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digitalmars.D.learn - immutable promise broken in unions?

reply Shriramana Sharma <samjnaa_dont_spam_me gmail.com> writes:
import std.stdio;
union EarthLocation
{
    struct { immutable double lon, lat, alt; }
    double[3] data;
}
void main()
{
    EarthLocation d = {data: [4, 5, 6]};
    writeln(d.data);
    d.data = [1, 2, 3];
    writeln(d.data);
}

I get the output:

[4, 5, 6]
[1, 2, 3]

I thought the promise of `immutable` was: never changes, whether via this 
interface or otherwise. How does then the above work?

Using DMD 2.0.69.2 on Kubuntu 64 bit.

-- 
Shriramana Sharma, Penguin #395953
Jan 02
next sibling parent reply John Colvin <john.loughran.colvin gmail.com> writes:
On Saturday, 2 January 2016 at 10:04:47 UTC, Shriramana Sharma 
wrote:
 import std.stdio;
 union EarthLocation
 {
     struct { immutable double lon, lat, alt; }
     double[3] data;
 }
 void main()
 {
     EarthLocation d = {data: [4, 5, 6]};
     writeln(d.data);
     d.data = [1, 2, 3];
     writeln(d.data);
 }

 I get the output:

 [4, 5, 6]
 [1, 2, 3]

 I thought the promise of `immutable` was: never changes, 
 whether via this interface or otherwise. How does then the 
 above work?

 Using DMD 2.0.69.2 on Kubuntu 64 bit.
You are manually breaking immutable by making a union of immutable and mutable data and then writing to the mutable reference. This is roughly equivalent to casting away immutable and then writing to the reference. It's a bug in your code. All references to the same data should be 1) either immutable or const or all the references should be 2) either mutable or const (assuming the data was never immutable). Anything else is dangerous.
Jan 02
parent reply Meta <jared771 gmail.com> writes:
On Saturday, 2 January 2016 at 12:07:31 UTC, John Colvin wrote:
 You are manually breaking immutable by making a union of 
 immutable and mutable data and then writing to the mutable 
 reference. This is roughly equivalent to casting away immutable 
 and then writing to the reference. It's a bug in your code.

 All references to the same data should be
 1) either immutable or const
 or all the references should be
 2) either mutable or const (assuming the data was never 
 immutable).
 Anything else is dangerous.
Surely the compiler should disallow this. It makes it trivial to break the type system otherwise.
Jan 02
next sibling parent Marc =?UTF-8?B?U2Now7x0eg==?= <schuetzm gmx.net> writes:
On Saturday, 2 January 2016 at 12:08:48 UTC, Meta wrote:
 On Saturday, 2 January 2016 at 12:07:31 UTC, John Colvin wrote:
 You are manually breaking immutable by making a union of 
 immutable and mutable data and then writing to the mutable 
 reference. This is roughly equivalent to casting away 
 immutable and then writing to the reference. It's a bug in 
 your code.

 All references to the same data should be
 1) either immutable or const
 or all the references should be
 2) either mutable or const (assuming the data was never 
 immutable).
 Anything else is dangerous.
Surely the compiler should disallow this. It makes it trivial to break the type system otherwise.
Right, this is a known bug: https://issues.dlang.org/show_bug.cgi?id=13537
Jan 02
prev sibling parent reply John Colvin <john.loughran.colvin gmail.com> writes:
On Saturday, 2 January 2016 at 12:08:48 UTC, Meta wrote:
 On Saturday, 2 January 2016 at 12:07:31 UTC, John Colvin wrote:
 You are manually breaking immutable by making a union of 
 immutable and mutable data and then writing to the mutable 
 reference. This is roughly equivalent to casting away 
 immutable and then writing to the reference. It's a bug in 
 your code.

 All references to the same data should be
 1) either immutable or const
 or all the references should be
 2) either mutable or const (assuming the data was never 
 immutable).
 Anything else is dangerous.
Surely the compiler should disallow this. It makes it trivial to break the type system otherwise.
Casting away immutable can sometimes be necessary (e.g. when talking to other languages), so I'm not sure it should be disallowed, but it'd be great if it was somehow easier to catch these bugs.
Jan 02
next sibling parent Shriramana Sharma <samjnaa_dont_spam_me gmail.com> writes:
John Colvin wrote:

 Casting away immutable can sometimes be necessary (e.g. when
 talking to other languages), so I'm not sure it should be
 disallowed, but it'd be great if it was somehow easier to catch
 these bugs.
Yes it was in the context of talking to C that I needed to make such a union. But already making a union is breaking the type system in a way, no? -- Shriramana Sharma, Penguin #395953
Jan 02
prev sibling parent tsbockman <thomas.bockman gmail.com> writes:
On Saturday, 2 January 2016 at 13:15:37 UTC, John Colvin wrote:
 Casting away immutable can sometimes be necessary (e.g. when 
 talking to other languages), so I'm not sure it should be 
 disallowed, but it'd be great if it was somehow easier to catch 
 these bugs.
It should be disallowed in safe code, though.
Jan 02
prev sibling parent Tobi G. <gruen_tobias web.de> writes:
On Saturday, 2 January 2016 at 10:04:47 UTC, Shriramana Sharma 
wrote:
 I thought the promise of `immutable` was: never changes.
The compiler doesn't protect you by carrying a bomb. :) But there is another usecase where it makes sense to allow writing to other union members despite the overlap with immutable data:
enum Types {Integer, Text, CalculationConstant}

struct oldStyleLimitedBadVariant // Don't use that! D offers 
std.variant.Algebraic..
{
    Types type;
    union
    {
        int integer;
        string text;
        immutable float calculationConstant;
    }
}
There i would expect that i can write to integer and text at any time. togrue
Jan 02