• Jason House <jason.james.house gmail.com> Mar 02 2009

Jason House <jason.james.house gmail.com> writes:

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?