I have been working with Scala for close to a year, but every now and then I come across a piece of code that I don't really understand. This time it is this one. I tried looking into documents on "scala methods with generic parameter type", but I am still confused.
def defaultCall[T](featureName : String) (block : => Option[T])(implicit name: String, list:Seq[String]) : Option[T] =
{
val value = block match {
case Some(n) => n match {
case i : Integer => /*-------Call another method----*/
case s : String => /*--------Call another method----*/
}
case _ => None
}
The method is called using the code shown below :
var exValue = Some(10)
val intialization = defaultCall[Integer]("StringName"){exValue}
What I don't understand in the above described code is the "case" statement in the defaultCall method.
I see that when the exValue has a value and is not empty, the code works as expected. But in case I change the exValue to None, then my code goes into the "case _ = None" condition. I don't understand why this happens since the match done here is against the "variable" which would be either an Integer or a String.
What happens here is that when you pass a None it will match on the second case, which "catches" everything that is not an instance of a Some[T]:
block match {
case Some(n) => // Will match when you pass an instance of Some[T]
case _ => // Will match on any other case
}
Note that None and Some are two different classes that inherit from Option.
Also, the variable match is only done if the first match succeeds, otherwise not. To achieve the type checking in the first match you could do:
block match {
case Some(n: Int) => // do stuff
case Some(n: String) => // do stuff
case _ => // Will match on any other case
}
Hope that helps
Related
I have a method that receives a function, but that function may be partial, in such case I don't want it to fail with MatchError.
def doSomething[X,Y](opt:Option[X])(f:X=>Y)={
f match {
case p:PartialFunction[X,Y]=> opt.flatMap(p.lift) //This doesn't seem to work
case _ => opt.map(f)
}
}
That way I can use the method like this
doSomething(x){
case t if predicate(t) => otherMethod(t)
}
so in case I don't have a predicate, I can use it like
this doSomething(x)(otherMethod) instead of
doSoemthing(x){
case t=> otherMethod(t)
}
Note: Looking for a solution that doesn't require catching MatchError exceptions
This isn't an answer because I don't think that what you want is possible in Scala.
The original method is fine and works as expected, though it could be a bit simpler:
def doSomething[X, Y](opt: Option[X])(f: X => Y): Option[Y] = {
f match {
case p: PartialFunction[X, Y] => opt.collect(p)
case _ => opt.map(f)
}
}
The problem is here:
doSomething(x){
case t if predicate(t) => otherMethod(t)
}
Scala is creating a Function rather than a PartialFunction from that match expression so the test is failing. If you pass a real PartialFunction the method works OK.
val p: PartialFunction[Int, Int] = {
case i: Int if i > 0 => i
}
doSomething(Some(0))(p) // Returns None
I don't think there is any way of doing what you want, mainly because doSomething has multiple argument lists which messes up type deduction for the second argument list.
My suggestion is just to use
x.map(f)
or
x.collect{
case ...
}
as appropriate in the calling code.
The syntax for partial function has been changed since 2.9 per SLS 8.5, so that even you do { case x => y}, it DOES NOT mean it is a partial function. Its type will be exact as you define it as.
In your case, you defined it as X=>Y (as in your function parameter), so it is just a X=>Y (it got compiled into a regular function, and non match cases will throw MatchError), and even you do isInstanceOf[PartialFunciton[_,_]], it won't match.
To make your scenario work, you can just simply cast the passed function as PartialFunction, like:
doSomething(Some(1))({case 2 => 0}: PartialFunction[Int,Int]) //This returns None without MatchError
while
doSomething(Some(1)){case 2 => 0} //This gives MatchError and it is not recognized as PartialFunction inside the body
This is probably not as convenient as you thought it is, but it is the only way to make it work. (or you define 2 separate functions for either case, like collect and map in standard library)
I'm not sure what you are passing as a Partial Function, but definitely you should have to define it with specific signature like this:
val positive: PartialFunction[Int, Option[Int]] = {
case x if x >= 0 => Some(x)
case _ => None
The positive function is defined only for positive numbers. In case of negative numbers, the function returns None and you won't get scala.MatchError in runtime.
