1 files changed, 14 insertions, 10 deletions
diff --git a/pbt/ts/src/property.ts b/pbt/ts/src/property.ts
index 2b69109..56683c1 100644
--- a/pbt/ts/src/property.ts
+++ b/pbt/ts/src/property.ts
@@ -12,28 +12,28 @@ type TestResult<A> = {
   counterexample: ShrinkResult<A> | null;
 };
 
-type Predicate<A> = (a: A) => boolean;
+type Predicate<A> = (a: A) => Promise<boolean>;
 
 export type Gen<A> = (rng: Prando) => (size: number) => A;
 
 type Shrinker<A> = (a: A) => A[];
 
-type Property<A> = (seed: number, size: number) => TestResult<A>;
+type Property<A> = (seed: number, size: number) => Promise<TestResult<A>>;
 
 const MAX_SUCCESS = 100;
 
-export function findMinimalCounterExample<A>(
+export async function findMinimalCounterExample<A>(
   x: A,
   predicate: Predicate<A>,
   shrinker: Shrinker<A>,
   depth = 0,
-): ShrinkResult<A> {
+): Promise<ShrinkResult<A>> {
   const xs = shrinker(x);
   let counterexample = x;
   let shrinks = depth;
   for (const y of xs) {
-    if (!predicate(y)) {
-      const shrink = findMinimalCounterExample(
+    if (!(await predicate(y))) {
+      const shrink = await findMinimalCounterExample(
         y,
         predicate,
         shrinker,
@@ -58,16 +58,20 @@ export function property<A>(
   predicate: Predicate<A>,
   shrinker?: Shrinker<A>,
 ): Property<A> {
-  return (seed: number, size: number) => {
+  return async (seed: number, size: number) => {
     const rng = new Prando(seed);
     const gen = generator(rng);
     let i = 0;
     for (; i < MAX_SUCCESS; i++) {
       const x = gen(size);
-      if (!predicate(x)) {
+      if (!(await predicate(x))) {
         let counterexample = {counterexample: x, shrinks: 0};
         if (shrinker !== undefined) {
-          counterexample = findMinimalCounterExample(x, predicate, shrinker);
+          counterexample = await findMinimalCounterExample(
+            x,
+            predicate,
+            shrinker,
+          );
         }
         return {seed, result: 'Falsified', tests: i + 1, counterexample};
       }
@@ -239,7 +243,7 @@ export function shrinkPair<A, B>(
 }
 
 export const Property = {
-  run<A>(prop: Property<A>): TestResult<A> {
+  run<A>(prop: Property<A>): Promise<TestResult<A>> {
     const seed = Math.floor(Math.random() * 1000000);
     return prop(seed, 0);
   },