What are (dis)advantages of using pure and immutable by default?
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?
Re: What are (dis)advantages of using pure and immutable by default?
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 =
immutable int* yi =
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.
Re: What are (dis)advantages of using pure and immutable by default?
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 =
immutable int* yi =
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.
Re: What are (dis)advantages of using pure and immutable by default?
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.
Re: What are (dis)advantages of using pure and immutable by default?
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.
