3 ms·

No way. Rust supports mutation, does not have referential transparency, etc.

SamReidHughes 8y ago
Rust functions perform mutation by consuming the parameter and emitting a new one in its place, effectively. You can take any data type and build an alternate version of it that can be used without using mut (and with a much less convenient API, but it’s basically a rote transformation of the code). Edit: I’d nominate as a good definition of pure functional programming the absence of undeclared data flow (i.e. side effects) by your code.
- deleted 8y ago
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- millstone 8y ago
  Give me a break. I can increment a variable in Rust. Yes you can perform an SSA-versioned-variables or linear-types-changing-the-world transformation. That's just sophism; we can conclude that C is pure functional via the same line of argument. In response to your edit: Rust does not encode side effects in its type system and does not require system calls to achieve side effects. If that's your point then we're in agreement.
  - SamReidHughes 8y ago
    You can’t do that with C. Nor can you do that with Rust, because it has UnsafeCell. In the absence of UnsafeCell and system calls (and, I guess, unsafe blocks), Rust encodes “all” its side effects (conservatively, and not trivially conservatively). All of its information flow, anyway.
    - millstone 8y ago
      It's trivial in Rust to share mutable memory via RefCell, Rc, others. These are core, not exotic. We can imagine a language in which these don't exist, but it's not Rust and would not enjoy the ergonomics of Rust. That's why Rust is not pure functional, or even "almost" pure functional.
      - SamReidHughes 8y ago
        RefCell uses UnsafeCell inside. Everything that does mutation, under shared ownership, has UnsafeCell somewhere -- IIRC this is or was a requirement to guarantee the compiler won't do certain optimizations. > but it's not Rust That's why I stipulated Rust without UnsafeCell.

Rust functions perform mutation by consuming the parameter and emitting a new one in its place, effectively. You can take any data type and build an alternate version of it that can be used without using mut (and with a much less convenient API, but it’s basically a rote transformation of the code). Edit: I’d nominate as a good definition of pure functional programming the absence of undeclared data flow (i.e. side effects) by your code.

deleted 8y ago
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millstone 8y ago
Give me a break. I can increment a variable in Rust. Yes you can perform an SSA-versioned-variables or linear-types-changing-the-world transformation. That's just sophism; we can conclude that C is pure functional via the same line of argument. In response to your edit: Rust does not encode side effects in its type system and does not require system calls to achieve side effects. If that's your point then we're in agreement.
- SamReidHughes 8y ago
  You can’t do that with C. Nor can you do that with Rust, because it has UnsafeCell. In the absence of UnsafeCell and system calls (and, I guess, unsafe blocks), Rust encodes “all” its side effects (conservatively, and not trivially conservatively). All of its information flow, anyway.
  - millstone 8y ago
    It's trivial in Rust to share mutable memory via RefCell, Rc, others. These are core, not exotic. We can imagine a language in which these don't exist, but it's not Rust and would not enjoy the ergonomics of Rust. That's why Rust is not pure functional, or even "almost" pure functional.
    - SamReidHughes 8y ago
      RefCell uses UnsafeCell inside. Everything that does mutation, under shared ownership, has UnsafeCell somewhere -- IIRC this is or was a requirement to guarantee the compiler won't do certain optimizations. > but it's not Rust That's why I stipulated Rust without UnsafeCell.