This specific function enables you to access to isDefinedAt method which is testing dynamically if a value is in the domain of the function.
postive(5).isDefinedAt // true
poistive.isInstanceOf[PartialFunction[Int, Option[Int]]] // true
I demonstrated here why you are always getting false when you check p.isInstanceOf
def doSomething[X,Y](opt:Option[X])(f:X=>Y)={
f match {
case p if p.isInstanceOf[PartialFunction[X,Y]] =>
println("I'm a pf")
println(s"Is it PartialFunction: ${p.isInstanceOf[PartialFunction[X,Y]]}")
opt.map(p)
case _ =>
println("I'm not a pf")
opt.map(f)
}
}
doSomething[Int, Option[Int]](Some(5))(positive) // partial function case
doSomething[Int, String](Some(5)) { // tricky case
case s => s.toString
}
You can play with it here:
In the following code the first expression returns a Result[String] which contains one of the strings "medical", "dental" or "pharmacy" inside of a Result. I can add .toOption.get to the end of the val statement to get the String, but is there a better way to use the Result? Without the .toOption.get, the code will not compile.
val service = element("h2").containingAnywhere("claim details").fullText()
service match {
case "medical" => extractMedicalClaim
case "dental" => extractDentalClaim
case "pharmacy" => extractPharmacyClaim
}
Hard to say without knowing what Result is. If it's a case class, with the target String as part of its constructor, then you could pattern match directly.
Something like this.
service match {
case Result("medical") => extractMedicalClaim
case Result("dental") => extractDentalClaim
case Result("pharmacy") => extractPharmacyClaim
case _ => // default result
}
If the Result class doesn't have an extractor (the upapply() method) you might be able to add one just for this purpose.
I'm assuming this Result[T] class has a toOption method which returns an Option[T] - if that's the case, you can call toOption and match on that option:
val service = element("h2").containingAnywhere("claim details").fullText().toOption
service match {
case Some("medical") => extractMedicalClaim
case Some("dental") => extractDentalClaim
case Some("pharmacy") => extractPharmacyClaim
case None => // handle the case where the result was empty
}
I have the following input string:
"0.3215,Some(0.5123)"
I would like to retrieve the tuple (0.3215,Some(0.5123)) with: (BigDecimal,Option[BigDecimal]).
Here is one of the thing I tried so far:
"\\d+\\.\\d+,Some\\(\\d+\\.\\d+".r findFirstIn iData match {
case None => Map[BigDecimal, Option[BigDecimal]]()
case Some(s) => {
val oO = s.split(",Some\\(")
BigDecimal.valueOf(oO(0).toDouble) -> Option[BigDecimal](BigDecimal.valueOf(lSTmp2(1).toDouble))
}
}
Using a Map and transforming it into a tuple.
When I try directly the tuple I get an Equals or an Object.
Must miss something here...
Your code has several issues, but the big one seems to be that the case None side of the match returns a Map but the Some(s) side returns a Tuple2. Map and Tuple2 unify to their lowest-common-supertype, Equals, which is what you're seeing.
I think this is what you're trying to achieve?
val Pattern = "(\\d+\\.\\d+),Some\\((\\d+\\.\\d+)\\)".r
val s = "0.3215,Some(0.5123)"
s match {
case Pattern(a,b) => Map(BigDecimal(a) -> Some(BigDecimal(b)))
case _ => Map[BigDecimal, Option[BigDecimal]]()
}
// Map[BigDecimal,Option[BigDecimal]] = Map(0.3215 -> Some(0.5123))
I don't use pattern matching as often as I should.
I am matching a domain name for the following:
1. If it starts with www., then remove that portion and return.
www.stackoverflow.com => "stackoverflow.com"
2. If it has either example.com or example.org, strip that out and return.
blog.example.com => "blog"
3. return request.domain
hello.world.com => "hello.world.com"
def filterDomain(request: RequestHeader): String = {
request.domain match {
case //?? case #1 => ?
case //?? case #2 => ?
case _ => request.domain
}
}
How do I reference the value (request.domain) inside the expression and see if it starts with "www." like:
if request.domain.startsWith("www.") request.domain.substring(4)
You can give the variable you pattern matching a name and Scala will infer its type, plus you can put an if statement in you case expression as follows
def filterDomain(request: RequestHeader): String = {
request.domain match {
case domain if domain.startsWith("www.") => domain.drop(4)
case domain if domain.contains("example.org") | domain.contains("example.com") => domain.takeWhile(_!='.')
case _ => request.domain
}
}
Note that the order of the case expressions matters.
