Confusing type mismatch in Scala
scala, scalatest
Solution
The method you're using accepts a function that transforms a single argument to a single output. What you're telling Scala in your second statement is that map should accept a function with two arguments! There's an important difference between a single argument that just happens to be a tuple of size two, and two distinct arguments.
So you have single argument, but you (and Scala) know it's a tuple. To access those two elements, you have to destructure or pattern match your argument against the tuple you want. You just do that by writing `forAll (map) { case (k: String, v: Int) => assert(v === expected(k)) }`. You're saying the argument you receive should match the pattern of a tuple `(String, Int)` and you want the first element bound to `k` and the second to `v`. You could probably eliminate mentioning the types explicitly here. Note that this is similar to the syntax used in pattern matching, which is essentially what you're doing here.
Problem
I have: ``` val words = List("all", "the", "words", "all", "the", "counts", "all", "day") val map = Exercise02.count(words.iterator) val expected = Map("all" -> 3, "the" -> 2, "words" -> 1, "counts" -> 1, "day" -> 1) ``` where `Exercise02.count` is `java.util.Iterator[String] => Map[String, Int]` and merely produces a count of each word in the input `java.util.Iterator[String]`. I wrote a test: ``` object Exercise02Spec extends FlatSpec with Inspectors { val words = List("all", "the", "words", "all", "the", "counts", "all", "day") val map = Exercise02.count(words.iterator) val expected = Map("all" -> 3, "the" -> 2, "words" -> 1, "counts" -> 1, "day" -> 1) "count" should "count the occurrence of each word" in { forAll (map) { kv => assert(kv._2 === expected(kv._1)) } // forAll (map) { (k: String, v: Int) => assert(v === expected(k)) } } } ``` The first line compiles just fine, and the test passes. If I replace the first line with the commented out second line, I get a compilation error. - sbt reports: `found : (String, Int) => Unit, required: ((String, Int)) => Unit` - IntelliJ IDEA reports: `Type mismatch, expected: ((String, Int)) => Unit, actual: (String, Int) => Unit` Why is this? And how do I fix it?