"))    println(pf(12))  }}

output

YesException in thread "main" scala.MatchError: 12 (of class java.lang.Integer)    at scala.PartialFunction$$anon$1.apply(PartialFunction.scala:253)at scala.PartialFunction$$anon$1.apply(PartialFunction.scala:251)    at com.brown.Hello$$anonfun$1.applyOrElse(Hello.scala:8)    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:36)    at com.brown.Hello$.main(Hello.scala:10)    at com.brown.Hello.main(Hello.scala)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:483)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

2.isDefinedAt()method

You can test if a Partial Function will match an input using the isDefinedAt method. This function avoids the risk of throwing a MatchError exception.

code example

package com.brown/**  * Created by BrownWong on 2016/9/29.  */object Hello {  def main(args: Array[String]): Unit = {    val pf: PartialFunction[Any, String] = {case s:String => "Yes"}    println(pf.isDefinedAt("HAHA"))    println(pf.isDefinedAt(12))  }}

output

truefalse

3.“chain” Partial Functions

You can “chain” PartialFunctions together: pf1 orElse pf2 orElse pf3 …. If pf1 doesn’t match, then pf2 is tried, then pf3, etc. A MathError is only thrown if none of them matches.

code example

package com.brown/**  * Created by BrownWong on 2016/9/29.  */object Hello {  def main(args: Array[String]): Unit = {    val pf1: PartialFunction[Any, String] = {case s:String => "Yes"}    val pf2: PartialFunction[Any, String] = {case s:Int => "Yes"}    val pf3 = pf1 orElse pf2    println(pf3.isDefinedAt("HAHA"))    println(pf3.isDefinedAt(12))  }}

output

truetrue

Ref

《Programming Scala》