In Scala 2.8.0 RC 2 this definition:
def buttonGroup[T](values: Array[T], textProvider: T => String = (t: T => t.toString)) = ...
gives the error message:
not found: value t
def buttonGroup[T](values: Array[T], textProvider: T => String = (_.toString)) = ...
gives
missing parameter type for expanded function ((x$1) => x$1{}.toString{}){}
Only this works:
textProvider: T => String = (_:T).toString
Why?
Either of these work, without type annotation:
def buttonGroup[T](values: Array[T], textProvider: T => String = (t: T) => t.toString) = 0
def buttonGroup[T](values: Array[T], textProvider: T => String = {t: T => t.toString}) = 0
But why don't your variations work?
The first one actually isn't valid Scala in any context:
scala> (t: Any => t.toString)
<console>:1: error: ';' expected but ')' found.
(t: Any => t.toString))
^
The second expression _.toString uses the placeholder syntax for anonymous functions, and only works if the expression has an expected type.
scala> def foo[T] = { (_.toString) : (T => String) }
foo: [T](T) => String
The problem is that the default expression for a parameter whose type depends an type parameter doesn't have an expected type. This seems counter-intuitive, why wouldn't it have the declared type of the argument as the expected type? It turns out that the expression can have a more specific type, and that type checking is deferred to the call-site:
scala> def foo[T](t: T = "string-t") = t
foo: [T](t: T)T
scala> foo(1)
res4: Int = 1
scala> foo()
res5: java.lang.String = string-t
scala> foo[Int]()
<console>:7: error: type mismatch;
found : java.lang.String
required: Int
Error occurred in an application involving default arguments.
foo[Int]()
If the type of textProvider doesn't include the type parameter T, the default expression has an expected type, and you can use the placeholder syntax:
scala> def buttonGroup[T](values: Array[T], textProvider: Any => String = _.toString) = 0
buttonGroup: [T](values: Array[T],textProvider: (Any) => String)Int
For a detailed explanation of the design of named and default parameters, I recommend Lucas Rytz's Scala Days presentation.