Re: Why can't function expecting immutable arg take mutable input?
Thanks all, for your kind explanations. Would then constString (for const(char)[]) and inoutString (for inout(char) []) be useful aliases if included in the runtime? -- Shriramana Sharma, Penguin #395953
Re: Why can't function expecting immutable arg take mutable input?
Ali Çehreli wrote: > Actually, others gave the answer to that question, which was apparently > not very clear. :) Yes it was clear and I did understand it: and I posted a reply thanking the others too, but for some reason it was still sitting in my outbox... -- Shriramana Sharma, Penguin #395953
Re: Why can't function expecting immutable arg take mutable input?
On Saturday, 17 October 2015 at 02:03:01 UTC, Shriramana Sharma
wrote:
Ali Çehreli wrote:
http://ddili.org/ders/d.en/const_and_immutable.html#ix_const_and_immutable.parameter,
%20const%20vs.%20immutable
Hi Ali – I take this chance to personally thank you sincerely
for your book which provides much-needed hand-holding in my
baby D-steps. I did read that chapter already and IMO you have
given clear instructions as to when to use const and when
immutable.
My question was however to the root of the issue, as to *why*
the compiler cannot consider mutable as immutable just like in
C/C++ any non-const can be taken as const.
It would seem that the answer is one related to optimization.
Obviously, labeling an argument as immutable can be done only
if we are sure that we will have to process only immutable
input, thereby paving the opportunity for the compiler to
optimize some memory access or allocation or such – I'm not
much clear beyond that but that's enough for me now...
It appears that the linked chapter doesn't explain *why* you
would want to receive immutable arguments.
In my experience, the most common motivation is a desire to
escape a reference to the argument. We want to read the data
later, but when we do, we want it to be unchanged from when we
received it:
---
struct S
{
immutable(int)[] numbers;
this(immutable(int)[] numbers)
{
this.numbers = numbers;
}
void printNumbers()
{
import std.stdio;
writeln(numbers);
}
}
immutable numbers = [1, 2, 3];
auto s = S(numbers);
/* ... */
s.printNumers(); // [1, 2, 3]
---
In the above code, *no matter what code is run between
construction and `printNumbers`*, it will always print the same
numbers it received at construction, as the numbers are
immutable. Because of this guarantee, S.numbers can simply alias
the constructor argument as seen in the constructor body, instead
of say, copying the numbers into a new heap-allocated copy of the
array. If we used const instead of immutable, there would be no
such guarantee, as const can refer to mutable data: the numbers
could have been overwritten between construction and the call to
`printNumbers`.
Another common use of immutable is to share data between multiple
threads. As immutable data never changes after initialization, it
can be passed between threads and read freely without worrying
about data races.
const in D simply exists to bridge mutable and immutable data. It
is different from C++'s const, despite sharing the same name.
Re: Why can't function expecting immutable arg take mutable input?
On 10/16/2015 07:02 PM, Shriramana Sharma wrote: > your book which provides I am glad that it is useful. :) > My question was however to the root of the issue, as to *why* the compiler > cannot consider mutable as immutable just like in C/C++ any non-const can be > taken as const. Actually, others gave the answer to that question, which was apparently not very clear. :) In the context of your question, immutable is a requirement of the function from its user. It is a demand that the argument shall not mutate. That's why mutable cannot be considered as mutable. For example, if you write a File struct and take the file name as string, you don't need to make a copy of the file name because you know that it will not mutate as long as your File object is alive. (You can cast immutable away with undefined consequences but it's a different issue. :) ) > It would seem that the answer is one related to optimization. Obviously, > labeling an argument as immutable can be done only if we are sure that we > will have to process only immutable input, thereby paving the opportunity > for the compiler to optimize some memory access or allocation or such – I'm > not much clear beyond that but that's enough for me now... That and what I said above: the user can rely on this guarantee as well. Ali
Re: Why can't function expecting immutable arg take mutable input?
