This code compiles fine, but fails at runtime:
val values = Set("a").toSeq.sorted
values match {
case Nil => println("empty")
case h::t => println(s"h = $h")
}
With the error message:
scala.MatchError: ArrayBuffer(a) (of class scala.collection.mutable.ArrayBuffer)
I understand that somewhere in the process an ArrayBuffer is created, on which I cannot pattern-match like this. However, why can't the compiler tell me that this is not going to work?
You are matching on an open (extensible) data type, Scala's Seq. It could be a List, so Scala doesn't complain with your List pattern. On the other hand, exhaustiveness cannot be checked because it could really be any class implementing Seq (that we may not even know statically), so Scala just trusts you on this one.
You can use generic Seq patterns instead:
values match {
case Seq() => println("empty")
case h +: t => println(s"h = $h")
}
Or just convert to a List and use the same patterns (but Lists are not very efficient data structures, so it's probably better with the first option).
val values = Set("a").toList.sorted
values match {
case Nil => println("empty")
case h::t => println(s"h = $h")
}
Getting errors from pattern matching can be tricky sometimes, but in this case:
scala> :type values
Seq[String]
scala> Seq(1,2,3) match { case h::t => "ok" }
res1: String = ok
There's not enough type info to say it can't work, and it errs on the side of not annoying you.
Related
I have a collection with elements that have a field field1. I want to get all field1s that are options of type MyType.
Currently this is my code.
elems.map(_.field1).map {case Some(found: MyType) => found}
I'm sure this can be done in a much nicer way.. It bugs me that I need to use map twice. Is there a way to do this with only one map/collect ?
EDIT: My code works. I'm just wondering if it can be done in a better (i.e. shorter or prettier way).
elems.flatMap(_.field1.collect { case x: MyType => x })
I believe utilising .flatMap may solve this issue for you
elems.flatMap(_.field1 match {
case myType: MyType => Some(myType)
case _ => None
}
Calling iterator before transforming the collection accumulates all the transformations into a single one so perhaps try
elems
.iterator
.flatMap(_.field1)
.collect { case v: MyType => v }
.toList
if your Seq type is case class you can use pattern matching with one collect function like so (see actual seq):
case class MyTypeWrapper(field1: Option[MyType])
case class MyType(x: String)
val elems = Seq.empty[MyTypeWrapper]
val expected: Seq[MyType] = elems.map(_.field1).map{ case Some(found: MyType) => found }
val actual: Seq[MyType] = elems.collect{ case MyTypeWrapper(Some(mt: MyType)) => mt }
// expected and actual should contains the same elements
How to do pattern matching on a Map in Scala ?
A (non working) attempt includes,
Map("a"->1, "b"->2, "c"->3) match {
case Map(a,b,_*) => a
}
which errs with
value Map is not a case class, nor does it have an unapply/unapplySeq member
case Map(a,b,_*) => a
The error is indicative enough, yet how to enrich Map with an unapply method for pattern matching ?
Many Thanks
Update
Following #Paul's comment, a neater use case may be like this,
Map("a"->1, "b"->2, "c"->3) match {
case Map("b"->2,_*) => "222"
}
namely, in this case, if map contains key b that maps onto value 2.
Most easy way is tramsform Map to List:
Map("a"->1, "b"->2, "c"->3).to[List] match {
case List(a,b,_*) => a
}
An approach to enriching Map with an unapplySeq method for pattern matching includes this,
object MapExtractor {
def unapplySeq[A <% Ordered[A], B <% Ordered[B]]
(s: Map[A,B]): Option[Seq[(A,B)]] = Some(s.toSeq.sorted)
}
where the sorting approach may be changed to any orderable (items comparable) logic. In this example,
Map("b"->2, "a"->1, "c"->3) match {
case MapExtractor ( x, xs # _* ) => println(s"x: $x") ; println(s"xs: $xs")
}
delivers
x: (a,1)
xs: ArrayBuffer((b,2), (c,3))
I have a Scala Option[T]. If the value is Some(x) I want to process it with a a process that does not return a value (Unit), but if it is None, I want to print an error.
