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Sorry, what do you mean by mutable vs immutable code? I don't think it necessarily has to do with mutability, because you can "mutate state" in regular callback

Sorry, what do you mean by mutable vs immutable code? I don't think it necessarily has to do with mutability, because you can "mutate state" in regular callbacks without useEffect. The only way to get true mutability in React is via refs, all other state is immutable.

paulddraper 4y ago
I mean pure vs impure. > 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.
- bayesian_horse 4y ago
  Also: even with hooks, the component is still a pure function in a less low-level kind of way. 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.
  - dgb23 4y ago
    That’s not pure anymore for any practical definition. 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.
    - ojkelly 4y ago
      What you’re missing here is that the hooks form part of the input to the hook/render function. There’s a reason they can’t be conditional (though with a compiler theoretically they could be). 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.
      - dgb23 4y ago
        I understand that, but your component is now an object that can be mutated via events and it tracks internal state via calls to useState. 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.
        bayesian_horse 4y ago
        no, it can't be mutated by events. The hooks only react to inputs in the side channel and only during execution. 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.
        dgb23 4y ago
        We're either talking about two different things or you are missing the forest in the trees. 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.
  - LudwigNagasena 4y ago
    Any function is a pure function if your consider the entire program state as your input and output.
    - satvikpendem 4y ago
      I see you've discovered the IO monad in Haskell.
    - bayesian_horse 4y ago
      You really don't need to give hooks access to some global variables (or all of them). You can, of course. But you would actually do that in a useEffect or use some setState in a callback somewhere. 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.
bayesian_horse 4y ago
The last statement isn't true unless you mean "mutability" in just the DOM elements. 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.
- phailhaus 4y ago
  Well, javascript is mutable so technically nothing is immutable. But I'm referring to working with React's hooks, where all props and state should be treated as immutable except for refs.
  - bayesian_horse 4y ago
    Refs are just as immutable in that perspective. And no, you're not supposed to mutate ref.current.anything outside useEffect or a callback or similar...
    - phailhaus 4y ago
      Of course you can! That's the whole point of refs, and why you can't use them as a dependency to callbacks or effects. They're totally mutable, which makes them very tricky to deal with. I mean, I wouldn't recommend updating them outside of effects/callbacks, but it's valid. For example: you can make a ref to count the number of times React calls your render loop by updating it in the body.

paulddraper 4y ago

I mean pure vs impure. > 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.

bayesian_horse 4y ago
Also: even with hooks, the component is still a pure function in a less low-level kind of way. 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.
- dgb23 4y ago
  That’s not pure anymore for any practical definition. 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.
  - ojkelly 4y ago
    What you’re missing here is that the hooks form part of the input to the hook/render function. There’s a reason they can’t be conditional (though with a compiler theoretically they could be). 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.
    - dgb23 4y ago
      I understand that, but your component is now an object that can be mutated via events and it tracks internal state via calls to useState. 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.
      - bayesian_horse 4y ago
        no, it can't be mutated by events. The hooks only react to inputs in the side channel and only during execution. 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.
        dgb23 4y ago
        We're either talking about two different things or you are missing the forest in the trees. 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.
- LudwigNagasena 4y ago
  Any function is a pure function if your consider the entire program state as your input and output.
  - satvikpendem 4y ago
    I see you've discovered the IO monad in Haskell.
  - bayesian_horse 4y ago
    You really don't need to give hooks access to some global variables (or all of them). You can, of course. But you would actually do that in a useEffect or use some setState in a callback somewhere. 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.

bayesian_horse 4y ago

The last statement isn't true unless you mean "mutability" in just the DOM elements. 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.

phailhaus 4y ago
Well, javascript is mutable so technically nothing is immutable. But I'm referring to working with React's hooks, where all props and state should be treated as immutable except for refs.
- bayesian_horse 4y ago
  Refs are just as immutable in that perspective. And no, you're not supposed to mutate ref.current.anything outside useEffect or a callback or similar...
  - phailhaus 4y ago
    Of course you can! That's the whole point of refs, and why you can't use them as a dependency to callbacks or effects. They're totally mutable, which makes them very tricky to deal with. I mean, I wouldn't recommend updating them outside of effects/callbacks, but it's valid. For example: you can make a ref to count the number of times React calls your render loop by updating it in the body.