Here's some code which looks reasonable enough to me:
val myMap: Map[Int, Int] = ((x: Int) => Map[Int, Int](1 -> x + 1, 2 -> x + 2))(4)
When I try to compile it, I get two errors like this:
Error:(20, 68) type mismatch;
found : Int(1)
required: String
val myMap: Map[Int, Int] = ((x: Int) => Map[Int, Int](1 -> x + 1, 2 -> x + 2))(4)
^
I understand that my compiler is trying to use the string addition implementation of +. But why is it doing that? How do I ask the compiler to use integer addition here?
(Changing Int to Integer doesn't help.)
Here is a simpler example that reproduces the error:
scala> def fail(x: Int) = 1 -> x + 1
<console>:10: error: type mismatch;
found : Int(1)
required: String
def fail(x: Int) = 1 -> x + 1
^
All of the operators in play are left-associative, so everything reads left-to-right. That is, the call chain looks like this:
1.->(x).+(1)
or using infix notation:
(1 -> x) + 1
1 -> x returns a tuple (1, x) and a tuple does not have a + method, so the compiler attempts to implicitly convert it to a String via toString, because String does have a + method, which eventually fails because Int is required.
Just use parentheses to group things appropriately:
scala> def foo(x: Int) = 1 -> (x + 1)
foo: (x: Int)(Int, Int)