manifold-cljs

Manifold implementation in Clojurescript

manifold-cljs

Build Status

A port of Manifold to Clojurescript.

This port tracks the latest Manifold version as closely as possible. As per Clojurescript port issue, Zach wanted to keep the port separate from the main project - so here it is. However, ~80% of the code was copied from the Clojure Manifold verbatim, so there’s a chance it might get ported back with reader conditionals.

The port hasn’t been used in any serious applications yet, but there are some tests. And they pass!

There are no blocking operations in Javascript, so some of the original Manifold functions had to go.

Usage

Add the following dependency to your project.clj or build.boot:

[manifold-cljs "0.1.6-0"]

Then use it in your project:

(ns example.project
  (:require [manifold-cljs.stream :as s]
            [manifold-cljs.deferred :as d]))

You can find several examples in the examples/ directory.

Extensions

Differences to Clojure implementation

Executors

manifold-cljs.executor defines an Executor protocol with implementations backed by goog.async.nextTick, setTimeout, synchronous execution and a batched variation which takes another executor as its implementation. An executor is selected while creating a Stream or a Deferred.

The call to manifold-cljs.executor/executor will always return the batched next-tick executor by default, so all of the callbacks on Streams and Deferreds are executed as tasks. Either as microtasks, if setImmediate is available, or as tasks (setTimeout). This means that the code will behave similarly to the way it would behave on the JVM if every stream and deferred were executed on an executor.

On the JVM, the call to manifold.executor/executor will return no executor by default, which will make the callbacks run on whichever thread triggered the completion of the Deferred.

The difference becomes apparent in the following snippet:

(let [s (s/stream), b (s/batch 2 s)]
  (s/put-all! s [1 2 3])
  (s/close! s)
  (assert (= [[1 2] [3]] (s/stream->seq b))))

The above succeeds when run on a single thread, but fails if run within a (manifold.executor/with-executor (manifold.executor/execute-pool) ...) block. The second result element - [3] - nevers gets delivered to the batched stream as the source stream gets closed first. All the Consumer events registered on the source get canceled.

Blocking put/take

stream/put and stream/take have the same signature as their Clojure counterparts, however setting the blocking? parameter to true will always trigger an assertion error. Puts and takes in manifold-cljs will always return a deferred result. Consequently there is no way to synchronously connect streams and stream/isSynchronous always returns false.

Metadata

I couldn’t find a protocol allowing mutation of metadata in Clojurescript. The default deferred and stream implementations are mutable, so there are no IMeta or IWithMeta implementations for streams/deferreds.

Deferred protocols

The protocols for the Deferred have been moved to manifold-cljs.deferred.core. The default implementation - to manifold-cljs.deferred.default. This is analogous to what has been done to streams. This was done in order to avoid a cyclic dependency between manifold-cljs.deferred, where the protocols used to live, and manifold-cljs.time. Clojurescript is compiled statically and can’t require a namespace in the middle of the file.

Ideally we should propagate this change to the Clojure Manifold.

Missing functions

  • manifold-cljs.stream/stream->seq - inherently blocking
  • manifold-cljs.deferred
    • let-flow - TODO: needs some advanced code walking
  • manifold-cljs.executor
    • instrumented-executor - TODO: do we want this in Cljs?
    • *stats-callbacks* - TODO: do we want this in Cljs?
    • utilization-executor - not applicable
    • execute-pool - not applicable
    • wait-pool - not applicable
  • manifold-cljs.time
    • format-duration - niche
    • floor - niche
    • add - niche
    • IClock - TODO: do we want this in Cljs?
    • IMockClock - TODO: do we want this in Cljs?
    • mock-clock - TODO: do we want this in Cljs?
    • *clock* - TODO: do we want this in Cljs?
    • with-clock - TODO: do we want this in Cljs?
    • scheduled-executor->clock - not applicable
  • manifold-cljs.utils
    • without-overflow - not used
    • fast-satisfies - don’t need
    • with-lock - don’t need

Cljs-only functions

  • manifold-cljs.deferred
    • time - measure time taken to evaluate the body in a deferred

TODO

  • WeakMap dependency - this can somehow be compiled in by the GCC - how?
  • unhandled error reporting - like goog.Deferred/Bluebird
  • DEBUG stack traces - like goog.Deferred/Bluebird
  • better logging - format to dev console?
  • performance - currently ~3x slower than core.async on the daisy example
  • deferred/let-flow - needs a different deep code walking impl/riddley replacement for Cljs

See Closure Promise for more ideas.

Patterns and Gotchas

Manifold is in need of best practices/patterns/gotchas library.

Writing d/loop-based stream combinators

Many of the stream combinators, like s/zip, use d/loop inside to take from a source stream and put into the destination stream. There is an additional step needed to make a combinator like that work well when the source stream is connected to other streams as well as combined via the combinator - passing values through an intermediary stream. See related Github issue for more info.

Error handling

Signalling “no more messages” upstream

Upstream is really closed only after an additional put

Most people expect the s/on-closed callback to get called once the s/close! is called on a stream. This is true if the callback is registered on the stream that is being closed. However, in case we s/close! a downstream stream and the callback is registered upstream, the close callback will only trigger once the producer tries to put another value into the upstream. See this and this Github issues for more info.

d/let-flow won’t handle deferred conditionals in a smart way

If you have a conditional clause where the condition is a deferred as well as a result - the result deferred will be awaited even if the condition is falsey, e.g.:

(let [x (deferred-never-realized-unless-y)]
  (let-flow [y (deferred-false)]
    (if y x :ok))

The above will block on x, even though y realizes to false.

See this Github issue for more info.

License

Copyright © 2016 Zach Tellman, Vadim Platonov

Distributed under the MIT License.

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