I can use the following code to do this, but I understand that the more idiomatic way is to treat the Option[T] as a sequence and use map, foreach, etc. How do I do this?
opt match {
case Some(x) => // process x with no return value, e.g. write x to a file
case None => // print error message
}
I think explicit pattern matching suits your use case best.
Scala's Option is, sadly, missing a method to do exactly this. I add one:
class OptionWrapper[A](o: Option[A]) {
def fold[Z](default: => Z)(action: A => Z) = o.map(action).getOrElse(default)
}
implicit def option_has_utility[A](o: Option[A]) = new OptionWrapper(o)
which has the slightly nicer (in my view) usage
op.fold{ println("Empty!") }{ x => doStuffWith(x) }
You can see from how it's defined that map/getOrElse can be used instead of pattern matching.
Alternatively, Either already has a fold method. So you can
op.toRight(()).fold{ _ => println("Empty!") }{ x => doStuffWith(x) }
but this is a little clumsy given that you have to provide the left value (here (), i.e. Unit) and then define a function on that, rather than just stating what you want to happen on None.
The pattern match isn't bad either, especially for longer blocks of code. For short ones, the overhead of the match starts getting in the way of the point. For example:
op.fold{ printError }{ saveUserInput }
has a lot less syntactic overhead than
op match {
case Some(x) => saveUserInput(x)
case None => printError
}
and therefore, once you expect it, is a lot easier to comprehend.
I'd recommend to simply and safely use opt.get which itself throws a NoSuchElementException exception if opt is None. Or if you want to throw your own exception, you can do this:
val x = opt.getOrElse(throw new Exception("Your error message"))
// x is of type T
as #missingfaktor says, you are in the exact scenario where pattern matching is giving the most readable results.
If Option has a value you want to do something, if not you want to do something else.
While there are various ways to use map and other functional constructs on Option types, they are generally useful when:
you want to use the Some case and ignore the None case e.g. in your case
opt.map(writeToFile(_)) //(...if None just do nothing)
or you want to chain the operations on more than one option and give a result only when all of them are Some. For instance, one way of doing this is:
val concatThreeOptions =
for {
n1 <- opt1
n2 <- opt2
n3 <- opt3
} yield n1 + n2 + n3 // this will be None if any of the three is None
// we will either write them all to a file or none of them
but none of these seem to be your case
Pattern matching is the best choice here.
However, if you want to treat Option as a sequence and to map over it, you can do it, because Unit is a value:
opt map { v =>
println(v) // process v (result type is Unit)
} getOrElse {
println("error")
}
By the way, printing an error is some kind of "anti-pattern", so it's better to throw an exception anyway:
opt.getOrElse(throw new SomeException)
I've obviously done a very poor job of explaining what I'm looking for in my original post so let's try this one more time. What I'm trying to accomplish is the ability to pass a sequence of items, extract one or more of the items, and then pass the REMAINDER of the sequence on to another extractor. Note that by sequence I mean sequence (not necessarily a List). My previous examples used list as the sequence and I gave some examples of extraction using cons (::), but I could just as well pass an Array as my sequence.
I thought I knew how pattern matching and extraction worked but I could be wrong so to avoid any more basic comments and links to how to do pattern matching sites here's my understanding:
If I want to return a single item from my extractor I would define an unapply method. This method takes whatever type I chose as input (the type could be a sequence...) and returns a single optional item (the return type could itself be a sequence). The return must be wrapped in Some if I want a match or None if I don't. Here is an example that takes a sequence as input and returns the same sequence wrapped in Some but only if it contains all Strings. I could very well just return the sequence wrapped in Some and not do anything else, but this seems to cause confusion for people. The key is if it is wrapped in Some then it will match and if it is None it will not. Just to be more clear, the match will also not happen unless the input also matches my unapply methods input type. Here is my example:
object Test {
// In my original post I just returned the Seq itself just to verify I
// had matched but many people commented they didn't understand what I
// was trying to do so I've made it a bit more complicated (e.g. match
// only if the sequence is a sequence of Strings). Hopefully I don't
// screw this up and introduce a bug :)
def unapply[A](xs: Seq[A]): Option[Seq[String]] =
if (xs forall { _.isInstanceOf[String] })
Some(xs.asInstanceOf[Seq[String]])
else
None
}
Using List as an example, I can now perform the following:
// This works
def test1(xs: List[_]) = xs match {
case (s: String) :: Test(rest) =>
println("s = " + s + ", rest = " + rest)
case _ =>
println("no match")
}
test1(List("foo", "bar", "baz")) // "s = foo, rest = List(bar, baz)"
My test1 function takes List as input and extracts the head and tail using cons via the constructor pattern (e.g. ::(s, rest)). It then uses type ascription (: String) to make sure the head (s) is a String. The tail contains List("bar", "baz"). This is a List which means it is also a Seq (sequence). It is then passed as input to my Test extractor which verifies that both "bar" and "baz" are strings and returns the List wrapped in Some. Since Some is returned it is considered a match (although in my original post where I inadvertently mixed up unapplySeq with unapply this didn't work as expected, but that aside...). This is NOT what I'm looking for. This was only an example to show that Test does in fact extract a Seq as input as expected.
