All Questions
Tagged with extractor pattern-matching
22
questions
12
votes
2
answers
1k
views
Scala extractors - skip unused parameters
given following code:
abstract class MyTuple
...
case class MySeptet(a: Int, b: Int, c: Int, d: Int, e: Int, f: Int, g: Int) extends MyTuple
case class MyOctet(a: Int, b: Int, c: Int, d: Int, e: ...
- 6,057
10
votes
6
answers
14k
views
Scala: Pattern matching when one of two items meets some condition
I'm often writing code that compares two objects and produces a value based on whether they are the same, or different, based on how they are different.
So I might write:
val result = (v1,v2) match {...
- 13.8k
9
votes
1
answer
3k
views
Understanding pattern matching on lists
I've been playing around with Extractors lately and was wondering how the List extractors work especially this:
List(1, 2, 3) match {
case x :: y :: z :: Nil => x + y + z // case ::(x, ::(y, ::(...
- 2,557
9
votes
3
answers
8k
views
Replacing case class inheritance with extractors preserving exhaustiveness checks in Scala
I have a simple class hierarchy that represents a graph-like structure with several distinct types of vertexes implemented using case classes:
sealed trait Node
sealed abstract case class Vertex ...
- 2,345
8
votes
5
answers
4k
views
Scala, partial functions
Is there any way to create a PartialFunction except through the case statement?
I'm curious, because I'd like to express the following (scala pseudo ahead!)...
val bi = BigInt(_)
if (bi.isValidInt) ...
- 417k
8
votes
1
answer
6k
views
Pattern matching against Scala Map entries
Is there any Scala trick to enable pattern matching against map keys? In other words, I'd like to have an extractor that beside the Map instance accepted also a key value that would mean I want this ...
- 1,324
7
votes
1
answer
2k
views
Nested Scala matchers why Some(Some(1),1) can't match?
It seems that nested matching doesn't work, which is a strange limitation.
An example of the behaviour follows:
Some(Some(1),2) match {
| case Some(Some(a),b) => a
| case e => e
| }
<...
- 3,743
6
votes
3
answers
2k
views
Modeling with Scala case class
I'm attempting to model responses from REST APIs as case classes which I can use pattern matching on.
I thought it would be a good fit assuming inheritance, but I see that this is deprecated.
I know ...
- 10k
4
votes
2
answers
2k
views
unapply method of a case class is not used by the Scala compiler to do pattern matching, why is that?
abstract class Animal
case class Cat(name: String) extends Animal
case class Dog(name: String) extends Animal
Say I have defined Cat and Dog, two case classes.
Then I use them like this:
val ...
- 8,167
4
votes
2
answers
2k
views
Implicit parameters won't work on unapply. How to hide ubiquitous parameters from extractors?
Apparently unapply/unapplySeq in extractor objects do not support implicit parameters. Assuming here an interesting parameter a, and a disturbingly ubiquitous parameter b that would be nice to hide ...
- 675
4
votes
1
answer
267
views
Scala 2.8.0.RC2 compiler issue on pattern matching statement?
Why does the following module not compile on Scala 2.8.RC[1,2]?
object Test {
import util.matching.Regex._
val pVoid = """\s*void\s*""".r
val pVoidPtr = """\s*(const\s+)?void\s*\*\s*""".r
val ...
- 1,686
3
votes
1
answer
3k
views
Scala alternative cases match syntax with different type of extracted value
object NoSense {
def main(args: Array[String]) {
val value = "true" match {
case value @ (IntValue(_) | BooleanValue(_)) => value
}
require(value == true)
}
}
...
- 5,703
3
votes
1
answer
144
views
Why and how to migrate case class to extractor in Scala
In the Programming in Scala discussion on when to use case classes and when extractors, they say
Extractors break [the] link between data representations and patterns ... This property is called ...
- 6,856
1
vote
1
answer
63
views
No pattern match for Seq[Seq[T]]
def example1(sss:Seq[Seq[String]]) = sss match {
case Seq(a, b) :+ Seq(c, d) :+ tail => true
}
val res1 = example1(Seq(Seq("a", "b"), Seq("c","d")))
// scala.MatchError: List(List(a, b), List(c, ...
- 50.9k
1
vote
1
answer
717
views
Fixing a match clause warning by getting rid of a type parameter
i want to implement the following exhaustive match, but i can't figure out to remove the type parameter and thus the warning that it is erased:
sealed trait Q[+V]
case object QEmpty extends Q[Nothing]...
- 67k
1
vote
2
answers
2k
views
How to match multiple case classes and extract same (-named) arguments?
Consider the following contrived example of implementing a unary & binary operations on real number expressions.
abstract class DoubleE
case class Negate(x: DoubleE) extends DoubleE
case class ...
- 6,012
1
vote
1
answer
92
views
Problems with scala regex extractor
I have a problem with the regex extrator, this is my regex
val regex = """(some\/params\/results\/\b[A-Z]{3}\/[A-Z]{3}\b\/)*""".r
And when I try to do this:
val regex(res) = "some/params/results/...
- 67
1
vote
2
answers
346
views
How to use case class define a extractor?
I want to use case class match Seq[Byte] as List defined, but a compiler error occurred.
use case class with compiler error
case class :: (head: Byte, tail: Seq[Byte])
def ...
- 2,473
1
vote
1
answer
154
views
Email Extractor Java Pattern excluding characters before a dot
I am currently using an email extractor which is working well, but I would like to change its pattern which is:
[^a-zA-Z0-9-](?<num>[a-zA-Z0-9_-]{2,20}@[A-Za-z0-9_-]{3,20}\.[\.a-zA-Z0-9_-]+)[^a-...
- 13
0
votes
3
answers
356
views
Scala: Workaround for unparameterizable extractor
Since extractors cannot take custom parameters (as answered in Stack Overflow: Can extractors be customized...), I try to find an alternative way of solving the following problem.
I have a lot of ...
- 3,973
0
votes
2
answers
231
views
How does the extractor work when the `unapply` function returns Boolean instead of Option?
I learned about extractors from the stairway book:
object Twice {
def apply(x: Int) = x * 2
def unapply(x: Int) = if(x % 2 == 0) Some(x / 2) else None
}
// outside pattern ...
- 22.7k
0
votes
0
answers
250
views
Kotlin Array/List extractor in Pattern Matching
In Scala one can extract array elements in a pattern matching statement. For example, I want to take the first two elements of a string input:
private def parseFieldSize(s: String): Option[(Int, Int)...
- 381