Use for questions about Scala's placeholder syntax for anonymous functions.
Questions tagged [scala-placeholder-syntax]
23 questions
92
votes
1 answer
Scala underscore - ERROR: missing parameter type for expanded function
I know there have been quite a few questions on this, but I've created a simple example that I thought should work,but still does not and I'm not sure I understand why
val myStrings = new Array[String](3)
// do some string initialization
// this…
Jeff Storey
- 56,312
- 72
- 233
- 406
15
votes
2 answers
Underscore in Named Arguments
Can someone explain me what exactly is going on here? I am not fully getting into it right now:
val s = Seq(1D,2D,3D,4D)
case class WithUnit(value: Double, unit: String)
s map { WithUnit(_,"cm") } // works
s map { WithUnit(value = _ , unit = "cm") }…
Peter Schmitz
- 5,824
- 4
- 26
- 48
8
votes
2 answers
What are the rules to govern underscore to define anonymous function?
I am using _ as placeholder for creating anonymous function, and the problem is I cannot predict how Scala is going to transform my code. More precisely, it mistakenly determines how "large" the anonymous function I want.
List(1,2,3) foreach…
WeiChing 林煒清
- 4,452
- 3
- 30
- 65
5
votes
1 answer
Scala underscore use to simplify syntax of function literals
I have the following code:
var x = Array(1,3,4,4,1,1,3)
var m = Int.MaxValue
x.foreach((x)=>(m = m min x))
I tried to simplify last sentence to:
x.foreach((m = _ min m))
But the interpreter says:
scala> x.foreach((m = _ min m))
:8:…
Stan
- 73
- 1
- 4
3
votes
1 answer
Why does ((_: Int, _: Int) => _ / _) not compile when ((_: Int) / (_: Int)) does?
I am learning Scala and have a very basic question. Consider the following two expressions using the placeholder syntax -
// Syntax A
val fnA = (_: Int, _: Int) => _ / _
// Syntax B
val fnB = (_: Int) / (_: Int)
and their attempted…
Jai Prabhu
- 245
- 3
- 10
3
votes
1 answer
Use of underscore in function call with Try parameters
I'm trying to understand particular use of underscore in Scala. And following piece of code I cannot understand
class Test[T, S] {
def f1(f: T => S): Unit = f2(_ map f)
def f2(f: Try[T] => Try[S]): Unit = {}
}
How is the _ treated in this…
yurybubnov
- 357
- 2
- 11
3
votes
1 answer
In Scala, what are the rules for making closures with underscores?
At first I had believed that using underscores to make closures (e.g. println _) were just shorthand for using an arrow (e.g. x => println x), but I just recently learned that you can also do the following:
def f(a: Int, b: Int) = a + 2 * b
List(1,…
math4tots
- 8,540
- 14
- 58
- 95
2
votes
2 answers
Explanation of "unbound placeholder parameter"
Question
Why case 2 causes "unbound placeholder parameter" while case 1 is OK?
Case 1
val h: (Int => Int) = (x:Int) => { scala.util.Random.nextInt(x) }
val _h: (Int => Int) = { scala.util.Random.nextInt(_) }
h(5) // 3
_h(5) // 3
Case 2
val…
mon
- 18,789
- 22
- 112
- 205
2
votes
2 answers
Scala underScore strange behavior: error: missing parameter type for expanded function
First, this:
"1 2".split(" ").toSet
and this:
Set("1", "2")
both evaluate to the same thing, namely
res1: scala.collection.immutable.Set[String] = Set(1, 2)
Why then, when I do:
Set("1", "2") map (_.toInt)
I get as expected this:
res2:…
GA1
- 1,568
- 2
- 19
- 30
2
votes
0 answers
Scala underscore: error: missing parameter type for expanded function
I recently started learning scala and I was a bit confused by how underscore works. While it in most case gives you convenient anonymous method, sometimes it just confuses compiler (and me).
For example,
This works
val randomList =…
Darin
- 143
- 1
- 8
1
vote
2 answers
trying to replace words in dataset (DataFrame)
I am trying to put some kind of placeholder for certain words in my dataset.
However, my method doesnt seem to do anything. I do not get an error but it also doesn't do what it is supposed to do. What am I doing wrong here?
CODE:
wordlist_urls…
smartypants97
- 11
- 3
1
vote
1 answer
Error for parentheses in higher order function definitions (Scala)
I am facing error with round brackets in high-order definition. The following code works fine:
val foo: Int => (Int => Int) = n => n + _*2
However, after adding parentheses compiler error arises
val foo1: Int => (Int => Int) = n => n +…
Loom
- 9,768
- 22
- 60
- 112
1
vote
1 answer
Specify the method signature of the method to apply the eta expansion
Is there a way to specify the signature of the method to which I want to apply the eta expansion?
For example:
val tupleNum = (1L,2L)
case class CaseClass(a:String, b:String)
object CaseClass {
def apply(a: Long, b: Long): CaseClass = new…
angelcervera
- 3,699
- 1
- 40
- 68
1
vote
1 answer
what is does it mean println(_)?
I have this piece of code in scala
val wordCounts = logData.flatMap(line => line.split(" ")).map(word => (word, 1)).reduceByKey((a, b) => a + b)
wordCounts.foreach(println(_))
So what does println(_) mean and what should it print?
Khaled
- 345
- 5
- 14
1
vote
1 answer
Underscores and string concatenation in List.map with Scala
Scala lets you use an underscore to do a simple map. So for example instead of writing:
def roleCall(people: String*){
people.toList.map(x => println(x))
}
...I can instead write:
def roleCall(people: String*){
people.toList.map(println(_))
}…
James Crosswell
- 648
- 4
- 13