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Eh, syntactically it hides the state as in `f x` has the same value every time, but calling functional programming stateless is a stretch, I consider it a half-

Eh, syntactically it hides the state as in `f x` has the same value every time, but calling functional programming stateless is a stretch, I consider it a half-truth told the newcomers to understand the concept. Functional languages still have state but they're abstracted out to function call. E.g. if you have a recursive descent parser written in Haskell, your program will still have state, not every time you're in `expr` function you'll have the same conceptual state. I think lack of mutation is a lot more pronounced, since it has very important implications to runtime (necessary O(logn) slow down) and requires a more sophisticated type system (linear type system) in order to emulate safe mutation.

stutonk 6y ago
In addition, every closure is also a 'state container,' as it were. This is how some functional languages e.g. Erlang emulate mutation. A function holds the desired state in a closure and evaluates to a new function closure with updated state. It's also how a stateful object system can be trivially implemented in Scheme. A few of pieces of the Erlang OTP infrastructure are actually just an elaborate mechanism to reintroduce global mutable state i.e. the process registry or Mnesia.
- toast0 6y ago
  You can use closures to store state in Erlang, but generally state is kept by passing it through to further function calls. I would say this makes Erlang mostly have explicit state, rather than saying Erlang is stateless. Ets is implemented in C as global mutable state, but you could implement it in Erlang as a process per table (and a process to hold the list of tables) with no loss of functionality. You would send messages to fetch data or update data, etc. It's a performance and memory efficiency optimization to do it with C, of course.
- kazagistar 6y ago
  Pure functional languages, aka Haskell or Idris or whatever, don't have this property. A closure closes over values, and thus is a value, no state mutation involved. Of course, your point about complex mechanisms to reintroduce mutation into specific contexts stands (usually via monads) but those are a very different Kind of object then the functions.
- elcritch 6y ago
  It’s important to note Erlang’s global state tools are about controlled global state. The process registry is more elaborate than a simple naming scheme in say Python or Java but really not much harder to use. However that controlled registry allows the process registry to become multi-node with minimal or no code changes for the clients. It’s comparable in power to but far simpler than Consul or etcd for finding services. I view micro services as fairly equivalent to Erlang actors as they both reinforce that state encapsulation. The same idea goes for mnesia or ETS which are comparable in power to Redis.

stutonk 6y ago

In addition, every closure is also a 'state container,' as it were. This is how some functional languages e.g. Erlang emulate mutation. A function holds the desired state in a closure and evaluates to a new function closure with updated state. It's also how a stateful object system can be trivially implemented in Scheme. A few of pieces of the Erlang OTP infrastructure are actually just an elaborate mechanism to reintroduce global mutable state i.e. the process registry or Mnesia.

toast0 6y ago
You can use closures to store state in Erlang, but generally state is kept by passing it through to further function calls. I would say this makes Erlang mostly have explicit state, rather than saying Erlang is stateless. Ets is implemented in C as global mutable state, but you could implement it in Erlang as a process per table (and a process to hold the list of tables) with no loss of functionality. You would send messages to fetch data or update data, etc. It's a performance and memory efficiency optimization to do it with C, of course.
kazagistar 6y ago
Pure functional languages, aka Haskell or Idris or whatever, don't have this property. A closure closes over values, and thus is a value, no state mutation involved. Of course, your point about complex mechanisms to reintroduce mutation into specific contexts stands (usually via monads) but those are a very different Kind of object then the functions.
elcritch 6y ago
It’s important to note Erlang’s global state tools are about controlled global state. The process registry is more elaborate than a simple naming scheme in say Python or Java but really not much harder to use. However that controlled registry allows the process registry to become multi-node with minimal or no code changes for the clients. It’s comparable in power to but far simpler than Consul or etcd for finding services. I view micro services as fairly equivalent to Erlang actors as they both reinforce that state encapsulation. The same idea goes for mnesia or ETS which are comparable in power to Redis.