Ali Çehreli wrote: http://ddili.org/ders/d.en/const_and_immutable.html#ix_const_and_immutable.parameter, %20const%20vs.%20immutable Hi Ali – I take this chance to personally thank you sincerely for your book which provides much-needed hand-holding in my baby D-steps. I did read that chapter already and IMO you have given clear instructions as to when to use const and when immutable. My question was however to the root of the issue, as to *why* the compiler cannot consider mutable as immutable just like in C/C++ any non-const can be taken as const. It would seem that the answer is one related to optimization. Obviously, labeling an argument as immutable can be done only if we are sure that we will have to process only immutable input, thereby paving the opportunity for the compiler to optimize some memory access or allocation or such – I'm not much clear beyond that but that's enough for me now... -- Shriramana Sharma, Penguin #395953
Re: Why can't function expecting immutable arg take mutable input?
On 10/16/2015 03:35 AM, Shriramana Sharma wrote: Hello. I still haven't wrapped my mind around the const/immutable thing yet and am still stuck in C/C++ mode. :-( Welcome to the club! :) You can read my understanding of the issue at the following link: http://ddili.org/ders/d.en/const_and_immutable.html#ix_const_and_immutable.parameter,%20const%20vs.%20immutable Ali
Re: Why can't function expecting immutable arg take mutable input?
On Fri, Oct 16, 2015 at 04:05:19PM +0530, Shriramana Sharma via Digitalmars-d-learn wrote: > Hello. I still haven't wrapped my mind around the const/immutable > thing yet and am still stuck in C/C++ mode. :-( > > A function that takes mutable arguments cannot be called with > immutable input at the call site since it does not promise to *not* > mutate the input. That's of course clear. > > Why can't a function that takes an immutable argument be called with a > mutable input at the call site? What you want here is const, not immutable. Const is a guarantee that the argument will not be modified *by the function*. Immutable, however, is a much stronger guarantee: that the argument will not be modified by *anyone*. That is, the compiler is free to assume that the result of calling a pure function that takes immutable arguments will never ever change, no matter what, so it's safe to elide all but the first call to that function and cache its result. You can't do this with const, because it's possible that somebody may hold a mutable reference to the data and mutate it between calls. [...] > I understand that const can refer to either mutable or immutable, so > does this mean I should replace all occurrences of `string` in > arguments and return values of functions by `const(char)[]`? [...] If your functions don't need the stronger guarantee of immutable, yes, use const(char)[] instead. It's what const was designed for -- to take arguments that can be either mutable or immutable. T -- Tech-savvy: euphemism for nerdy.
Re: Why can't function expecting immutable arg take mutable input?
On Friday, October 16, 2015 12:35 PM, Shriramana Sharma wrote: > Why can't a function that takes an immutable argument be called with a > mutable input at the call site? > > IOW, why isn't mutable implicitly convertible to immutable? immutable says that the data won't ever change. If references to mutable data would be implicitly convertible to immutable, then you could break the immutable guarantee by mutating through the mutable reference which would also be visible through the immutable reference. Keep in mind that functions/methods may store references and reuse them on subsequent calls. If the parameter is immutable, then the function can assume that the data doesn't change in between calls. If you could pass mutable data, then you could change it between calls, and the function's immutability expectation would fail. Also, multiple threads may work concurrently on the same data. But if you could have an immutable reference in one thread and a mutable one in another, then "immutable" data could change while the function runs. An immutable parameter is as much a demand by the function from the caller as it is a guarantee to the caller. If you don't need the demand part, and only want to guarantee that the function does not alter the argument (through that reference), const does that. > I just finished writing a string processing module which calls multiple > subroutines, and all of them carrying arguments with type `string` viz. > `immutable(char)[]` IIUC, and I tried to pass it something which came from > File.byLine(), then got the error: > > function textattr.applyTextAttr (string text) is not callable using > argument types (char[]) > > I understand that const can refer to either mutable or immutable, so does > this mean I should replace all occurrences of `string` in arguments and > return values of functions by `const(char)[]`? If the functions don't actually require immutable data, then yes, use const instead.
Re: Why can't function expecting immutable arg take mutable input?
On Friday, 16 October 2015 at 12:48:42 UTC, Meta wrote: This doesn't work for char because it has indirections (a pointer to its data). Whoops, should be char[], not char.
Re: Why can't function expecting immutable arg take mutable input?
