Om Sweet Om: functional front-end engineering with ClojureScript and React
blog.getprismatic.com
blog.getprismatic.com
Your post explained it in general terms. I will probably apply the same in plain JS now. It sounds like an interesting challenge to implement the Cursors idea using React's Update addon [1] instead of depending on Mori. (I don't like Mori because I don't get the feeling that it's intended to be used without ClojureScript - its API is rather cumbersome from plain JS)
I believe that it had something to do with it being relatively much work to update things deep in a tree.
Like, if I have Model -> Users -> User -> WallUpdates -> WallUpdate -> Comment, and I want to make an update to a comment, I'd have to do a lot of code writing. Is that true, or did I just miss the magic function that helps me do this?
But I'm now realizing that maybe, if I want to "manually" do that, I'm just doing it wrong - this should partly be what Om's cursors are meant to avoid, right? (this, and locality to components)
In general, if it helps you as feedback, I found Mori a little difficult to approach, having no Clojure experience. I suspect that some more examples of typical usage patterns at the top of the docs would do a lot. Like, which methods are supposed to be commonplace, and which ones are more for edge cases? What kind of things am I supposed to do all the time, what kind of things should I wish to avoid except in these and these cases?
FWIW, I feel like the same holds for Underscore/Lo-Dash's docs, except it's been less of a problem for me because I'm more familiar with typical usage patterns of mutable data structures.
In the simple little project where I used mori, I did this:
const {
get,
conj,
js_to_clj: map,
clj_to_js: pretty
} = mori;
I can't imagine changing the cljs compiler or adding a piece to mori that must be maintained is important enough for this. I'd imagine Swannodette's answer would be "you should use ClojureScript anyway" :)
[1] -- https://github.com/swannodette/mori/blob/master/src/mori.clj...
This is a nitpick but JavaScript convention is to use camelCase, not underscores, and it feels weird whenever you encounter a library that uses them, usually it's a library that is written by Ruby or Python programmers. It's irritating if it's a library you need to use a lot.
* single mutable ref to normalized app state
* cursors for encapsulation and modularity
* O(1) deep equality checks
* fastest possible react performance
Interestingly, as your app state gets bigger, you are actually forced to use something like cursors for all state updates, for performance reasons. Naive immutability in javascript means clone-before-modify, which is far too slow when your state values are large. It quickly bubbled up in our profiler. Funneling all immutable updates through a cursor means the cursor can use structure sharing on the state value, skipping the defensive clone and preserving reference equality for equal subtrees. Also afaik, implementing a general shouldComponentUpdate (edit: in js) without something like cursors is not possible, because you need === equality for onChange props.
I'm currently building an app where the core state is simply a tree of plain old mutable JS objects. We have all the expected disadvantages (mostly: no easy shouldComponentUpdate), but things are easy to reason about, otherwise. We found that stuff like "has the user expanded a dropdown" or "which auto-completeable search term has just been entered" works perfectly in a local component's setState. The idea is, basically, anything that the user doesn't care losing when navigating away from the current screen can go in a local component's state. This seems to go entirely against the functional immutability idea, but it significantly reduces the amount of updates we need to do to our big central data structure.
I'm going to build a new thing and I'm considering going the nice functional way. However, I can't see any downsides to using directly mutable state to simple, local, unimportant stuff.
Would you do it like that? Or would you avoid component state altogether and go with props only?
In my own application, I started off storing everything in app state, using local state only for short-lived transient data, but over time I found that having a bunch of information like what tab is currently visible in the app state became difficult to manage and made the app state a nightmare. So now I like to keep a strict separation: app state is basically my model - the data being processed (if I were writing a text editor, its the document being edited) and local state is everything else: which dialog is open, which tab is selected, button toggle state, mouse state etc (in a text editor, whether bold is selected, whether I'm in print preview mode or edit mode, etc). Basically app state is what is being processed and local state is how to process or display it. This separation also means that the app state is what gets synced with the server. I like to think of it as "if its something I might want to persist in the database, then it lives in app state, if its something I would never want in the database, then it lives in local state"
I'm curious what other peoples experience with this is.
Another Om question: how do you sync state between client and server, or how do you handle communication?
I looked at how om-sync does this (using tx-listen to automatically sync changes), but ended up building a more traditional request/response model using sente[0] and having my app ask for sub-trees of my app state as needed. Again, just wondering about peoples experience with this and different approaches tried.
