Named Type Arguments - More Details
In this section we give more details about the named type arguments (experimental).
Syntax
The addition to the grammar is:
SimpleExpr1 ::= ...
| SimpleExpr (TypeArgs | NamedTypeArgs)
NamedTypeArgs ::= ‘[’ NamedTypeArg {‘,’ NamedTypeArg} ‘]’
NamedTypeArg ::= id ‘=’ Type
Note in particular that named arguments cannot be passed to type constructors:
class C[T]
val x: C[T = Int] = // error
new C[T = Int] // error
class E extends C[T = Int] // error
Compatibility considerations
Named type arguments do not have an impact on binary compatibility, but they have an impact on source compatibility: if the name of a method type parameter is changed, any existing named reference to this parameter will break. This means that the names of method type parameters are now part of the public API of a library.
(Unimplemented proposal: to mitigate this, scala.deprecatedName could be extended to also be applicable on method type parameters.)
In this article