Now, here's where I caused mass confusion last time when I inadvertently used unapplySeq instead of unapply in my write up. After much confusion trying to understand the comments that were posted I finally picked up on the mistake. Many thanks to Dan for pointing me in the right direction...
But just be avoid any more confusion, let me clarify my understanding of unapplySeq. Like unapply, unapplySeq takes in whatever argument I choose as input, but instead of returning a single element it returns a sequence of elements. Each item in this sequence can then be used for additional pattern matching. Again, to make a match happen the input type must match and my returned sequence must be wrapped in Some and not be None. When extracting over the sequence of items returned from unapplySeq, you can use _* to match any remaining items not yet matched.
Ok, so my extractor takes a sequence as input and returns a sequence (as a single item) in return. Since I only want to return a single item as a match I need to use unapply NOT unapplySeq. Even though in my case I'm returning a Seq, I don't want unapplySeq because I don't want to do more pattern matching on the items in the Seq. I just want to return the items as a Seq on its own to then be passed to the body of my case match. This sounds confusing, but to those that understand unapply vs unapplySeq I hope it isn't.
So here is what I WANT to do. I want to take something that returns a sequence (e.g. List or Array) and I want to extract a few items from this sequence and then extract the REMAINDER of the items (e.g. _*) as a sequence. Let's call it the remainder sequence. I want to then pass the remainder sequence as input to my extractor. My extractor will then return the remaining items as a single Seq if it matches my criteria. Just to be 100% clear. The List (or Array, etc) will have its unapplySeq extractor called to create the sequence of items. I will extract a one or more of these items and then pass what is left as a sequence to my Test extractor which will use unapply (NOT unapplySeq) to return the remainder. If you are confused by this, then please don't comment...
Here are my tests:
// Doesn't compile. Is there a syntax for this?
def test2(xs: Seq[_]) = xs match {
// Variations tried:
// Test(rest) # _* - doesn't compile (this one seems reasonable to me)
// Test(rest # _*) - doesn't compile (would compile if Test had
// unapplySeq, but in that case would bind List's
// second element to Test as a Seq and then bind
// rest to that Seq (if all strings) - not what I'm
// looking for...). I though that this might work
// since Scala knows Test has no unapplySeq only
// unapply so # _* can be tied to the List not Test
// rest # Test(_*) - doesn't compile (didn't expect to)
case List(s: String, Test(rest) # _*) =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
// This works, but messy
def test3(xs: List[_]) = xs match {
case List(s: String, rest # _*) if (
rest match { case Test(rest) => true; case _ => false }
) =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
I created test3 based on comments from Julian (thanks Julian..). Some have commented that test3 does what I want so they are confused what I'm looking for. Yes, it accomplishes what I want to accomplish, but I'm not satisfied with it. Daniel's example also works (thanks Daniel), but I'm also not satisfied with having to create another extractor to split things and then do embedded extractions. These solutions seem too much work in order to accomplish something that seems fairly straight forward to me. What I WANT is to make test2 work or know that it can't be done this way. Is the error given because the syntax is wrong? I know that rest # _* will return a Seq, that can be verified here:
def test4(xs: List[_]) = xs match {
case List(s: String, rest # _*) =>
println(rest.getClass) // scala.collection.immutable.$colon$colon
case _ =>
println("no match")
}
It returns cons (::) which is a List which is a Seq. So how can I pass the _* Seq on to my extractor and have is return bound to the variable rest?