On Friday, 16 October 2015 at 10:35:23 UTC, Shriramana Sharma wrote: Hello. I still haven't wrapped my mind around the const/immutable thing yet and am still stuck in C/C++ mode. :-( A function that takes mutable arguments cannot be called with immutable input at the call site since it does not promise to *not* mutate the input. That's of course clear. Why can't a function that takes an immutable argument be called with a mutable input at the call site? IOW, why isn't mutable implicitly convertible to immutable? I just finished writing a string processing module which calls multiple subroutines, and all of them carrying arguments with type `string` viz. `immutable(char)[]` IIUC, and I tried to pass it something which came from File.byLine(), then got the error: function textattr.applyTextAttr (string text) is not callable using argument types (char[]) I understand that const can refer to either mutable or immutable, so does this mean I should replace all occurrences of `string` in arguments and return values of functions by `const(char)[]`? This actually *is* possible, if the type you're passing is a value type with no indirections. Then you can just pass it by value and it will be implicitly convertible to immutable. This doesn't work for char because it has indirections (a pointer to its data).
Re: Why can't function expecting immutable arg take mutable input?
On Friday, 16 October 2015 at 10:35:23 UTC, Shriramana Sharma wrote: I understand that const can refer to either mutable or immutable, so does this mean I should replace all occurrences of `string` in arguments and return values of functions by `const(char)[]`? Use `inout` attribute for that: take inout(char)[] parameter and return inout(char)[] result.
Re: Why can't function expecting immutable arg take mutable input?
On Friday, 16 October 2015 at 10:35:23 UTC, Shriramana Sharma wrote: Hello. I still haven't wrapped my mind around the const/immutable thing yet and am still stuck in C/C++ mode. :-( A function that takes mutable arguments cannot be called with immutable input at the call site since it does not promise to *not* mutate the input. That's of course clear. Why can't a function that takes an immutable argument be called with a mutable input at the call site? The contract of immutable is such that any reference to immutable data is a guarantee that no reference to the same data will modify it anywhere in the program. Passing mutable data to a function where an immutable parameter is declared would break that contract, since the data could be modified through the original reference. The compiler can take advantage of such a strict contract to make optimizations it would otherwise be unable to. const only guarantees const data will not be modifed through a single reference, but says nothing about other references to the same data. A const parameter can accept const, immutable, and mutable arguments.
Re: Why can't function expecting immutable arg take mutable input?
Shriramana Sharma píše v Pá 16. 10. 2015 v 16:05 +0530: > Hello. I still haven't wrapped my mind around the const/immutable > thing yet > and am still stuck in C/C++ mode. :-( > > A function that takes mutable arguments cannot be called with > immutable > input at the call site since it does not promise to *not* mutate the > input. > That's of course clear. > > Why can't a function that takes an immutable argument be called with > a > mutable input at the call site? > > IOW, why isn't mutable implicitly convertible to immutable? > > I just finished writing a string processing module which calls > multiple > subroutines, and all of them carrying arguments with type `string` > viz. > `immutable(char)[]` IIUC, and I tried to pass it something which came > from > File.byLine(), then got the error: > > function textattr.applyTextAttr (string text) is not callable using > argument > types (char[]) > Because immutable means it could resist in ROM so there could be some optimalization. Const on the other hand is what you are looking for. > I understand that const can refer to either mutable or immutable, so > does > this mean I should replace all occurrences of `string` in arguments > and > return values of functions by `const(char)[]`? > Yes you could, but be carefull that you do not store result from File.byLine() without dup or idup, because with next calling of File.byLine() it will probably modificate the previous result.
Why can't function expecting immutable arg take mutable input?
Hello. I still haven't wrapped my mind around the const/immutable thing yet and am still stuck in C/C++ mode. :-( A function that takes mutable arguments cannot be called with immutable input at the call site since it does not promise to *not* mutate the input. That's of course clear. Why can't a function that takes an immutable argument be called with a mutable input at the call site? IOW, why isn't mutable implicitly convertible to immutable? I just finished writing a string processing module which calls multiple subroutines, and all of them carrying arguments with type `string` viz. `immutable(char)[]` IIUC, and I tried to pass it something which came from File.byLine(), then got the error: function textattr.applyTextAttr (string text) is not callable using argument types (char[]) I understand that const can refer to either mutable or immutable, so does this mean I should replace all occurrences of `string` in arguments and return values of functions by `const(char)[]`? -- Shriramana Sharma, Penguin #395953
