And as a side note: can we *please* stop shitting on every React post that appears on HN? All this hater energy is really starting to be a drag.
The most basic example is making an external request based on props: can't do that in the render loop because it has to be pure, since React may run it multiple times before committing to the DOM. `useEffect` is the only way you can get a guarantee that it will only be run once after rendering, and only when its dependencies change.
But there is a lot of mutable code that could be immutable, and `useEffect` is how that happens in React.
> you can "mutate state" in regular callbacks without useEffect
Yes, via `useState`. That too gets overused. (Note: useEffect and useState are 100% necessary, but also easily overused.)
useEffect for when you want to manipulate state outside the component. I.e. change the document title, exchange data with an HTTP server, store data in localStorage.
The hooks yield effect descriptions, to be carried out later, on a side channel. So the component is still a pure map of the inputs to the outputs, just that the outputs are not just comprised of the javascript function's return value, but also the effects on the side channel.
When you get different answers for the same question, then you’re not calling a function.
When you can only get the same result by recreating the same internal state through external manipulation, you’re dealing with side effectful, imperative code.
Hooks are ergonomic and easy to reason about and that’s great. But they turn functions into objects.
The whole idea of the useState hook, is that it’s _not_ internal state to the hook. That state isn’t stored on the stack of the hook function, but against the component.
You cannot call the component with the same arguments getting the same results anymore AKA it’s not pure. You indirectly mutate it via event handlers, which are effectively methods.
If you attach an event handler, that "attaching" is an effect and executed outside the rendering. If the handler changes some state of the context, a rerender is triggered.
The value proposition of FP doesn't come from whether you feel like you're doing FP during implementation. It can only be assessed from the perspective of the caller. The caller doesn't care about the philosophical differences of how you achieve internal mutation or the implementation thereof.
They only care about whether you are returning the same result when they give you the same arguments. That's how you satisfy a function interface.
A component that keeps track of internal state with useState and modifies it via event handlers does not satisfy a functional interface, because given the same props, it may or may not return the same docoument fragment.
One cannot pass in the same state, nor can one see the state from the return value. The signature of a component that uses useState and event handlers to modify it, _hides_ internal state from the caller. A component like that doesn't satisfy a functional interface but does in fact hide an object interface via local retention and message passing.
Vice versa, a component that bangs on an internal variable in order to derive/calculate values that it returns, can satisfy a functional interface from the caller's perspective. Similarly you can use useState and useEffect to implement a functional interface. It all comes down to whether you are giving the same answer for the same question, which is typically not what you're doing with hooks.
Note that I'm not making a value judgement about whether a component is a function or an object (via hooks). If you need an object, then write an object. But be aware of the tradeoffs of writing a function vs an object and be upfront about it to your caller.
And a very helpful abstractions is to say that the context that you are saying is impure is just another parameter and another part of the return value. Because that's how React is treating context, and that's why you can treat React components as functionally pure if you respect the rules of hooks.
FP doesn't demand the parameters stay the same. It also allows for outputs of a function to get put back in as a parameter in a subsequent invocation, which is what happens to the context. If you insist on seeing it another way, I can't stop you, it's just more complicated and I don't see how that's helping.
My point is that _you as the caller_ are _not_ doing this when calling a stateful component Foo. Foo mutates between events and state is entirely encapsulated, so you are getting different results from the same question. The caller doesn't care about the internal execution model of Foo and React. A functional interface is _only_ satisfied if the function is referentially transparent. Which in this case it is not.
If all you can do is send messages and observe behavior from outside. It behaves exactly like an Object and not like a Function from the perspective of the caller.
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You are arguing in terms of the implementer of a stateful component. You are thinking in terms of how React/useState behaves from inside your component and further up the call stack. And in this context I agree. You can think of useState as part of your signature so to speak. In fact the hooks rules prevent you from doing things that violate this mental model.
But that's not what I'm talking about. I'm talking about the practical, real world perspective of a caller and whether they observe functional or object oriented behavior from your component.
They are input, but they are not input parameters.
A function is pure if it returns same result for same input parameters. Hooks make it possible to return different result for same input parameters, therefore making the function impure.
And that's why the context manipulated by hooks looks like a sideeffect, and it often does describe sideeffects, but it doesn't. That context is both an input and an output, only the rendering logic in React decides what to do with that output. Without the rendering logic deciding to execute those sideeffect, the context isn't actually changed at all. Which is why you usually don't need to care about when or how often the component is executed.
The whole point of pure function is that the caller can understand its dependencies without looking at its implementation, because all dependencies are listed as parameters.
You can redefine what "pure" means if you wish, but that pretty much defeats the purpose.
It's a cheap shot to complain that Javascript isn't a purely functional language... React can be used in a pretty pure way and if you don't you'll don't get the full benefit.
