Scala: can't write setter without getter? - scala

Question

This works:
class ButtonCountObserver {
private var cnt = 0 // private field
def count = cnt // reader method
def count_=(newCount: Int) = cnt = newCount // writer method
// ...
}
val b = new ButtonCountObserver
b.count = 0
But this doesn't
class ButtonCountObserver {
private var cnt = 0 // private field
def count_=(newCount: Int) = cnt = newCount // writer method
// ...
}
val b = new ButtonCountObserver
b.count = 0
I get: error: value count is not a member of ButtonCountObserver
Is it possible to create a setter (with the syntactic sugar) without a getter?

Answer 1

The spec requires that both a setter and getter are defined to be able to use the syntactic sugar for calling the setter:
The interpretation of an assignment to
a simple variable x = e depends on the
definition of x. If x denotes a
mutable variable, then the assignment
changes the current value of x to be
the result of evaluating the
expression e. The type of e is
expected to conform to the type of x.
If x is a parameterless function
defined in some template, and the same
template contains a setter function
x_= as member, then the assignment x =
e is interpreted as the invocation
x_=(e ) of that setter function.
Analogously, an assignment f .x = e to
a parameterless function x is
interpreted as the invocation f .x_=(e
). An assignment f (args) = e with a
function application to the left of
the ‘=’ operator is interpreted as f
.update(args, e ), i.e. the invocation
of an update function defined by f .
Furthermore, the getter must be visible in order to use the setter. I'm not sure if this is specified
Getter not visible #1
// error: method x cannot be accessed in x.Test
object x {
class Test {
private[this] var x0: Int = 0
private[Test] def x = x0
def x_=(a: Int) = x0 = a
}
val t = new Test
t.x = 1
}
Getter not visible #2
//<console>:11: error: type mismatch; found : x.Test required: ?{val x: ?}
object x {
class Test {
private[this] var x0: Int = 0
private[this] def x = x0
def x_=(a: Int) = x0 = a
}
val t = new Test
t.x = 1
}
Getter visible
object x {
class Test {
private[this] var x0: Int = 0
private[x] def x = x0
def x_=(a: Int) = x0 = a
}
val t = new Test
t.x = 1
}

Answer 2

As retronym pointed out, there must be a getter present. As a workaround however (if you don't want to provide a getter), you can make the getter return Unit
object x {
class Test {
private[this] var x0: Int = 0
def x: Unit = ()
def x_=(a: Int) = x0 = a
}
val t = new Test
t.x = 1
}
Don't think that that is considered good style (!), but it works.