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.
A member function must have at least one parameter. Here it's "parameter0":
parameter0.membfun()
It doesn't really matter whether the parameter name is separated from the function name by a dot, or a bracket. In a different syntax, it might have been written as "membfun(parameter0)" without any practical difference.
This means that Foo::length() can be pure no problem, because Foo is its parameter, and, as such, in its scope.
I see your point, but you are
still passing the entire state of the entire object to your function if you want to follow that line of thought. This is also discussed in the article, and it’s not really what functional programming is about. What other state is in there that the function has access to? Maybe on a case-by-case basis you can say that particular object only has one piece of data, but in the general case this is definitely not what purity and functional programming is about.
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.
i find this slightly patronising - i have studied languages such as scheme and haskell. and i don't see how writing a function such as length(string) could be pure if the member function length() was not, as you would pass a copy of the string (ok, pure) and then call a member function (impure, according to you) on it to actually get the length.
No, you would not typically call a member function on anything in a pure language. Instead, you would pass a sequence of characters and the function would return the length, by counting them.
I didn’t mean to sound patronizing, I apologize for that. Your question suggested that you aren’t that familiar with functional programming, so I was just trying to steer you in the right direction. A function that takes no parameters is not a pure function in general, unless all it does is return a constant value, which wouldn’t make much sense. length() does not make sense in a functional programming style.
well, the length function for c++ strings does return a constant object - an integer value.
and passing a sequence of characters? hello, C strlen( const char * s); ! with all the problems that has. and if you don't want to pass some horrible pointer, you have to pass some object containing the characters which must have a length function itself. hello, std::string!
what you are ignoring here is how functional types either ignore functionality under the hood, or sacrifice it for performance.
which is why functional programming will never catch on (look at lisps), imho. programmers really like to know wtf their code is actually doing on a mutable machine.
> the length function for c++ strings does return a constant object -
No, I mean, a standalone function with no parameters can only be functional if it always returns the same value, which is effectively no different than declaring a constant. The function always returns the same value all time (which wouldn't be so useful).
I think you are trying to find C++ examples to match the broader implications of functional programming (which is what I'm talking about), which will be difficult.
As far as whether "functional programming will catch on", I don't think it will for C++ so much, but it certainly has been very successful for many other languages. I earn my living writing only functional code, but most of the time it is not in C++.
