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> Correct, but you as the component writer don't care. From your perspective, you are writing a purely functional component because you have no control over the

by ww520 2y ago

> Correct, but you as the component writer don't care. From your perspective, you are writing a purely functional component because you have no control over the injected state. So your function is "pure" with respect to the props that are passed in and the state that is injected by React. It's actually UI = F(props, state) This is like calling the constructor of an OO class a pure function. The class constructor takes in parameters (props) and the created instances don't interfere with each other's states. With all due respect, this stretching of the definition of pure function is beyond recognition. > * the order that your function is called somehow affects the final render. Now you need to worry about how your function is called, and with what values. Not an issue with React. The order of the function invocations calling setXX() matters anyway in React or non-React libraries. The queued up setXX() calls pending state updates in React has an order. There's no difference in handling state update ordering between React and others. > * the underlying state is out of sync with the UI, because you changed it but didn't trigger a re-render. Not possible in React (except for refs). > * the internal state of the component ends up in an invalid state because of the ordering of set-state calls and renders. Again, not possible in React because all set-states are batched together and applied consistently to the next render cycle. There's no such thing as a "partial render". Both of these are because React's inability to detect changes to state updates in a comprehensive and fine grained way.

phailhaus 2y ago
> This is like calling the constructor of an OO class a pure function. No, because again I must stress that you cannot control the injected state. You merely ask React to set state in the next render. The next call to your function may not have the new state! React can and will call your component function multiple times with different values of state, so you must assume it could be anything. This is why component functions must be written without any side effects, because there is no way for you to know which invocation React will actually persist to the DOM. For this reason, you really are writing a pure function with respect to props and state. It's a requirement in React! > There's no difference in handling state update ordering between React and others. The huge difference is that because state is fixed within a render cycle, it is not possible to read an intermediate value. You can't get in a situation where one handler updates a state value, and another (or the same) handler reads that new value within the same render cycle. That's the sort of order-dependent nonsense that can make a component very buggy and fall into an inconsistent state. > Both of these are because React's inability to detect changes to state updates in a comprehensive and fine grained way. That's because React is built in a way to provide a set of very specific guarantees when it comes to managing state and rendering it. You are free to come up with "simpler" solutions, but you are going to have to sacrifice some of these guarantees for that simplicity. React component functions have no side effects, and are defined completely declaratively. That is extremely powerful with a lot of advantages, such as being trivially composable. It is extremely easy to extract functionality out of a component and turn it into a reusable hook, because everything is already stateless. If you ditch all that just so you can use variables, you lose all of those benefits. You'll have to deal with truly stateful components, ones that are path dependent and difficult to reproduce for debugging. By contrast, you can reproduce any React component's UI by passing in the same props and state. This is only possible because state is external to the component and out of your control.
- ww520 2y ago
  Thank you for the thoughtful replies. It helps a lot in understanding the kinds of problems React was facing. With all due respect, > For this reason, you really are writing a pure function with respect to props and state. A pure function by definition is one which returns the same output for the same input. The output (html) of the React functional component with the same props can be different based on the changed state. It is not a pure function. It's really no different from an OO object with internal state. > There's no difference in handling state update ordering between React and others. > > it is not possible to read an intermediate value. There is no intermediate value. All value changes are in the final form as there is no multi-threads reading and mutating the state. The block of code mutating the value is executed fully before another block of code can read it. As for multiple handlers reading and rendering the state and some might miss the change by another handler, again it's a problem of React's inability to detect change for each handler. If every handler can detect change by any other code (other handlers included), there's no problem. > That's because React is built in a way to provide a set of very specific guarantees when it comes to managing state and rendering it. React requires that set of guarantees because its design decision - its use of diff to detect changes. It's a non-issue in other frameworks and solutions.
  - phailhaus 2y ago
    > A pure function by definition is one which returns the same output for the same input. Yes, and the input includes state. React manages it and sets it before your function runs; your function then reads those values when it calls the hooks, and it is read-only. If I adjusted the React syntax to pass in state as a function argument instead, wouldn't you agree that it is pure? It's the same! Just because it's not passed in via arguments does not mean it is not an input. And a pure function is still pure even if the caller is stateful. Consider the "rules of hooks" in React: you cannot conditionally use a hook, and hooks must be used in the same order every time. That's exactly the same as regular arguments, making them functionally identical but with better dev ergonomics. That's on purpose: state is an input, but having it passed via function arguments would suck to use. Question: have you written any React components? This is a basic rule that is one of the first things you learn: components should be pure functions with respect to props and state. They're not even allowed to set state as a side-effect. If you're still convinced it's not pure, can you tell me how a component function's output can change given the same props and state? > The block of code mutating the value is executed fully before another block of code can read it. And that's the problem: with arbitrary code, there is no such thing as a distinct "block of code". One function can read value A, set it to B, then call another function, which sees value B, does something as a result, and then it renders. But that's wrong: value B has not been persisted to the DOM yet, so it should not have been read by the second function. Now you're in trouble, because your component is behaving as if both A and B are true at the same time. This is not possible with React. But to pull that off, they had to externalize state and make it read-only to your component.

