Questions tagged [unapply]
37 questions
3
votes
1 answer
Scala custom unapply with generics
I would like to condense my evaluator thanks to a custom generic unapply function, which evaluates the argument and returns the value if it succeeds.
But this fails with the error error: not found: type Eval
Any way of achieving this? I have looked…
Mikaël Mayer
- 10,425
- 6
- 64
- 101
3
votes
2 answers
scala unapply that returns boolean
Reading this, I still have questions about unapply() that returns Boolean.
If take a look at Scala Programming Book (2nd edition), page 602. There is an example:
case Email(Twice(x @ UpperCase()), domain) => ...
Where UpperCase defined as an object…
ses
- 13,174
- 31
- 123
- 226
2
votes
1 answer
Why is there a `null` check in the implementation of the `unapply` method for case classes?
I'm working to replace the unapply method on a case class's companion object with my own implementation. And after investigating lots of different tangents related to implementing unapply, it appears there is a null guard in most of them, both in…
chaotic3quilibrium
- 5,661
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- 86
2
votes
1 answer
why do I have to use the unapply operator in maple, when defining a function by diff?
I am an absolute beginner in maple and have problems to understand the following:
The following does not work:
f:=(x)->x^2;
df_wrong:=(x)->diff(f(x),x);
Since df_wrong(1); always yields the following "Error, (in df_wrong) invalid input: diff…
DonkeyKong
- 465
- 1
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2
votes
1 answer
scala Nil type matchs against arraybuffer
I found a strange working construct in scala:
(ArrayBuffer[Int]():Seq[Int]) match {
case Nil => "whoo"
case _ => "nayyy"
}
which returns
"whoo"
Apparently this already works partially for Vectors, but not pattern matching. can someone…
Mikaël Mayer
- 10,425
- 6
- 64
- 101
2
votes
2 answers
Casting String to Int using scala extractors
I am trying to cast a String to Int using extractors. My code looks as follows.
object Apply {
def unapply(s: String): Option[Int] = try {
Some(s.toInt)
} catch {
case _: java.lang.Exception => None
}
}
object App {
def toT[T](s:…
vikraman
- 358
- 7
- 14
2
votes
2 answers
Scala match error with unapply
I'm trying out the code at http://www.scala-lang.org/node/112 and I'm getting a match error for something that doesn't look like it should throw one.
This is the original code:
object Twice {
def apply(x: Int): Int =…
JasonMond
- 1,440
- 1
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- 30
1
vote
2 answers
Scala shapeless Generic.Aux implicit parameter not found in unapply
I encountered the following problem with implicits in Scala, using Shapeless's Generic.Aux:
case class Complex(re: Double, im: Double)
object Prod2 {
def unapply[C, A, B](c: C)(implicit C: Generic.Aux[C, A :: B :: HNil]) =…
Tongfei Chen
- 613
- 4
- 14
1
vote
2 answers
Why can't I reuse "unapply" without repeating the method signature
The following Scala code compiles fine:
val f = (input: String) => Some("result")
object Extract {
def unapply(input: String): Option[String] = f(input)
}
val Extract(result) = "a string"
But if I replace the extractor by:
object Extract {
…
Victor Basso
- 5,556
- 5
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1
vote
1 answer
Scala - not a case class nor does it have method .unapply
I am quite new to Scala and got a few unresolved problems with the following code:
object exprs{
println("Welcome to the Scala worksheet")
def show(e: Expr): String = e match {
case Number(x) => x.toString
case…
mj1261829
- 1,200
- 3
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- 53
1
vote
2 answers
Scala implicit unapply
Am trying to implicitly deconstruct a class instance into a tuple to create a nicer DSL syntax.
Here's a simplified example of what I'm trying to do:
class Pair[A,B](a: A, b: B){
def left = a
def right = b
}
val pair = new Pair(1,2)
implicit def…
virtualeyes
- 11,147
- 6
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- 91
1
vote
1 answer
Is it possible to use implicit conversions for parameters to extractors (unapply) in Scala?
I have created a class called CaseInsensitive which wraps a string (see Implementing a string class that does case insensitive comparisions in Scala).
I've created a case class which has a member variable of type CaseInsensitive, so it gets a…
waterlooalex
- 13,642
- 16
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1
vote
0 answers
Scala-Lift: ambiguous reference to overloaded definition - unapply method in child class
Compiler says
error: ambiguous reference to overloaded definition,
both method unapply in object UserDto of type (in: Any)Option[(String, String, String, String)]
and method unapply in class AbstractJsonConversion of type (in:…
Eugene Goncharuk
- 26
- 3
0
votes
1 answer
Why does this compile? (Scala Regex unapplySeq)
Why does this compile?
val regex = raw"a*".r
def matchRegex(str: String): Boolean =
str match {
case regex("abc") => true
case _ => false
}
As you can see, I don't try to extract any values from the string but…
Devigny
- 3
- 2
0
votes
2 answers
Syntactic sugar explanation of Scala'a unapply method
I am getting an error in the extractor step (unapply method call).
The error message is: Wrong number of arguments for the extractors. found 2; expected 0
Can someone please help what is causing the error (where my misunderstanding is).
class…
user3103957
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