prefer non-strict defaults;

--- a/src/Pure/General/json.scala	Thu Mar 15 11:20:17 2018 +0100
+++ b/src/Pure/General/json.scala	Thu Mar 15 11:27:32 2018 +0100
@@ -325,7 +325,7 @@
       case _ => None
     }
 
-  def value_default[A](obj: T, name: String, unapply: T => Option[A], default: A): Option[A] =
+  def value_default[A](obj: T, name: String, unapply: T => Option[A], default: => A): Option[A] =
     value(obj, name) match {
       case None => Some(default)
       case Some(json) => unapply(json)
@@ -333,31 +333,31 @@
 
   def string(obj: T, name: String): Option[String] =
     value(obj, name, Value.String.unapply)
-  def string_default(obj: T, name: String, default: String = ""): Option[String] =
+  def string_default(obj: T, name: String, default: => String = ""): Option[String] =
     value_default(obj, name, Value.String.unapply, default)
 
   def double(obj: T, name: String): Option[Double] =
     value(obj, name, Value.Double.unapply)
-  def double_default(obj: T, name: String, default: Double = 0.0): Option[Double] =
+  def double_default(obj: T, name: String, default: => Double = 0.0): Option[Double] =
     value_default(obj, name, Value.Double.unapply, default)
 
   def long(obj: T, name: String): Option[Long] =
     value(obj, name, Value.Long.unapply)
-  def long_default(obj: T, name: String, default: Long = 0): Option[Long] =
+  def long_default(obj: T, name: String, default: => Long = 0): Option[Long] =
     value_default(obj, name, Value.Long.unapply, default)
 
   def int(obj: T, name: String): Option[Int] =
     value(obj, name, Value.Int.unapply)
-  def int_default(obj: T, name: String, default: Int = 0): Option[Int] =
+  def int_default(obj: T, name: String, default: => Int = 0): Option[Int] =
     value_default(obj, name, Value.Int.unapply, default)
 
   def bool(obj: T, name: String): Option[Boolean] =
     value(obj, name, Value.Boolean.unapply)
-  def bool_default(obj: T, name: String, default: Boolean = false): Option[Boolean] =
+  def bool_default(obj: T, name: String, default: => Boolean = false): Option[Boolean] =
     value_default(obj, name, Value.Boolean.unapply, default)
 
   def list[A](obj: T, name: String, unapply: T => Option[A]): Option[List[A]] =
     value(obj, name, Value.List.unapply(_, unapply))
-  def list_default[A](obj: T, name: String, unapply: T => Option[A], default: List[A] = Nil)
+  def list_default[A](obj: T, name: String, unapply: T => Option[A], default: => List[A] = Nil)
     : Option[List[A]] = value_default(obj, name, Value.List.unapply(_, unapply), default)
 }