const?? When and why? This is ugly!

[hasen]

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.