CODEWITHSUNDEEP
scalaset
Andrey Izmaylov
Andrey Izmaylov

Reputation: 39

Scala. Sets as functions

I have a problem. I'm representing a set by its characterisitc function, so I've defined a type alias for this representation. The function filterHead should take a set and a predicate as an input and return a result of function f .

 type Set = Int => Boolean
 def filterHead(s: Set, f: Int => Boolean): Boolean = f(s.head)

Then the following error occures: "value head is not a member of Int => Boolean". And the error related to type allias definition, not to input function f

Upvotes: 0

Views: 168

Answers (1)

Patryk Ćwiek
Patryk Ćwiek

Reputation: 14318

When you define the alias for Set, the Set parameter will be 'unrolled' to the full type:

scala> type Set = Int => Boolean
defined type alias Set

scala> def foo(s : Set, i : Int) = ???
foo: (s: Int => Boolean, i: Int)Nothing

And when you're using the Set(1,2,3), you're using the companion object's apply method which returns a different type:

scala> val set = Set(4,1,2)
set: scala.collection.immutable.Set[Int] = Set(4, 1, 2)

Also, as you might notice, the collection set is generic here. You could make a generic type alias too though (the alias would be Set[T]), so there still could be some confusion.

Solution? Use a full type name:

scala> def filterHead(s : scala.collection.immutable.Set[Int], setFunc : Set) = setFunc(s.head)
filterHead: (s: scala.collection.immutable.Set[Int], setFunc: Int => Boolean)Boolean

Or give your alias a different name:

type SetFunc = Int => Boolean

or, in a generic way:

type SetFunc[T] = T => Boolean

Or even import the scala.collection.immutable.Set[T] under a different name - alias on import.

Upvotes: 1

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