infer an empty collection literal's type from the first value stored into it

Makefile

@@ -1123,6 +1123,7 @@ MEMCHECK_CASES := \
 	tests/cases/test_list_transform_ownership \
 	tests/cases/test_result_unwrap_or_fallback_leak \
 	tests/cases/test_borrowed_operand_extraction \
+	tests/cases/test_task_result_payloads \
 	tests/cases/test_struct_closure_field tests/cases/test_generic_struct_field_dtor \
 	tests/cases/test_iterator_drain tests/cases/test_method_fresh_string_return \
 	tests/cases/test_lazy_adapter_pipeline tests/cases/test_named_local_adapter_chain \

bench/grpc/pith/client.pith

@@ -58,17 +58,16 @@ fn echo_request(payload: Bytes) -> Bytes:
     w.write_bytes(1, payload) catch false
     return w.bytes()
 
-# time each call, pushing the latency of every call into `lat` (a shared list
-# handle created by the caller — task results with list payloads are avoided on
-# purpose, the handle is the simpler contract).
-fn timed_batch(ch: grpc.Conn, method: String, request: Bytes, count: Int, lat: List[Int]) -> Int:
+# time each call and return the latency of every call.
+fn timed_batch(ch: grpc.Conn, method: String, request: Bytes, count: Int) -> List[Int]:
+    mut lat := []
     mut i := 0
     while i < count:
         t0 := time.mono_nanos()
         ch.unary(method, request) catch bytes.empty()
         lat.push(time.mono_nanos() - t0)
         i = i + 1
-    return count
+    return lat
 
 # each worker gets its OWN connection from the pool (round-robin) and keeps its
 # calls on that one connection's pipeline, so the pool spreads load across
@@ -77,18 +76,14 @@ fn timed_batch(ch: grpc.Conn, method: String, request: Bytes, count: Int, lat: L
 fn run_concurrent(pool: grpc.PoolConn, method: String, request: Bytes, calls: Int, conc: Int, merged: List[Int]):
     per := calls / conc
     mut tasks := []
-    mut buckets: List[List[Int]] := []
     mut w := 0
     while w < conc:
         c := pool.at(w % pool.size())
-        bucket: List[Int] := []
-        buckets.push(bucket)
-        tasks.push(spawn timed_batch(c, method, request, per, bucket))
+        tasks.push(spawn timed_batch(c, method, request, per))
         w = w + 1
     for t in tasks:
-        await t
-    for bucket in buckets:
-        for v in bucket:
+        lat := await t
+        for v in lat:
             merged.push(v)
 
 # the value at `pct` percent of the way through a sorted list, matching the
@@ -160,7 +155,8 @@ fn main():
     start := time.now()
     latencies: List[Int] := []
     if conc <= 1:
-        timed_batch(ch.at(0), method, request, calls, latencies)
+        for v in timed_batch(ch.at(0), method, request, calls):
+            latencies.push(v)
     else:
         run_concurrent(ch, method, request, calls, conc, latencies)
     elapsed := time.now() - start

docs/idiomatic_pith.md

@@ -69,6 +69,12 @@ few elements, switch to the lazy adapters in `std.iter`. The chain
 fuses and the source is walked once. See `docs/iterators.md` for the
 protocol and worked examples.
 
+An empty literal takes its element type from an annotation when you give one
+(`mut names: List[String] := []`), and otherwise from the first value stored
+into it — `mut tasks := []` followed by `tasks.push(spawn worker())` types
+`tasks` as a list of tasks. Annotate when the first store sits far from the
+declaration and a reader would have to hunt for it.
+
 Collections are shared handles. If a function needs to mutate its own top-level
 container, start with `copy_list`, `copy_map`, or `copy_set`.
 

examples/concurrency.pith

@@ -27,6 +27,16 @@ fn main() -> Int!:
     value := await spawn compute(21)
     print(value.to_string())
 
+    # fan out over a list of tasks; the list needs no annotation, the
+    # first push names its element type
+    mut work := []
+    for n in 1..=3:
+        work.push(spawn compute(n))
+    mut doubled := []
+    for t in work:
+        doubled.push(await t)
+    print("doubled: {doubled[0]} {doubled[1]} {doubled[2]}")
+
     jobs := Channel[Int](1)
     jobs.send(7)
     picked := select:

examples/expected/concurrency.txt

@@ -1,5 +1,6 @@
 response from https://example.com
 42
+doubled: 2 4 6
 select picked: 7
 await_ctx timed out: true
 send_ctx ok: true