When writing case clauses you can do something like:
case someVar if someVar.length < 2 => someVar.toLowerCase
This should make pretty clear how grabbing matched values works.
So in this case, you would need to write something like:
case d if d.startsWith("www.") => d.substring(4)
If you're dead set on using a regex rather than String methods such as startsWith and contains, you can do the following:
val wwwMatch = "(?:www\\.)(.*)".r
val exampleMatch = "(.*)(?:\\.example\\.(?:(?:com)|(?:org)))(.*)".r
def filterDomain(request: String): String = {
request.domain match {
case wwwMatch(d) => d
case exampleMatch(d1, d2) => d1 + d2
case _ => request.domain
}
}
Now, for maintainability's sake, I wouldn't go this way, because a month later, I will look at this and not remember what it's doing, but that's your call.
you don't need pattern matching for that:
request.domain
.stripPrefix("www.")
.stripSuffix(".example.org")
.stripSuffix(".example.com")
I found myself writing something like this quite often:
a match {
case `b` => // do stuff
case _ => // do nothing
}
Is there a shorter way to check if some value matches a pattern? I mean, in this case I could just write if (a == b) // do stuff, but what if the pattern is more complex? Like when matching against a list or any pattern of arbitrary complexity. I'd like to be able to write something like this:
if (a matches b) // do stuff
I'm relatively new to Scala, so please pardon, if I'm missing something big :)
This is exactly why I wrote these functions, which are apparently impressively obscure since nobody has mentioned them.
scala> import PartialFunction._
import PartialFunction._
scala> cond("abc") { case "def" => true }
res0: Boolean = false
scala> condOpt("abc") { case x if x.length == 3 => x + x }
res1: Option[java.lang.String] = Some(abcabc)
scala> condOpt("abc") { case x if x.length == 4 => x + x }
res2: Option[java.lang.String] = None
The match operator in Scala is most powerful when used in functional style. This means, rather than "doing something" in the case statements, you would return a useful value. Here is an example for an imperative style:
var value:Int = 23
val command:String = ... // we get this from somewhere
command match {
case "duplicate" => value = value * 2
case "negate" => value = -value
case "increment" => value = value + 1
// etc.
case _ => // do nothing
}
println("Result: " + value)
It is very understandable that the "do nothing" above hurts a little, because it seems superflous. However, this is due to the fact that the above is written in imperative style. While constructs like these may sometimes be necessary, in many cases you can refactor your code to functional style:
val value:Int = 23
val command:String = ... // we get this from somewhere
val result:Int = command match {
case "duplicate" => value * 2
case "negate" => -value
case "increment" => value + 1
// etc.
case _ => value
}
println("Result: " + result)
In this case, you use the whole match statement as a value that you can, for example, assign to a variable. And it is also much more obvious that the match statement must return a value in any case; if the last case would be missing, the compiler could not just make something up.
It is a question of taste, but some developers consider this style to be more transparent and easier to handle in more real-world examples. I would bet that the inventors of the Scala programming language had a more functional use in mind for match, and indeed the if statement makes more sense if you only need to decide whether or not a certain action needs to be taken. (On the other hand, you can also use if in the functional way, because it also has a return value...)
This might help:
class Matches(m: Any) {
def matches[R](f: PartialFunction[Any, R]) { if (f.isDefinedAt(m)) f(m) }
}
implicit def any2matches(m: Any) = new Matches(m)
scala> 'c' matches { case x: Int => println("Int") }
scala> 2 matches { case x: Int => println("Int") }
Int
Now, some explanation on the general nature of the problem.
Where may a match happen?