For client/server, we've started building up some nice abstractions around our API (like fetching more data in a generic paginated list stored in a cursor), but the write side is still mostly manual. We're trying to use the same API for iOS and web, which probably prevents us from doing some fancier things that continue the same architecture back into the server.
Easy replay/undo was one reason. Another reason was that I wanted to be able to take a _complete_ snapshot of the app state, for debugging purposes. A third was that I wanted to keep all the state that a component needs to know in order to render itself in one place, not many places.
I haven't used Om yet, but a potential approach could be to maintain two stateful objects: one for the model (e.g. stuff that you'd persist permanently), one for the view model (e.g. which tab is selected).
If you wanted your app to come back to exactly the same state after a page reload, you could persist the model in databases, and the view model in browser local storage.
Just like you want to control a react.dom.input, you also may want to control a tabstrip. Maybe you have some biz logic that says "When you click save, select the next record, and if record.type===2, select the third tab". You never know in advance what state needs to be controlled from the top, I found myself refactoring my viewmodel state higher and higher.
The mouse state is never going to be controlled from the top, so that is component local state.
So, to answer your question, we don't propagate events :)
And, how do you go for teaching new comer on the project ? I do suppose you don't only hire Clojure or functional dev, so the transition part seem very interesting, especially with Om getting more (deserved) attention !
most popular databases have native libraries developed by the community (postgres, redis, mongo, etc)
but since Clojure runs on top of the JVM, you can probably access any database through the Java World
It takes a bit of getting used to coming from an ORM, but you'll love the lack of overhead soon enough :)
I'm wondering if this is an issue for you (or perhaps everyone is comfortable enough with the ClojureScript/Om syntax that this isn't a problem)?
But my dream is to be a really good Clojure(Script) lisp programmer. I'm not so good at any lisp, but have went thru a couple of books, try to do every tutorial and to play in the LightTable instarepl and implement things that I can easily do in JS.
How can I, who am far from a lisp pro, get a job in a lisp/clojure shop? I think by doing something 8h/day in a team is the best way to learn, but of course no one will hire you if you slow down the rest (at least in the beginning).
when going all the way down and then up, it gets stuck around the 2nd or 3rd panel
I have heard of it and used it myself.
The primary difference is really the intended use case. Where ETM is focusing on highly-specific (usually on a nutrient/caloric basis) meal plans to hit a target nutritional intake, we are more centered around using general preferences about food (ingredients, cuisine, cooking methods, etc.) in a "browsing" capacity -- currently in the form of recipes aggregated from other sites and bloggers. The goal is eventually to also incorporate nearby restaurant dishes.
It takes away a lot of the data-binding cruft that you find in Backbone.View and works well with React without making you switch languages, design paradigms, or add new libraries. The Backbone component makes it easier to build data hierarchies than with plain JS, but I assume other MVC frameworks could be retrofitted to work with React.
cljsbuild app.cljs > app.js
I think what GP is complaining about is the compilation step, which requires a JVM and isn't exactly the fastest.
Touche - You don't need project.clj and leiningen and can run a command basically like you're asking for. I'm not sure how much faster it is than using leiningen and cljsbuild, as I haven't tried it: https://github.com/clojure/clojurescript/wiki/Quick-Start#cl...
You could also use something like Nailgun too, if that is more desirable.
When I develop in ClojureScript, I generally avoid rebuilding altogether whenever possible (auto or otherwise) and instead have my editor send changes to the browser REPL. I don't know if you can do this with node too, but it would seem odd if not. This seems to be the preferred approach by most Clojure devs.
Btw, I ran: time lein cljsbuild once on a project with a minor change (added a line break) and it took 22 seconds.... so this is also what I'm talking about.
Transpiling a bucket load of any language in a warm jvm will be less than 2-3 seconds, and could easily be done on page refresh.
to give you some perspective, we have massive closure apps, first compilation on my (very powerful) desktop takes a minute 30 seconds or so (advanced optimizations and several other extremely stringent flags along with quite a few compiler plugins we wrote) keeping the jvm warm, and compiling with these same flags + plugins, the time comes down to under 5 seconds.
This is a good analysis of why Clojure startup is slow: http://nicholaskariniemi.github.io/2014/02/25/clojure-bootst...
This is why I think ClojureScript/Node should become the "default" environment for writing Clojure. It would open up Clojure to a new class of apps that are currently not practical.
I can't see them porting the cross compilation of clojurescript to node.
I guess the way React handles input is by attaching event handlers to child components, so that changes to the child properties bubble up to the root component state. Is this the optimal pattern for handling events in a React app? Or is there an advantage to be gained by using core.async channels to process event input for Om/react?
