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digitalmars.D - Re: const?? When and why? This is ugly!

hasen Wrote:

 hasen wrote:
 I haven't been following D for over a year .. now I notice there's a 
 const!!
 
 [...]

It seems the argument is centered around making "pure functions" possible. But, surely there's a better way of doing this than borrowing const from C++. Just like import is a better approach than #include and `namespace` Yesterday I noticed this page http://www.digitalmars.com/d/2.0/const-faq.html which I haven't seen before posting the topic yesterday. Argument #2 reads: "It makes for interfaces that can be relied upon, which becomes increasingly important the more people that are involved with the code. In other words, it scales very well." I think this is not worth the complexity, but then again, Argument#4 renders #2 as insiginificant, so I won't bother too much arguing against #2. #4 reads: "The future of programming will be multicore, multithreaded. Languages that make it easy to program them will supplant languages that don't." So, this is driven by the need to popularize D as a very good tool for multicore applications! "Transitive const is key to bringing D into this paradigm." Really? have you considered other possibilities? How about, adding a new attribute to functions: `pure` pure real sin( real x ) { ... } and design the rest around this concept. The compiler must make sure that this function is really pure: - native types must be passed by value, not by reference - doesn't accept pointers - <insert some condition for objects and arrays> If these requirements aren't met, the compiler will spit some error message "in function <F>: doing <X> voilates the `pure` attribute" objects and arrays will need some special treatment or requirements in order to be passed to pure functions. What are those requirements? I have given it a very deep thought yet(*), but I'm sure this approach is better than `const`. Also, make `string` a builtin type that's immutable (like python), and discourage the use of char[] as a type for strings, (unless used to implement a special string class). The need for a special string type also stems from the fact that char[a..b] slices bytes, not characters! (*) A quick idea might be: - For arrays, any statement of the type `a[b] = c` will be illegal inside a pure function. and pure functions can only call pure functions. - For objects, any statement of the form `a.b = c` is illegal, and there must be a way to know if a method call will change the object or not. (It would be best if the compiler could detect this automatically if it's possible). - Another approach, is to prohibit passing objects all together, and introduce some new construct that's immutable (such as a tuple, again, like python). `pure` will probably have the same viral effects as `const`, in that any function called from a pure function must also be pure, and this viral nature will propagate to classes/objects as well. However, the advantage is not complicating the type system more than is needed. Also, what's the deal with const pointers?? Why should `pure` function be able to use pointers at all? Is there any real-life use case where a pure function needs to access memory instead of some abstract concept like a variable/array/tuple? If you think about it, CTFE (compile time function execution) are `pure` already, and they're detected automatically by the compiler, with no need for explicitly marking them as such.

You're getting closer... Your scheme uses a lot of deep copying which can kill performance. What if there was a way to skip such deep copies? Or at least force the caller to copy the data before use? How do you implement thread safe deep copies?
Mar 02 2009