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John Carmack on Functional Programming in C++ (2018)

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Re: John Carmack on Functional Programming in C++ (2018)

#31

Post by zabzonk »

rhn_mk1 wrote: post #25
FpUser wrote: post #15

Earlier quoted context omitted.

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.

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.

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.

Re: John Carmack on Functional Programming in C++ (2018)

#32

Post by tmsln »

inductive_magic wrote: post #18
baby wrote: post #8

He talked about it a bit during his lex friedman interview. He said he spent some time there, IIRC, and found it somewhat intriguing, perhaps interesting, but needed to get shit done eventually and so stopped his investigation. Very pragmatic. From what I can see in this article he mostly seemed to like the purity aspect of FP. It seems to me like a number of FP concepts aren’t really “FP things”, as Carmack points o…

>He said he spent some time there, IIRC, and found it somewhat intriguing, perhaps interesting, but needed to get shit done eventually and so […] I think this resonates with many. My take is: don’t be dogmatic. Regardless of the language, one should write as beautiful, composable and pure as possible - but no more than that. Where only performance matters, oop wins. When clarity is the only priority, Haskell wins. Bu…

> Where only performance matters, oop wins

If you tell that Mike Acton he will give you an unhappy face. [0]

In fact OOP can hurt performance a lot.

[0] https://m.youtube.com/watch?v=rX0ItVEVjHc

Re: John Carmack on Functional Programming in C++ (2018)

#33

Post by jb1991 »

rhn_mk1 wrote: post #25
FpUser wrote: post #15

Earlier quoted context omitted.

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.

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.

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.

Re: John Carmack on Functional Programming in C++ (2018)

#34

Post by pjmlp »

forrestthewoods wrote: post #16
bmitc wrote: post #13

Earlier quoted context omitted.

> but needed to get shit done eventually and so stopped his investigation. Very pragmatic. That's a strange take by him and on pragmatism. There's also almost nothing pragmatic about C++. Many functional languages are actually quite pragmatic.

> There's also almost nothing pragmatic about C++. Huh? We’re talking about game dev. C or C++ is the primary language for just about every game engine ever. Even Unity is written C++. I can’t say functional languages haven’t ever shipped a game. I’m sure there’s an example or three. But yeah when it comes to shipping games C++ is the definition of pragmatic.

Unity is partially written in C++, and since HPC# introduction they have been slowly migrating code from C++ into it.

Unity is one of the reasons why Microsoft started to take Midori lessons into regular C#.

Ultimately if we want to be pedantic, all game engines are written in C and C++, as that is the only way current OSes expose their graphics APIs, no alternative implementation can change that unless they also write an OS in the process.

Re: John Carmack on Functional Programming in C++ (2018)

#35

Post by zabzonk »

RichieAHB wrote: post #26

In terms of applying this to languages other than C++, as someone who’s relatively new (4 years) to Java I expected to see a lot more side effects in the Java I’ve worked with in that time. In fact, the opposite has been true, and much of the points discussed in this article seems to have been adhered to; even going back to some of the older codebases. That said, we mainly have stateless microservices running on k8s,…

i am by no means a java expert, but i would have thought that the fact that most objects in java are shipped between functions as references makes purity more difficult to effect and to diagnose than in c++ where everything is by default shipped as a value - of course c++ allows you to change this , java less so?

Re: John Carmack on Functional Programming in C++ (2018)

#36

Post by noduerme »

As a 25+ years coder who's very accustomed to thinking in my own ways, I have a problem with functional-purism similar to the problem I had with globals and factory functions. The fact is that most programs have a lot of states and these states have to be represented somehow. Allowing the objects to contain functions that act on those states is a perfectly good analogy for most business logic or game logic you're modeling. Abstracting what you can abstract into atomic functions is great. But so many functions are more comprehensible when they take place bound to the scope of what it is they're acting upon... again, not to produce an answer, but to change/track/maintain state which is a critical part of what people experience when they interact with a program. I don't think that thinking in terms of objects or even global states should be considered a code smell when what you need to do is to get things done. If/when you need to make some functions synchronous across asynchronous systems, by atomizing their logic, then you will hit upon functional programming methods by default.

Re: John Carmack on Functional Programming in C++ (2018)

#37

Post by zabzonk »

jb1991 wrote: post #28
zabzonk wrote: post #27

Earlier quoted context omitted.

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?

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…

why do you think the length member function is not pure?

Re: John Carmack on Functional Programming in C++ (2018)

#38

Post by jb1991 »

zabzonk wrote: post #37
jb1991 wrote: post #28

Earlier quoted context omitted.

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…

why do you think the length member function is not pure?

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.

Re: John Carmack on Functional Programming in C++ (2018)

#39

Post by thom »

noduerme wrote: post #36

As a 25+ years coder who's very accustomed to thinking in my own ways, I have a problem with functional-purism similar to the problem I had with globals and factory functions. The fact is that most programs have a lot of states and these states have to be represented somehow. Allowing the objects to contain functions that act on those states is a perfectly good analogy for most business logic or game logic you're mod…

The argument isn’t that state is bad or doesn’t need to be managed. It’s more that spreading that state over a large number of areas leads to a lot of complexity and cognitive overhead in terms of expected behaviour at any given point of execution. Functional programming gives you ways to be much more explicit about the transformation being performed and the before/after states (except arguably when you start getting overly cute with zippers and lenses and such). To the extent that this approach hasn’t magically transformed programmer productivity or software quality, we know that both approaches are admissible.

Re: John Carmack on Functional Programming in C++ (2018)

#40

Post by jb1991 »

jb1991 wrote: post #38
zabzonk wrote: post #37

Earlier quoted context omitted.

why do you think the length member function is not pure?

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.

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.
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