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best article i've seen on pragmatic purity - i would only dissent on c++ not providing facilities for pure functions (i don't here mean pure in the virtual=0 se

best article i've seen on pragmatic purity - i would only dissent on c++ not providing facilities for pure functions (i don't here mean pure in the virtual=0 sense) - you can, and should, do quite a lot.

jb1991 4y ago
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 is not checked by the compiler, that is what has been meant here.
- 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.
leni536 4y ago
There is one place where purity is effectively enforced: constexpr functions called in constexpr context. It's a little bit limited, as it's only enforced for compile time evaluations.
HybridCurve 4y ago
GCC has a pure attribute for functions which is documented as being prohibitive of impure behavior. Though, I honestly cannot say how thoroughly it is enforced or if there are significant benefits outside of the obvious and some minor optimizations which the compiler might be able to perform.
- leni536 4y ago
  It's not enforced. It's a promise. If you mark a non-pure function pure, then you are going to have a bad time. I would also be reluctant to mark any function pure that takes pointers as arguments (directly or indirectly).

jb1991 4y ago

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 is not checked by the compiler, that is what has been meant here.

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.

leni536 4y ago

There is one place where purity is effectively enforced: constexpr functions called in constexpr context. It's a little bit limited, as it's only enforced for compile time evaluations.

HybridCurve 4y ago

GCC has a pure attribute for functions which is documented as being prohibitive of impure behavior. Though, I honestly cannot say how thoroughly it is enforced or if there are significant benefits outside of the obvious and some minor optimizations which the compiler might be able to perform.

leni536 4y ago
It's not enforced. It's a promise. If you mark a non-pure function pure, then you are going to have a bad time. I would also be reluctant to mark any function pure that takes pointers as arguments (directly or indirectly).