by ww520 2y ago

phailhaus 2y ago

> This is like calling the constructor of an OO class a pure function. No, because again I must stress that you cannot control the injected state. You merely ask React to set state in the next render. The next call to your function may not have the new state! React can and will call your component function multiple times with different values of state, so you must assume it could be anything. This is why component functions must be written without any side effects, because there is no way for you to know which invocation React will actually persist to the DOM. For this reason, you really are writing a pure function with respect to props and state. It's a requirement in React! > There's no difference in handling state update ordering between React and others. The huge difference is that because state is fixed within a render cycle, it is not possible to read an intermediate value. You can't get in a situation where one handler updates a state value, and another (or the same) handler reads that new value within the same render cycle. That's the sort of order-dependent nonsense that can make a component very buggy and fall into an inconsistent state. > Both of these are because React's inability to detect changes to state updates in a comprehensive and fine grained way. That's because React is built in a way to provide a set of very specific guarantees when it comes to managing state and rendering it. You are free to come up with "simpler" solutions, but you are going to have to sacrifice some of these guarantees for that simplicity. React component functions have no side effects, and are defined completely declaratively. That is extremely powerful with a lot of advantages, such as being trivially composable. It is extremely easy to extract functionality out of a component and turn it into a reusable hook, because everything is already stateless. If you ditch all that just so you can use variables, you lose all of those benefits. You'll have to deal with truly stateful components, ones that are path dependent and difficult to reproduce for debugging. By contrast, you can reproduce any React component's UI by passing in the same props and state. This is only possible because state is external to the component and out of your control.

ww520 2y ago
Thank you for the thoughtful replies. It helps a lot in understanding the kinds of problems React was facing. With all due respect, > For this reason, you really are writing a pure function with respect to props and state. A pure function by definition is one which returns the same output for the same input. The output (html) of the React functional component with the same props can be different based on the changed state. It is not a pure function. It's really no different from an OO object with internal state. > There's no difference in handling state update ordering between React and others. > > it is not possible to read an intermediate value. There is no intermediate value. All value changes are in the final form as there is no multi-threads reading and mutating the state. The block of code mutating the value is executed fully before another block of code can read it. As for multiple handlers reading and rendering the state and some might miss the change by another handler, again it's a problem of React's inability to detect change for each handler. If every handler can detect change by any other code (other handlers included), there's no problem. > That's because React is built in a way to provide a set of very specific guarantees when it comes to managing state and rendering it. React requires that set of guarantees because its design decision - its use of diff to detect changes. It's a non-issue in other frameworks and solutions.
- phailhaus 2y ago
  > A pure function by definition is one which returns the same output for the same input. Yes, and the input includes state. React manages it and sets it before your function runs; your function then reads those values when it calls the hooks, and it is read-only. If I adjusted the React syntax to pass in state as a function argument instead, wouldn't you agree that it is pure? It's the same! Just because it's not passed in via arguments does not mean it is not an input. And a pure function is still pure even if the caller is stateful. Consider the "rules of hooks" in React: you cannot conditionally use a hook, and hooks must be used in the same order every time. That's exactly the same as regular arguments, making them functionally identical but with better dev ergonomics. That's on purpose: state is an input, but having it passed via function arguments would suck to use. Question: have you written any React components? This is a basic rule that is one of the first things you learn: components should be pure functions with respect to props and state. They're not even allowed to set state as a side-effect. If you're still convinced it's not pure, can you tell me how a component function's output can change given the same props and state? > The block of code mutating the value is executed fully before another block of code can read it. And that's the problem: with arbitrary code, there is no such thing as a distinct "block of code". One function can read value A, set it to B, then call another function, which sees value B, does something as a result, and then it renders. But that's wrong: value B has not been persisted to the DOM yet, so it should not have been read by the second function. Now you're in trouble, because your component is behaving as if both A and B are true at the same time. This is not possible with React. But to pull that off, they had to externalize state and make it read-only to your component.