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The language does not offer the features that you were referring to, and that the author of the article mentions. You can write pure functions, but the purity i

• zabzonk 4y ago
  yes, i know. but there is nothing that stops you from writing pure functions, and quite a lot of features that encourage it, even if they are not checked by the compiler. i'd probably like a c++ compiler that warned me about lack of purity, but as the article says, sometimes you have to bite the bullet.
  • mejutoco 4y ago
    > there is nothing that stops you from writing pure functions If they are not checked, they will be forgotten. When reading a function, if I do not have the guarantee that it is pure, it means the same as an old comment (not much). If the only way to enforce it is convention and force of will it will not be enforced. As an exaggerated illustration: we might add a "without-bugs" annotation to a function, to indicate it has no bugs. After all, nothing stops you from writing bug-free functions :)
• FpUser 4y ago
  For example declaring function of the class as const pretty much turns it into pure function and compiler makes sure that it does not modify state of said class. So you are wrong from a practical standpoint. try to modify state of the class inside const function and it'll be compiler error.
  • jb1991 4y ago
    That doesn’t prevent you from accessing global variables or reading member variables (which many const functions do). Marking a member function as const doesn’t in anyway guarantee purity, and is actually quite unrelated. In fact, most const class functions are by definition not pure at all, because they will read internal state, otherwise they wouldn’t even need to be members of the class. So I would be willing to argue that nearly all const class functions are impure.
    • zabzonk 4y ago
      um, i seriously doubt that many const functions access global variables. if those in your code do, then i suggest your code has problems - the typical use of a const function is doing something like length() does on a std::string. how, or why. would such a function access a global variable?
      • jb1991 4y ago
        I agree, but most would access internal state, which is equally impure. A pure function has no access to anything outside of its function parameters, and it changes nothing outside the scope of the function. In the context of a pure function, a local member variable is no different than a global variable because it’s outside the scope of the actual function. If you can’t take the function away from the class and have it still work perfectly, then it’s not pure. length() is not pure, but a function like length(string) could be.
        • zabzonk 4y ago
          why do you think the length member function is not pure?
          • jb1991 4y ago
            If you read the article and see what pure functions are, you will understand. The length function has no parameters, by definition it must access data outside of the function if it’s going to do anything. This means it is not pure. Can you take that length function outside of the class and run it and have it do something? No, it is tightly coupled to the class that it is attached to.
            • jb1991 4y ago
              Incidentally if this has whetted your appetite for functional programming, I encourage you to read a lot more about it and study languages that are truly functional. It could be quite liberating to see this approach to software development. It is not a panacea but it is a very useful perspective to acquire on software engineering, regardless of language, which is what this article discusses.
  • rhn_mk1 4y ago
    The "pretty much" does all the work here though. It's not pure if you can modify the state of a global, the file system, the environment, the standard output, etc., and this is half of the reason why purity is desired.
    • zabzonk 4y ago
      certainly desired, but not always possible, seeing as we actually want to do state changes, for example output, unless with much hand-waving monads, which of course really do change state.
    • jb1991 4y ago
      And it’s not just about modification, it’s also not pure if you simply read data that is not passed into the function via parameters. Because in a pure function, you don’t need to access anything, reading or writing, that is outside of the function itself.