diff --git a/sjsonnet/src/sjsonnet/Evaluator.scala b/sjsonnet/src/sjsonnet/Evaluator.scala
index d9f1d1ea..735c3f59 100644
--- a/sjsonnet/src/sjsonnet/Evaluator.scala
+++ b/sjsonnet/src/sjsonnet/Evaluator.scala
@@ -80,7 +80,7 @@ class Evaluator(resolver: CachedResolver,
 
   def visitAsLazy(e: Expr)(implicit scope: ValScope): Lazy = e match {
     case v: Val => v
-    case e => () => visitExpr(e)
+    case e => new LazyWithComputeFunc(() => visitExpr(e))
   }
 
   def visitValidId(e: ValidId)(implicit scope: ValScope): Val = {
@@ -104,7 +104,7 @@ class Evaluator(resolver: CachedResolver,
           val b = bindings(i)
           newScope.bindings(base+i) = b.args match {
             case null => visitAsLazy(b.rhs)(newScope)
-            case argSpec => () => visitMethod(b.rhs, argSpec, b.pos)(newScope)
+            case argSpec => new LazyWithComputeFunc(() => visitMethod(b.rhs, argSpec, b.pos)(newScope))
           }
           i += 1
         }
@@ -490,9 +490,9 @@ class Evaluator(resolver: CachedResolver,
       val b = bindings(i)
       arrF(i) = b.args match {
         case null =>
-          (self: Val.Obj, sup: Val.Obj) => () => visitExpr(b.rhs)(scope(self, sup))
+          (self: Val.Obj, sup: Val.Obj) => new LazyWithComputeFunc(() => visitExpr(b.rhs)(scope(self, sup)))
         case argSpec =>
-          (self: Val.Obj, sup: Val.Obj) => () => visitMethod(b.rhs, argSpec, b.pos)(scope(self, sup))
+          (self: Val.Obj, sup: Val.Obj) => new LazyWithComputeFunc(() => visitMethod(b.rhs, argSpec, b.pos)(scope(self, sup)))
       }
       i += 1
     }
@@ -564,7 +564,7 @@ class Evaluator(resolver: CachedResolver,
             case null =>
               visitAsLazy(b.rhs)(newScope)
             case argSpec =>
-              () => visitMethod(b.rhs, argSpec, b.pos)(newScope)
+              new LazyWithComputeFunc(() => visitMethod(b.rhs, argSpec, b.pos)(newScope))
           }
           i += 1
           j += 1
diff --git a/sjsonnet/src/sjsonnet/Materializer.scala b/sjsonnet/src/sjsonnet/Materializer.scala
index c6bea8f5..31dc9a5e 100644
--- a/sjsonnet/src/sjsonnet/Materializer.scala
+++ b/sjsonnet/src/sjsonnet/Materializer.scala
@@ -73,7 +73,7 @@ abstract class Materializer {
     case ujson.Null => Val.Null(pos)
     case ujson.Num(n) => Val.Num(pos, n)
     case ujson.Str(s) => Val.Str(pos, s)
-    case ujson.Arr(xs) => new Val.Arr(pos, xs.map(x => (() => reverse(pos, x)): Lazy).toArray[Lazy])
+    case ujson.Arr(xs) => new Val.Arr(pos, xs.map(x => new LazyWithComputeFunc(() => reverse(pos, x))).toArray[Lazy])
     case ujson.Obj(xs) =>
       val builder = new java.util.LinkedHashMap[String, Val.Obj.Member]
       for(x <- xs) {
diff --git a/sjsonnet/src/sjsonnet/Val.scala b/sjsonnet/src/sjsonnet/Val.scala
index f8f3fd2e..e6aaaa0b 100644
--- a/sjsonnet/src/sjsonnet/Val.scala
+++ b/sjsonnet/src/sjsonnet/Val.scala
@@ -16,7 +16,7 @@ import scala.reflect.ClassTag
  * evaluated dictionary values, array contents, or function parameters
  * are all wrapped in [[Lazy]] and only truly evaluated on-demand
  */
-abstract class Lazy {
+protected abstract class Lazy {
   protected[this] var cached: Val = null
   def compute(): Val
   final def force: Val = {
@@ -25,6 +25,24 @@ abstract class Lazy {
   }
 }
 
+/**
+ * Thread-safe implementation that discards the compute function after initialization.
+ */
+final class LazyWithComputeFunc(@volatile private[this] var computeFunc: () => Val) extends Lazy {
+  def compute(): Val = {
+    val f = computeFunc
+    if (f != null) {  // we won the race to initialize
+      val result = f()
+      cached = result
+      computeFunc = null  // allow closure to be GC'd
+    }
+    // else: we lost the race to compute, but `cached` is already set and
+    // is visible in this thread due to the volatile read and writes via
+    // piggybacking; see https://stackoverflow.com/a/8769692 for background
+    cached
+  }
+}
+
 /**
   * [[Val]]s represented Jsonnet values that are the result of evaluating
   * a Jsonnet program. The [[Val]] data structure is essentially a JSON tree,
@@ -395,7 +413,7 @@ object Val{
             if(argVals(j) == null) {
               val default = params.defaultExprs(i)
               if(default != null) {
-                argVals(j) = () => evalDefault(default, newScope, ev)
+                argVals(j) = new LazyWithComputeFunc(() => evalDefault(default, newScope, ev))
               } else {
                 if(missing == null) missing = new ArrayBuffer
                 missing.+=(params.names(i))