manifold

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Case in point:

(require '[manifold.deferred :as md])

(let [d (md/deferred)]
  (-> d
      (md/timeout! 1000 :timeout)
      (md/chain println))

  ;; Succeeding d with a deferred that never succeeds.
  (md/success! d (md/deferred)))

;; println never triggers

Is this an expected behavior? If not, would it be a breaking change to fix it?

Looks like this is expected https://github.com/ztellman/manifold/blob/master/docs/deferred.md#composing-with-deferreds

Note that if a timeout is placed on a deferred returned by chain, the timeout elapsing will prevent any further stages from being executed.

This ticket is about a different thing, it's about timeout! not working for nested deferreds.

In the example, I use 3-arity timeout! which doesn't short-circuit the chain:

(-> (md/deferred) (md/timeout! 1000 :timeout) (md/chain println))
;; Prints :timeout after a second.

@alexander-yakushev This doesn't have anything to do with timeout!. If you remove the timeout! call from the let, it still exhibits the same behavior:

(let [d (md/deferred)]
  (-> d
      (md/chain println))

  ;; Succeeding d with a deferred that never succeeds.
  (md/success! d (md/deferred)))

=> true

The reason is that chain is designed to pause on unrealized inputs. From the docstring: "When composing, either x or the returned values may be values which can be converted to a deferred, causing the composition to be paused." (Maybe not the clearest language there...)

Since you resolve d with another deferred immediately, the timeout doesn't kick in, and d's value is passed down to (md/chain println). But, since the value of d was an unrealized deferred, the chain pauses.