I am new to Scala and to funcprog.
I have a piece of code (some of you might recognize it):
trait SwingApi {
type ValueChanged <: Event
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
...
}
where I do not underestand what val ValueChanged: {...} is.
From this post I sort of learned that
type ValueChanged <: Event
and
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
are two unrelated things because they are in different namespaces, etc, and type ValueChanged is an abstract subtype of Event.
Good, then I try in a Scala worksheet:
type myString <: String
val myString: {
def myfunc(x: String): String
}
and it shows me an error "only classes can have declared and undefined members"... Isn't it a similar construction?
Finally, the questions are:
what is ValueChanged in val ValueChanged part of code?
is it truely so unrelated to type ValueChanged <: Event
what does this syntax mean:
val myVal:{def func{x:T}:T}
? What's the name of the value, its type and its actual value here?
Thanks!
{def unapply(x: Event): Option[TextField]}
is a structural Type, it means that it accepts any Object that has an unapply method with a Event as Parameter and Option[TextField] as return value. It is most often used similar to Duck typing, e.g.:
def foo(canQuack: {def quack(): Unit}) = {
canQuack.quack()
}
object Bar{
def quack(): Unit = print("quack")
}
object Baz{
def bark(): Unit = print("bark")
}
foo(Bar) //works
foo(Baz) //compile error
so
type StructuralType = {def unapply(x: Event): Option[TextField]}
val ValueChanged: StructuralType
declares a val with the name ValueChanged and with the Type StructuralType, but assigns no value, that makes only sense in a trait or abstract class, thats why your example doesnt work.
So what
trait SwingApi {
...
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
...
}
means is that the trait SwingApi can only be applied to Objects/Classes that have a val with the name ValueChanged and whatever value is assigned to it, has an unapply method
trait SwingApi {
val ValueChanged: {
def unapply(x: Event): Option[TextField]
}
}
//works:
object Bar extends SwingApi{
val ValueChanged = {
def unapply(x: Event): Option[TextField] = None
}
}
//compile error:
object Baz extends SwingApi{
val ValueChanged = {
//wrong name
def foo(x: Event): Option[TextField] = None
}
}
//compile error:
object Baz2 extends SwingApi{
val ValueChanged = {
//wrong input/output type
def unapply(): Unit = {}
}
}
All code untested
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