Note that I've also tried passing varargs to my unapply constructor (e.g. unapply(xs: A*)...) but that won't match either.
So, I hope it is clear now when I say I want to extract the remainder of a sequence in pattern matching. I'm not sure how else I can word it.
Based on the great feedback from Daniel I'm hoping he is going to have an answer for me :)
I'd like to extract the first item and pass the remainder on to another extractor.
OK. Your test1 does that, exactly. first_item :: Extractor(the_rest). The weird behavior you're seeing comes from your Test extractor. As you already had the answer to your stated question, and as expected behavior from your Test strikes you as a problem with test1, it seems that what you really want is some help with extractors.
So, please read Extractor Objects, from docs.scala-lang.org, and Pattern Matching in Scala (pdf). Although that PDF has an example of unapplySeq, and suggests where you'd want to use it, here are some extra examples:
object Sorted {
def unapply(xs: Seq[Int]) =
if (xs == xs.sortWith(_ < _)) Some(xs) else None
}
object SortedSeq {
def unapplySeq(xs: Seq[Int]) =
if (xs == xs.sortWith(_ < _)) Some(xs) else None
}
Interactively:
scala> List(1,2,3,4) match { case Sorted(xs) => Some(xs); case _ => None }
res0: Option[Seq[Int]] = Some(List(1, 2, 3, 4))
scala> List(4,1,2,3) match { case Sorted(xs) => Some(xs); case _ => None }
res1: Option[Seq[Int]] = None
scala> List(4,1,2,3) match { case first :: Sorted(rest) => Some(first, rest); case _ => None }
res2: Option[(Int, Seq[Int])] = Some((4,List(1, 2, 3)))
scala> List(1,2,3,4) match { case SortedSeq(a,b,c,d) => (a,b,c,d) }
res3: (Int, Int, Int, Int) = (1,2,3,4)
scala> List(4,1,2,3) match { case _ :: SortedSeq(a, b, _*) => (a,b) }
res4: (Int, Int) = (1,2)
scala> List(1,2,3,4) match { case SortedSeq(a, rest # _*) => (a, rest) }
res5: (Int, Seq[Int]) = (1,List(2, 3, 4))
Or maybe -- I only have the faint suspicion of this, you haven't said as much -- you don't want extractor help, but actually you want a terse way to express something like
scala> List(1,2,3,4) match { case 1 :: xs if (xs match { case Sorted(_) => true; case _ => false }) => xs }
res6: List[Int] = List(2, 3, 4)
Erlang has a feature like this (although, without these crazy extractors):
example(L=[1|_]) -> examine(L).
, which pattern-matches the same argument twice - to L and also to [1|_]. In Erlang both sides of the = are full-fledged patterns and could be anything, and you can add a third or more patterns with more =. Scala seems to only support the L=[1|_] form, having a variable and then a full pattern.
scala> List(4,1,2,3) match { case xs # _ :: Sorted(_) => xs }
collection.immutable.::[Int] = List(4, 1, 2, 3)
Well, the easiest way is this:
case (s: String) :: Test(rest # _*) =>
If you need this to work on general Seq, you can just define an extractor to split head from tail:
object Split {
def unapply[T](xs: Seq[T]): Option[(T, Seq[T])] = if (xs.nonEmpty) Some(xs.head -> xs.tail) else None
}
And then use it like
case Split(s: String, Test(rest # _*)) =>
Also note that if you had defined unapply instead of unapplySeq, then # _* would not be required on the pattern matched by Test.
:: is an extractor. For how it works (from a random googling), see, for example, here.
def test1(xs: List[_]) = xs match {
case s :: rest =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
scala> test1(List("a", "b", "c"))
s = a rest = List(b, c)
I think this is what you wanted?
Messing around with this, it seems that the issue has something to do with unapplySeq.
object Test {
def unapply[A](xs: List[A]): Option[List[A]] = Some(xs)
}
def test1(xs: List[_]) = xs match {
case (s: String) :: Test(s2 :: rest) =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
test1(List("foo", "bar", "baz"))
produces the output:
s = foo rest = List(baz)
I'm havng trouble googling up docs on the difference between unapply and unapplySeq.
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).