There are three places where pattern matching might happen: val, case and for. The rules for them are:
// throws an exception if it fails
val pattern = value
// filters for pattern, but pattern cannot be "identifier: Type",
// though that can be replaced by "id1 # (id2: Type)" for the same effect
for (pattern <- object providing map/flatMap/filter/withFilter/foreach) ...
// throws an exception if none of the cases match
value match { case ... => ... }
There is, however, another situation where case might appear, which is function and partial function literals. For example:
val f: Any => Unit = { case i: Int => println(i) }
val pf: PartialFunction[Any, Unit] = { case i: Int => println(i) }
Both functions and partial functions will throw an exception if called with an argument that doesn't match any of the case statements. However, partial functions also provide a method called isDefinedAt which can test whether a match can be made or not, as well as a method called lift, which will turn a PartialFunction[T, R] into a Function[T, Option[R]], which means non-matching values will result in None instead of throwing an exception.
What is a match?
A match is a combination of many different tests:
// assign anything to x
case x
// only accepts values of type X
case x: X
// only accepts values matches by pattern
case x # pattern
// only accepts a value equal to the value X (upper case here makes a difference)
case X
// only accepts a value equal to the value of x
case `x`
// only accept a tuple of the same arity
case (x, y, ..., z)
// only accepts if extractor(value) returns true of Some(Seq()) (some empty sequence)
case extractor()
// only accepts if extractor(value) returns Some something
case extractor(x)
// only accepts if extractor(value) returns Some Seq or Tuple of the same arity
case extractor(x, y, ..., z)
// only accepts if extractor(value) returns Some Tuple2 or Some Seq with arity 2
case x extractor y
// accepts if any of the patterns is accepted (patterns may not contain assignable identifiers)
case x | y | ... | z
Now, extractors are the methods unapply or unapplySeq, the first returning Boolean or Option[T], and the second returning Option[Seq[T]], where None means no match is made, and Some(result) will try to match result as described above.
So there are all kinds of syntactic alternatives here, which just aren't possible without the use of one of the three constructions where pattern matches may happen. You may able to emulate some of the features, like value equality and extractors, but not all of them.
Patterns can also be used in for expressions. Your code sample
a match {
case b => // do stuff
case _ => // do nothing
}
can then be expressed as
for(b <- Some(a)) //do stuff
The trick is to wrap a to make it a valid enumerator. E.g. List(a) would also work, but I think Some(a) is closest to your intended meaning.
The best I can come up with is this:
def matches[A](a:A)(f:PartialFunction[A, Unit]) = f.isDefinedAt(a)
if (matches(a){case ... =>}) {
//do stuff
}
This won't win you any style points though.
Kim's answer can be “improved” to better match your requirement:
class AnyWrapper[A](wrapped: A) {
def matches(f: PartialFunction[A, Unit]) = f.isDefinedAt(wrapped)
}
implicit def any2wrapper[A](wrapped: A) = new AnyWrapper(wrapped)
then:
val a = "a" :: Nil
if (a matches { case "a" :: Nil => }) {
println("match")
}
I wouldn't do it, however. The => }) { sequence is really ugly here, and the whole code looks much less clear than a normal match. Plus, you get the compile-time overhead of looking up the implicit conversion, and the run-time overhead of wrapping the match in a PartialFunction (not counting the conflicts you could get with other, already defined matches methods, like the one in String).
To look a little bit better (and be less verbose), you could add this def to AnyWrapper:
def ifMatch(f: PartialFunction[A, Unit]): Unit = if (f.isDefinedAt(wrapped)) f(wrapped)
and use it like this:
a ifMatch { case "a" :: Nil => println("match") }
which saves you your case _ => line, but requires double braces if you want a block instead of a single statement... Not so nice.
Note that this construct is not really in the spirit of functional programming, as it can only be used to execute something that has side effects. We can't easily use it to return a value (therefore the Unit return value), as the function is partial — we'd need a default value, or we could return an Option instance. But here again, we would probably unwrap it with a match, so we'd gain nothing.
Frankly, you're better off getting used to seeing and using those match frequently, and moving away from this kind of imperative-style constructs (following Madoc's nice explanation).