All sorts of things can carry state and be mutable in a React application. Even the properties of a component are actually mutable, and it sometimes happens that people pass around arrays or objects and mutate them when they shouldn't.
The react docs literally call it an escape hatch [0]. First sentence.
[0]https://beta.reactjs.org/learn/you-might-not-need-an-effect
useEffect let’s you safely communicate with the world outside react (ie cause side effects). It’s the only reliable way to …escape.
The original docs made no such claim.
Which brings us back to the biggest problem with useEffect. Even the react devs don’t seem to have a clue how it should actually used, until only recently, despite it being among the 2 most used hooks ever since hooks were introduced.
The original docs are outdated
The abstraction itself is ripe for misuse
Tons of devs use it when they shouldn’t.
Insert shrug emoji here.
Ie writing reactive code but relying on the React render loop to trigger reactions. The problem is that you can no longer step through your code in full and as you say, your code now mandates flushes to the DOM that are unnecessary (mid computation).
It's better to setRows at explicit places or learn and use a library like rxjs if you really need to encapsulate that complexity.
Kind of ironic when this post kicked off a long argumentative thread shining a light on the exact things people don't like about React... "escape hatch", "impure side effects", "useEffect... huge point of failure... it sucks", arguments whether hooks and refs are immutable or not... and these comments are coming from people who claim to like React!
E.g. an outspokenly pro-climate politician who passed the 'frack through orphanages with baby seals bill' will not be taken seriously because actions don't match reality.
IIRC useEffect was not originally called an escape hatch and that terminology was pegged on once they realised that a fundamental hook was actually a 30mm footgun.
To make it worse they haven't provided a clear way to deal with the problem instead relying on 3rd party packages
Don’t take my word for it though. Dan Abramov himself has said pretty much what I’m saying (you’d have to find it on Twitter). Ken C Dodds shared a talk (not run by him) called “Goodbye UseEffect”. Look this stuff up if you haven’t. It’s out there. There are often better ways than useEffect. And when those don’t work you can always use the esc… well you get the idea.
EDIT: spelling
So like many other functional programming langs/libs/frameworks react had a controlled means to run side effects.
That’s the escape hatch of useEffect. It’s not a bad thing. It’s thematically same as the unsafe block in Rust. It allows you to reach out of the main paradigm, when required.
React is complex because views aren't pure; slow because it has to rerender things that didn't change, because it doesn't actually know what changed, because views are impure; hard to program because impure views create bugs that even that extra complexity can't fix; and has all those escape hatches to change execution time and memoize things because the execution time actually has semantic value.
And that's react, that incredibly well designed thing with incredibly knowledgeable authors, that made a huge effort to think about every problem.
Non-pure reactive systems just do not work.
The idea is to have an Observable object that can store arbitrary trees of data (each itself an Observable). UI drawing is generally split into a function call for each DOM element that has children. These functions are executed such that they track the use of observables and will be rerun when they change.
I've more recently, after having suffered React and friends, started creating an Open Source implementation of the pattern we used. Currently, it's mostly still lacking in documentation and marketing, and that's unlikely to change if I'm honest. But this should give you an impression if you're interested:
https://github.com/vanviegen/aberdeen/blob/master/examples/t...
Curious to know your thoughts about this being an effective non-pure reactive system.
Because in React, the smallest unit is the component.
In Solid the smallest unit is the whatever reactive value you use, and only that changes. Components are there just to organize your code and do the initial render.
React components can, and I would argue in vast majority of cases should, be pure functions. You just need to displace state management outside of React.
This removes the need for class components and hooks and returns React to its roots as a view library. Most complications in React seem to stem from its desire to also be a state library.
Case in point: we used MobX with React to build a UI for an entire document management system (essentially a file system with versioning, security and workflows) - I don’t think we have more than a handful of impure React components in the entire codebase.
You can even have a lot of pure state interdependence. What React doesn't even try (because yes, it's a view library).
Anyway, one can certainly keep all views pure. It's a bit hard in Javascript where a lot of things are hidden, but it's perfectly doable. It's even the recommended way to use React. But that doesn't save React from being way larger, slower, and harder to use than it could otherwise be.
The beta react docs have a number of great examples of this!
cf. "can be please stop hyping .. [favoured tech of the month/year/decade]"
Talk about pissing into the wind.
[0]https://beta.reactjs.org/learn/you-might-not-need-an-effect
This term is typically used for ways of breaking the rules of a framework. With useEffect you’re still very much in React world and need to follow the rules to achieve much of anything with it.
The norm would be not to use an escape hatch. But I doubt there are many React applications that aren’t using useEffect.
I think HN is averse to it because it's blamed for "how complicated the web has become". If only we would all settle for simple static websites!