Introduce another version of Signals that has no uninitialized field.

tests/run/Signals2.check

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+0
+10
+30

tests/run/Signals2.scala

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+// An alternative to Signals1 that does not rely on an uninitialized variable
+import annotation.unchecked._
+
+package frp:
+
+  trait Signal[+T]:
+    def apply()(using caller: Signal.Caller): T
+
+  object Signal:
+
+    abstract class AbstractSignal[+T] extends Signal[T]:
+      private var observers: Set[Caller] = Set()
+      private var currentValue: T = eval(this)
+
+      protected def eval(caller: Caller): T
+
+      protected def computeValue(): Unit =
+        val newValue = eval(this)
+        val observeChange = observers.nonEmpty && newValue != currentValue
+        currentValue = newValue
+        if observeChange then
+          val obs = observers
+          observers = Set()
+          obs.foreach(_.computeValue())
+
+      def apply()(using caller: Caller): T =
+        observers += caller
+        assert(!caller.observers.contains(this), "cyclic signal definition")
+        currentValue
+    end AbstractSignal
+
+    def apply[T](expr: Caller ?=> T): Signal[T] =
+      new AbstractSignal[T]:
+        protected def eval(caller: Caller) = expr(using caller)
+        computeValue()
+
+    class Var[T](private var expr: Caller ?=> T) extends AbstractSignal[T]:
+      protected def eval(caller: Caller) = expr(using caller)
+
+      def update(expr: Caller ?=> T): Unit =
+        this.expr = expr
+        computeValue()
+    end Var
+
+    opaque type Caller = AbstractSignal[?]
+    given noCaller: Caller = new AbstractSignal[Unit]:
+      protected def eval(caller: Caller) = ()
+      override def computeValue() = ()
+
+  end Signal
+end frp
+
+import frp._
+class BankAccount:
+  def balance: Signal[Int] = myBalance
+
+  private val myBalance: Signal.Var[Int] = Signal.Var(0)
+
+  def deposit(amount: Int): Unit =
+    if amount > 0 then
+      val b = myBalance()
+      myBalance() = b + amount
+
+  def withdraw(amount: Int): Int =
+    if 0 < amount && amount <= balance() then
+      val b = myBalance()
+      myBalance() = b - amount
+      myBalance()
+    else throw new AssertionError("insufficient funds")
+end BankAccount
+
+@main def Test() =
+  def consolidated(accts: List[BankAccount]): Signal[Int] =
+    Signal(accts.map(_.balance()).sum)
+
+  val a = BankAccount()
+  val b = BankAccount()
+  val c = consolidated(List(a, b))
+  println(c())
+  a.deposit(10)
+  println(c())
+  b.deposit(20)
+  println(c())
+end Test