Error on invalid dispose function.

src/generators/disposable.js

@@ -2,6 +2,9 @@ import that from "../that.js";
 
 export default function disposable(value, dispose) {
   let done = false;
+  if (typeof dispose !== "function") {
+    throw new Error("dispose is not a function");
+  }
   return {
     [Symbol.iterator]: that,
     next: () => done ? {done: true} : (done = true, {done: false, value}),

src/generators/observe.js

@@ -6,6 +6,12 @@ export default function(initialize) {
   let resolve;
   const dispose = initialize(change);
 
+  if (dispose != null && typeof dispose !== "function") {
+    throw new Error(typeof dispose.then === "function"
+        ? "async initializers are not supported"
+        : "initializer returned something, but not a dispose function");
+  }
+
   function change(x) {
     if (resolve) resolve(x), resolve = null;
     else stale = true;

src/generators/queue.js

@@ -5,6 +5,12 @@ export default function(initialize) {
   const queue = [];
   const dispose = initialize(push);
 
+  if (dispose != null && typeof dispose !== "function") {
+    throw new Error(typeof dispose.then === "function"
+        ? "async initializers are not supported"
+        : "initializer returned something, but not a dispose function");
+  }
+
   function push(x) {
     queue.push(x);
     if (resolve) resolve(queue.shift()), resolve = null;