Reputation: 726
in scala play framework I seen this code:
abstract class AnalyserInfo
case class ColumnC(typeName:String,fieldName:String) extends AnalyserInfo
case class TableC(typeName:String) extends AnalyserInfo
val asIs :PartialFunction[AnalyserInfo,String] = {
case ColumnC(_,f) => f;
case TableC(typeName) => typeName
}
What is the difference with:
val asIs: (AnaliserInfo)=>String = (info) => info match {
case ColumnC(_,f) => f;
case TableC(typeName) => typeName
}
There is a preferred style? and why in the first case the match keyword can be omitted?
Thank you for the support.
Upvotes: 5
Views: 462
Reputation: 4475
Double => Double
is just a shorthand for Function[Double, Double]
. PartialFunction
inherits from Function
but adds a few methods. Most importantly, it adds the method isDefinedAt
which allows you to query if the function is defined for some parameter.
The case
s without a match are a special syntax to define partial functions, which generates an isDefinedAt
that returns true
for all matching case
s.
Say we have a function that returns 1/x, but only for positive values of x, we could it define as:
scala> val f: (Double => Double) = { case x if x > 0 => 1/x }
f: (Double) => Double = <function1>
or as:
scala> val g: PartialFunction[Double, Double] = { case x if x > 0 => 1/x }
g: PartialFunction[Double,Double] = <function1>
The second version has the benefit that we could check if the function is applicable to some parameter:
scala> g.isDefinedAt(-3)
res0: Boolean = false
This feature is for example used in Scala to implement the actor library where an Actor might only consume certain types of messages.
Upvotes: 8