Currently this code doesn't work, because the B array is const, so B[i] is const:


import std.algorithm: schwartzSort;
void main() {
    auto A = [0, 1, 2];
    const B = [10, -20, 30];
    schwartzSort!(i => B[i])(A);
}


dmd gives:

...\dmd2\src\phobos\std\conv.d(3734): Error: static assert  "Don't know how to emplace a const(int) with a const(int)."
...\dmd2\src\phobos\std\algorithm.d(9210):        instantiated from here: emplace!(const(int), const(int))
test.d(5):        instantiated from here: schwartzSort!(__lambda2, "a < b", cast(SwapStrategy)0, int[])


But the array A is mutable. So is it acceptable for me to ask in Bugzilla this code to work?

Bye,
bearophile

On 09/03/2013 03:22 PM, bearophile wrote:

> Currently this code doesn't work, because the B array is const, so B[i]
> is const:
>
>
> import std.algorithm: schwartzSort;
> void main() {
>      auto A = [0, 1, 2];
>      const B = [10, -20, 30];
>      schwartzSort!(i => B[i])(A);

The return type of the lambda is deduced as const(int). I think that is silly. A copied value type should not preserve the const attribute. (I vaguely remember discussions on this topic.)

Explicit return type works:

    schwartzSort!(delegate int (i) => B[i])(A);

Playing along with the compiler and casting away const also works:

    schwartzSort!(i => cast()B[i])(A);

> }
>
>
> dmd gives:
>
> ...\dmd2\src\phobos\std\conv.d(3734): Error: static assert "Don't know
> how to emplace a const(int) with a const(int)."
> ...\dmd2\src\phobos\std\algorithm.d(9210):        instantiated from
> here: emplace!(const(int), const(int))
> test.d(5):        instantiated from here: schwartzSort!(__lambda2, "a <
> b", cast(SwapStrategy)0, int[])
>
>
> But the array A is mutable. So is it acceptable for me to ask in
> Bugzilla this code to work?

I would think so but I think this issue may already be in bugzilla.

>
> Bye,
> bearophile

Thanks,
Ali

On Tue, Sep 03, 2013 at 03:57:09PM -0700, Ali Çehreli wrote:
> On 09/03/2013 03:22 PM, bearophile wrote:
> 
> > Currently this code doesn't work, because the B array is const, so B[i] is const:
> >
> >
> > import std.algorithm: schwartzSort;
> > void main() {
> >      auto A = [0, 1, 2];
> >      const B = [10, -20, 30];
> >      schwartzSort!(i => B[i])(A);
> 
> The return type of the lambda is deduced as const(int). I think that
> is silly. A copied value type should not preserve the const
> attribute. (I vaguely remember discussions on this topic.)
> 
> Explicit return type works:
> 
>     schwartzSort!(delegate int (i) => B[i])(A);
> 
> Playing along with the compiler and casting away const also works:
> 
>     schwartzSort!(i => cast()B[i])(A);
> 
> > }
> >
> >
> > dmd gives:
> >
> > ...\dmd2\src\phobos\std\conv.d(3734): Error: static assert "Don't know
> > how to emplace a const(int) with a const(int)."
> > ...\dmd2\src\phobos\std\algorithm.d(9210):        instantiated from
> > here: emplace!(const(int), const(int))
> > test.d(5):        instantiated from here: schwartzSort!(__lambda2, "a <
> > b", cast(SwapStrategy)0, int[])
> >
> >
> > But the array A is mutable. So is it acceptable for me to ask in Bugzilla this code to work?
> 
> I would think so but I think this issue may already be in bugzilla.
[...]

I filed a new issue just in case:

	http://d.puremagic.com/issues/show_bug.cgi?id=10960


T

-- 
"Outlook not so good." That magic 8-ball knows everything! I'll ask about Exchange Server next. -- (Stolen from the net)

H. S. Teoh:

> I filed a new issue just in case:
>
> 	http://d.puremagic.com/issues/show_bug.cgi?id=10960

Thank you. But the idea of stripping away the const from copied values misses my main point, so I have added a note there.

Bye,
bearophile