Quoted var assignment

To be able to use var we need to extend Expr[T] with a VarExpr[T] that allows the expression to be used in the left-hand side of an expression.

def whileNotZero(body: VarExpr[Int] => Expr[Unit]) = '{
  var x = 0
  while (x == 0) ~body('(x))
}
whileNotZero(x => '{ ~x = ~x - 1 })

Currently in '{ ~x = ~x - 1 }, the left hand side cannot be spliced in. The VarExpr[T] would allow x to be spliced on the left hand side of the assignment.

[nicolasstucki]

An alternavie library implementation is possible

sealed abstract class VarRef[T] {
  def update(expr: Expr[T]): Expr[Unit]
  def expr: Expr[T]
}

object VarRef {
  def apply[T: Type, U](init: Expr[T])(body: VarRef[T] => Expr[U]): Expr[U] = '{
    var x = ~init
    ~body(
      new VarRef {
        def update(e: Expr[T]): Expr[Unit] = '{ x = ~e }
        def expr: Expr[T] = '(x)
      }
    )
  }

}

object Test {
  VarRef(4)(varRef => '{ ~varRef.update(3); ~varRef.expr })
}

But there is a bug when splicing ~e, it emits an error with value 'apply' is not a member of scala.quoted.Expr[Unit] (see #4044).

[nicolasstucki]

Fixed by #4081