> Threading mostly-irrelevant state through a bunch of different functions is a sign that your program is under-abstracted.
The problem is that often you do want fairly complex state in the leaves of the tree, but want very little of it in anything else. Web browsers are a classic example of this. Pure FP solutions such as Elm that completely eschew the idea of local mutable state require a lot more ceremony to implement something like a form (the classic thorn for Elm users). By forcibly moving up the state to the root, you sometimes end up needing to pull some fairly severe contortions.
E.g. the usual answer to move the state back up to the root in the land of statically-typed, pure FP is to express it in a return type (e.g. a reader or state monad, culminating in the famous ReaderT handler strategy in Haskell) or in the limit bolt on an effect system instead. The usual answer in impure FP is to accept some amount of mutable state and just rely on programmers not to "overdo" it.
But from a certain point of view, writing an elaborate effect system whose very elaborateness might cause performance issues and inscrutable error messages sounds suspiciously like trying to work around a problem in visualization with an over-engineered code solution. And from another perspective it feels a bit like a trick. If some function has a lot of state, then I would hope by opening up the definition of the function I'd see how it all works, but with an effect system all of a sudden I've split things up into an interpreter that actually performs the mutation and an interface that merely marks what mutation is to be done. It feels like I've strewn logic around in even more places than if I just had direct stateful, mutable calls there!
I will say plainly that I think there are situations in which mutability offers more elegant solutions than immutability, but I think most languages that offer it do it badly. I’m most experienced programming the Erlang platform via Elixir, and I think it offers a really nice midpoint between locality of state and purity. Within a process everything is immutable, and mutation requires sending a message to a process that will have a function specifying an explicit, pure state transformation from that message. Just about the only thing I don’t love about Elixir is the lack of real types.
I’m also very pragmatic and to the example of a web browser I would say, most applications are not web browsers. The overwhelming majority aren’t, in fact. I’ve chosen at this point in my career to mostly focus on enterprise software development, which I believe was Rich’s original field as well, and I’ve seen an enormous number of solutions with too much state cast about everywhere that benefit massively from centralizing the state high in the tree and really thinking through the data model carefully. So I stand by the principle I advocated originally, but it’s not universally applicable. It’s my belief that one of the core virtues of software development is knowing when to apply which principles.
It’s a good point. UI is a situation where the classic OOP-style frameworks work really well when they’re carefully designed. I think we’re still waiting on a model for doing that with FP that doesn’t rely on passing state deep down into an expression tree like React and its descendants encourage you to do. There’s stuff like Redux but it has its own problems.
The problem is that often you do want fairly complex state in the leaves of the tree, but want very little of it in anything else. Web browsers are a classic example of this. Pure FP solutions such as Elm that completely eschew the idea of local mutable state require a lot more ceremony to implement something like a form (the classic thorn for Elm users). By forcibly moving up the state to the root, you sometimes end up needing to pull some fairly severe contortions.
E.g. the usual answer to move the state back up to the root in the land of statically-typed, pure FP is to express it in a return type (e.g. a reader or state monad, culminating in the famous ReaderT handler strategy in Haskell) or in the limit bolt on an effect system instead. The usual answer in impure FP is to accept some amount of mutable state and just rely on programmers not to "overdo" it.
But from a certain point of view, writing an elaborate effect system whose very elaborateness might cause performance issues and inscrutable error messages sounds suspiciously like trying to work around a problem in visualization with an over-engineered code solution. And from another perspective it feels a bit like a trick. If some function has a lot of state, then I would hope by opening up the definition of the function I'd see how it all works, but with an effect system all of a sudden I've split things up into an interpreter that actually performs the mutation and an interface that merely marks what mutation is to be done. It feels like I've strewn logic around in even more places than if I just had direct stateful, mutable calls there!