I haven't seen any examples which use both core.async and Om/react, but I'm very curious about the possibility.
1. http://rigsomelight.com/2013/07/18/clojurescript-core-async-...
2. http://swannodette.github.io/2013/07/12/communicating-sequen...
that's strange, because Om's tutorials use core.async
https://github.com/swannodette/om/wiki/Basic-Tutorial
https://github.com/swannodette/om/wiki/Intermediate-Tutorial
I'm relieved to know it was there the whole time and I simply missed it, thank you.
The basic [2] tutorial on the Om wiki uses core.async heavily as well. Also, there are a ton of Om examples [3] within the repo itself that are a good reference.
1. https://github.com/jalehman/omtut-angular
If your application state is large but you only care about a few parts of it at a time (a few "cursors") how does Om deal with that?
--
To give more background, let me give more specifics over what I'm thinking about when I ask this.
I've been working on a JS accounting app. One thing about accounting is that the balance of an account is dependent on every prior transaction that the account has ever had. So modeled in functional terms, it seems like you'd have to make this a tree of depth "N" where N is the total number of transactions in the account. It seems like you have to model it this way to get the data dependencies right (a change in an early transaction affects every subsequent transaction).
A functional approach with such a deep tree seems to break down (at least from the relatively little I know about Om). For one, I don't want to have to load every transaction into memory. Even though the most recent transaction depends logically on the first one, I don't want to be forced to load the first one just to show the most recent one. One possible way to work around this is to collapse all transactions prior to the ones I want to display into a single "fake" transaction that stores the accumulated balance as of this point. But now my in-memory model has to differ from my real model, and as the two diverge things start to get more complicated.
A bigger problem: I can't think of a good way, with this design, to indicate to the data layer which transactions the UI actually cares about and wants to have loaded into memory. If the tree has only one logical root (the most recent transaction) and is logically N transactions deep, how does the UI indicate which of those N it actually cares about? Maybe Om cursors can do this somehow?
I've been using a more traditional mutation-based/observable design, which seems to be working well. The UI can "subscribe" to certain transactions/balances, and this serves as a signal to the data layer about what needs to be loaded and what derived values need to be kept up to date. The data layer can be smart about how many transactions it actually needs to load to compute what the UI cares about.
With regard to Om or Clojure I'm not sure of the specifics of such an implementation, but doing something like that with other languages that have functional programming support is one of the Major Selling Points.
In my application, there is a lot of data that doesn't always need to be loaded - for example, various metrics can be viewed in charts, but since there can be a lot of data points, you wouldn't want to load metrics which aren't being viewed (since there could be many dozen different metrics, yet you may only care about 3 or 4 of them). So I think this is a good example for your question.
In itself, Om does not deal with this and assumes that the entire application state is loaded at all times. As far as Om is concerned, everything is always there in a big tree structure.
But since Om lets you update the state easily through cursors, it is easy to update sub-trees when needed. So there are two very similar approaches that I've tried out:
1. If you have data where you basically select one of a collection datasets (eg in my application, if I can only chart one metric at a time), then you can have an end-point in the tree for this data and you attach the currently selected data when it is needed, swapping the old for the new. You can easily do this with an om/update! command. It is also easy to then send the old data to the server while the new data is loaded into the state, if you have changes you need to keep in sync (om/transact! gives you the old value).
2. I have experimented with creating Om components that read the value of their cursor for some sentinel value (maybe nil or a special keyword or something) that tells it the data is not loaded, which then requests the data from the server before passing it to its child component for consumption (perhaps throwing up a loading indicator in the meantime). You can then also implement logic to unload data again (maybe a timeout after the child stops using it). This works quite well for me.
A possible improvement on #2 that I want to try out when I get a few hours is to make use of Clojures lazy sequences, rather than using a sentinel value to implement lazy loading. It seems like a natural fit, but I have yet to try it.
There are, of course, many other approaches one could take (eg, keeping track of what's loaded in a collection instead of using sentinels), but I have not tried any others.
So I guess, in summary, no, Om doesn't deal with this for you, but it gives you the tools to build it yourself to suit your needs. Personally, I like it this way because I know my access patterns better than Om does.
e.g. are you using something like:
app component <-> page components <-> (widget) components
where the app creates the page and the page creates the widgets and communication is flowing through channels?
and how do you deal with url transitions?
do you capture and flow up the click from a widget back to the app so it knows it's time to render a different page? and have some (case ...) at the app level?
or using some kind of mix with secretary and changing the location.href/a hrefs?
etc