feat: add SafeAsyncIter chaining

Author: gmega

codex/utils/safeasynciter.nim

@@ -232,3 +232,28 @@ proc empty*[T](_: type SafeAsyncIter[T]): SafeAsyncIter[T] =
     true
 
   SafeAsyncIter[T].new(genNext, isFinished)
+
+proc chain*[T](iters: seq[SafeAsyncIter[T]]): SafeAsyncIter[T] =
+  if iters.len == 0:
+    return SafeAsyncIter[T].empty
+
+  var curIdx = 0
+
+  proc ensureNext(): void =
+    while curIdx < iters.len and iters[curIdx].finished:
+      inc(curIdx)
+
+  proc isFinished(): bool =
+    curIdx == iters.len
+
+  proc genNext(): Future[?!T] {.async: (raises: [CancelledError]).} =
+    let item = await iters[curIdx].next()
+    ensureNext()
+    return item
+
+  ensureNext()
+
+  return SafeAsyncIter[T].new(genNext, isFinished)
+
+proc chain*[T](iters: varargs[SafeAsyncIter[T]]): SafeAsyncIter[T] =
+  chain(iters.toSeq)

tests/codex/utils/testsafeasynciter.nim

@@ -373,7 +373,7 @@ asyncchecksuite "Test SafeAsyncIter":
     # Now, to make sure that this mechanism works, and to document its
     # cancellation semantics, this test shows that when the async predicate
     # function is cancelled, this cancellation has immediate effect, which means
-    # that `next()` (or more precisely `getNext()` in `mapFilter` function), is 
+    # that `next()` (or more precisely `getNext()` in `mapFilter` function), is
     # interrupted immediately. If this is the case, the the iterator be interrupted
     # before `next()` returns this locally captured value from the previous
     # iteration and this is exactly the reason why at the end of the test
@@ -404,14 +404,29 @@ asyncchecksuite "Test SafeAsyncIter":
 
     expect CancelledError:
       for fut in iter2:
-        if i =? (await fut):
+        without i =? (await fut), err:
           collected.add(i)
-        else:
-          fail()
 
     check:
       # We expect only values "0" and "1" to be collected
       # and not value "2" that - although resolved and ready to be returned -
       # will not be returned because of the cancellation.
       collected == @["0", "1"]
       iter2.finished
+
+  test "should allow chaining":
+    let
+      iter1 = SafeAsyncIter[int].new(0 ..< 5)
+      iter2 = SafeAsyncIter[int].new(5 ..< 10)
+      iter3 = chain[int](iter1, SafeAsyncIter[int].empty, iter2)
+
+    var collected: seq[int]
+
+    for fut in iter3:
+      without i =? (await fut), err:
+        fail()
+      collected.add(i)
+
+    check:
+      iter3.finished
+      collected == @[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]