Add select future combinator

examples/workflow.py

@@ -13,9 +13,13 @@
 # pylint: disable=W0613
 # pylint: disable=C0301
 
+from datetime import timedelta
 
 from restate import Workflow, WorkflowContext, WorkflowSharedContext
-from restate.exceptions import TerminalError
+from restate import select
+from restate import TerminalError
+
+TIMEOUT = timedelta(seconds=10)
 
 payment = Workflow("payment")
 
@@ -38,13 +42,17 @@ def payment_gateway():
     ctx.set("status", "waiting for the payment provider to approve")
 
     # Wait for the payment to be verified
-    result = await ctx.promise("verify.payment").value()
-    if result == "approved":
-        ctx.set("status", "payment approved")
-        return { "success" : True }
 
-    ctx.set("status", "payment declined")
-    raise TerminalError(message="Payment declined", status_code=401)
+    match await select(result=ctx.promise("verify.payment").value(), timeout=ctx.sleep(TIMEOUT)):
+        case ['result', "approved"]:
+            ctx.set("status", "payment approved")
+            return { "success" : True }
+        case ['result', "declined"]:
+            ctx.set("status", "payment declined")
+            raise TerminalError(message="Payment declined", status_code=401)
+        case ['timeout', _]:
+            ctx.set("status", "payment verification timed out")
+            raise TerminalError(message="Payment verification timed out", status_code=410)
 
 @payment.handler()
 async def payment_verified(ctx: WorkflowSharedContext, result: str):

python/restate/__init__.py

@@ -22,7 +22,7 @@
 # pylint: disable=line-too-long
 from .context import DurablePromise, RestateDurableFuture, RestateDurableCallFuture, RestateDurableSleepFuture, SendHandle
 from .exceptions import TerminalError
-from .asyncio import as_completed, gather, wait_completed
+from .asyncio import as_completed, gather, wait_completed, select
 
 from .endpoint import app
 
@@ -56,4 +56,5 @@ def test_harness(app, follow_logs = False, restate_image = ""): # type: ignore
     "gather",
     "as_completed",
     "wait_completed",
+    "select"
 ]

python/restate/asyncio.py

@@ -27,6 +27,38 @@ async def gather(*futures: RestateDurableFuture[Any]) -> List[RestateDurableFutu
         pass
     return list(futures)
 
+async def select(**kws: RestateDurableFuture[Any]) -> List[Any]:
+    """
+    Blocks until one of the futures is completed.
+
+    Example:
+
+    who, what = await select(car=f1, hotel=f2, flight=f3)
+    if who == "car":
+        print(what)
+    elif who == "hotel":
+        print(what)
+    elif who == "flight":
+        print(what)
+
+    works the best with matching:
+
+    match await select(result=ctx.promise("verify.payment"), timeout=ctx.sleep(timedelta(seconds=10))):
+    case ['result', "approved"]:
+        return { "success" : True }
+    case ['result', "declined"]:
+        raise TerminalError(message="Payment declined", status_code=401)
+    case ['timeout', _]:
+        raise TerminalError(message="Payment verification timed out", status_code=410)
+
+    """
+    if not kws:
+        raise ValueError("At least one future must be passed.")
+    reverse = { f: key for key, f in kws.items() }
+    async for f in as_completed(*kws.values()):
+        return [reverse[f], await f]
+    assert False, "unreachable"
+
 async def as_completed(*futures: RestateDurableFuture[Any]):
     """
     Returns an iterator that yields the futures as they are completed.