It seems to me a good practice to mark all functions that I write as `pure` and define all the variables as `immutable`, unless there is a reason not to.
I can see some serious advantages of this, most notable of which is minimum side-effect and predictability of the code.  However I suppose it's going to impact the performance and memory footprint as well, though I have no idea how deep the impact will be.

I'm pretty sure I'm not the first one to think about this; so what would you seasoned D'ers say?

On Monday 07 September 2015 12:40, Bahman Movaqar wrote:

> It seems to me a good practice to mark all functions that I write as `pure` and define all the variables as `immutable`, unless there is a reason not to.

I agree.

> I can see some serious advantages of this, most notable of which is minimum side-effect and predictability of the code.  However I suppose it's going to impact the performance and memory footprint as well, though I have no idea how deep the impact will be.

I don't see how merely marking things immutable/pure would affect performance negatively. They're just marks on the type. If anything, you could get a performance boost from the stricter guarantees. But realistically, there won't be a difference.

If you change your algorithms to avoid mutable/impure, then you may see worse performance than if you made use of them. But I suppose that would be "a reason not to" mark everything immutable/pure.

On Monday, 7 September 2015 at 10:55:13 UTC, anonymous wrote:
> On Monday 07 September 2015 12:40, Bahman Movaqar wrote:
>> I can see some serious advantages of this, most notable of which is minimum side-effect and predictability of the code.  However I suppose it's going to impact the performance and memory footprint as well, though I have no idea how deep the impact will be.
>
> I don't see how merely marking things immutable/pure would affect performance negatively. They're just marks on the type. If anything, you could get a performance boost from the stricter guarantees. But realistically, there won't be a difference.

Just "marks", eh?
I was under the impression that when a variable, that is declared as `immutable`, is passed to a function, a copy of the value is passed.
However based on "marks" I can imagine that since the data is marked as `immutable` only a reference is passed; and the compiler guarantees that what is referenced to never changes.  Am I right?

> If you change your algorithms to avoid mutable/impure, then you may see worse performance than if you made use of them. But I suppose that would be "a reason not to" mark everything immutable/pure.

True.
Nowadays that more algorithms are designed with parallelism and distribution in mind, though, I believe mutating values and impure functions, at least in certain domains, will cease to exist.

On Monday 07 September 2015 13:12, Bahman Movaqar wrote:

> I was under the impression that when a variable, that is declared as `immutable`, is passed to a function, a copy of the value is passed.
>
> However based on "marks" I can imagine that since the data is marked as `immutable` only a reference is passed; and the compiler guarantees that what is referenced to never changes.  Am I right?

Generally immutable doesn't affect how things are passed around. An immutable type is passed the same as a mutable version of it.

Compilers may optimize based on immutability. This may mean that a reference is passed instead of a copy, or the other way around. I don't know if compilers do such things currently, or how much potential is in that. Also, I don't think the language has a stance on that; it's purely an optimization.

immutable does affect how you can pass things around, though. An example:

----
void f(int a) {}
void g(int* a) {}

void main()
{
    int xm;
    immutable int xi;
    f(xm); /* ok, obviously */
    f(xi); /* ok */

    int* ym = &xm;
    immutable int* yi = ξ
    g(ym); /* ok, obviously */
    g(yi); /* doesn't compile */
}
----

f(xi) is ok because when passing an int or an immutable(int), it's copied completely. So the f cannot possibly mutate the original immutable variable xi.

g(yi) is not ok, because when passing an int* or an immutable(int*), only the pointer is copied. So g could dereference yi and mutate the referenced immutable variable xi.

The same applies to other forms of references: classes, ref parameters, arrays.

On Monday, 7 September 2015 at 11:49:32 UTC, anonymous wrote:
> void f(int a) {}
> void g(int* a) {}
>
> void main()
> {
>     int xm;
>     immutable int xi;
>     f(xm); /* ok, obviously */
>     f(xi); /* ok */
>
>     int* ym = &xm;
>     immutable int* yi = ξ
>     g(ym); /* ok, obviously */
>     g(yi); /* doesn't compile */
> }
> ----
>
> f(xi) is ok because when passing an int or an immutable(int), it's copied completely. So the f cannot possibly mutate the original immutable variable xi.
>
> g(yi) is not ok, because when passing an int* or an immutable(int*), only the pointer is copied. So g could dereference yi and mutate the referenced immutable variable xi.
>
> The same applies to other forms of references: classes, ref parameters, arrays.

Thanks for the explanation.
So from what I can gather, `immutable`, apart from possible compiler-level optimisations, is mainly there to help the programmer catch otherwise runtime errors at compile time.

This is nice.  Indeed very helpful in a language when one deals with pointers and references.