src/HOL/Library/Tools/lazy.scala

+/* Author: Pascal Stoop, ETH Zurich
+   Author: Andreas Lochbihler, Digital Asset */
+
+object Lazy {
+  final class Lazy[A] (f: Unit => A) {
+    var evaluated = false;
+    lazy val x: A = f(())
+
+    def get() : A = {
+      evaluated = true;
+      return x
+    }
+  }
+
+  def force[A] (x: Lazy[A]) : A = {
+    return x.get()
+  }
+
+  def delay[A] (f: Unit => A) : Lazy[A] = {
+    return new Lazy[A] (f)
+  }
+
+  def termify_lazy[Typerep, Term, A] (
+    const: String => Typerep => Term,
+    app: Term => Term => Term,
+    abs: String => Typerep => Term => Term,
+    unitT: Typerep,
+    funT: Typerep => Typerep => Typerep,
+    lazyT: Typerep => Typerep,
+    term_of: A => Term,
+    ty: Typerep,
+    x: Lazy[A],
+    dummy: Term) : Term = {
+    x.evaluated match {
+      case true => app(const("Code_Lazy.delay")(funT(funT(unitT)(ty))(lazyT(ty))))(abs("_")(unitT)(term_of(x.get())))
+      case false => app(const("Code_Lazy.delay")(funT(funT(unitT)(ty))(lazyT(ty))))(const("Pure.dummy_pattern")(funT(unitT)(ty)))
+    }
+  }
+}