I'm confused, what am I doing wrong here? This example came from the std.algorithm documentation.

import std.algorithm;

void main()
{
    immutable int[] arr = [ 2, 3, 1, 5, 0 ]; // index using pointers
    auto index1 = new immutable(int)*[arr.length];
    makeIndex!("a < b")(arr, index1);
}

Error is:

main.d(7): Error: template std.algorithm.makeIndex(alias less = "a < b",SwapStrategy ss = SwapStrategy.unstable,Range,RangeIndex) if (isForwardRange!(Range) && isRandomAccessRange!(RangeIndex) && is(ElementType!(RangeIndex) : ElementType!(Range)*)) does not match any function template declaration
main.d(7): Error: template std.algorithm.makeIndex(alias less = "a < b",SwapStrategy ss = SwapStrategy.unstable,Range,RangeIndex) if (isForwardRange!(Range) && isRandomAccessRange!(RangeIndex) && is(ElementType!(RangeIndex) : ElementType!(Range)*)) cannot deduce template function from argument types !("a < b")(immutable(int[]),immutable(int)*[])
main.d(7): Error: template instance errors instantiating template

Johann MacDonagh:

> I'm confused, what am I doing wrong here?

I think that Phobos needs way more&better unittests.

I think the error you receive reduces to this:

import std.range: isForwardRange;

void foo(R)(R r) if (isForwardRange!R) {}

void main() {
     immutable arr = [1, 2];
     foo(arr);
}

Bye,
bearophile

On 7/2/2011 9:00 PM, bearophile wrote:
> Johann MacDonagh:
>
>> I'm confused, what am I doing wrong here?
>
> I think that Phobos needs way more&better unittests.
>
> I think the error you receive reduces to this:
>
> import std.range: isForwardRange;
>
> void foo(R)(R r) if (isForwardRange!R) {}
>
> void main() {
>       immutable arr = [1, 2];
>       foo(arr);
> }
>
> Bye,
> bearophile

Strangely enough there is a unit test for this...
https://github.com/D-Programming-Language/phobos/blob/phobos-2.053/std/algorithm.d#L6742

Does this code fail to compile for you too? I want to make sure I didn't mess up my config here.

On 7/2/2011 9:00 PM, Johann MacDonagh wrote:
> On 7/2/2011 9:00 PM, bearophile wrote:
>> Johann MacDonagh:
>>
>>> I'm confused, what am I doing wrong here?
>>
>> I think that Phobos needs way more&better unittests.
>>
>> I think the error you receive reduces to this:
>>
>> import std.range: isForwardRange;
>>
>> void foo(R)(R r) if (isForwardRange!R) {}
>>
>> void main() {
>> immutable arr = [1, 2];
>> foo(arr);
>> }
>>
>> Bye,
>> bearophile
>
> Strangely enough there is a unit test for this...
> https://github.com/D-Programming-Language/phobos/blob/phobos-2.053/std/algorithm.d#L6742
>
>
> Does this code fail to compile for you too? I want to make sure I didn't
> mess up my config here.

Ah, here we go:

http://d.puremagic.com/issues/show_bug.cgi?id=6148

Replacing immutable with auto works. I'm still confused how the phobos unit tests pass though...

On 2011-07-02 18:14, Johann MacDonagh wrote:
> On 7/2/2011 9:00 PM, Johann MacDonagh wrote:
> > On 7/2/2011 9:00 PM, bearophile wrote:
> >> Johann MacDonagh:
> >>> I'm confused, what am I doing wrong here?
> >> 
> >> I think that Phobos needs way more&better unittests.
> >> 
> >> I think the error you receive reduces to this:
> >> 
> >> import std.range: isForwardRange;
> >> 
> >> void foo(R)(R r) if (isForwardRange!R) {}
> >> 
> >> void main() {
> >> immutable arr = [1, 2];
> >> foo(arr);
> >> }
> >> 
> >> Bye,
> >> bearophile
> > 
> > Strangely enough there is a unit test for this... https://github.com/D-Programming-Language/phobos/blob/phobos-2.053/std/al gorithm.d#L6742
> > 
> > 
> > Does this code fail to compile for you too? I want to make sure I didn't mess up my config here.
> 
> Ah, here we go:
> 
> http://d.puremagic.com/issues/show_bug.cgi?id=6148
> 
> Replacing immutable with auto works. I'm still confused how the phobos unit tests pass though...

That's because they're using immutable(int)[], not immutable int[]. The elements in the range are immutable, but the range is not. Whereas with immutable int[], the whole thing is immutable, so it doesn't work. The range has to be mutable.

- Jonathan M Davis

Jonathan M Davis wrote:
> The range has to be mutable.

Is there any easy way around this, aside from casting away the outer immutable?

It's a very annoying limitation that doesn't really have to be there - I believe the language itself would let you pass immutable int[] to a regular function that expects immutable(int)[], since the outermost reference is passed by value anyway.

On 7/2/2011 10:42 PM, Adam D. Ruppe wrote:
> Jonathan M Davis wrote:
>> The range has to be mutable.
>
> Is there any easy way around this, aside from casting away
> the outer immutable?
>
> It's a very annoying limitation that doesn't really have to be
> there - I believe the language itself would let you pass immutable
> int[] to a regular function that expects immutable(int)[], since the
> outermost reference is passed by value anyway.

Just the pointer is passed by value, referencing elements in the array mutate the source.

On 7/2/2011 10:22 PM, Jonathan M Davis wrote:
> On 2011-07-02 18:14, Johann MacDonagh wrote:
>> On 7/2/2011 9:00 PM, Johann MacDonagh wrote:
>>> On 7/2/2011 9:00 PM, bearophile wrote:
>>>> Johann MacDonagh:
>>>>> I'm confused, what am I doing wrong here?
>>>>
>>>> I think that Phobos needs way more&better unittests.
>>>>
>>>> I think the error you receive reduces to this:
>>>>
>>>> import std.range: isForwardRange;
>>>>
>>>> void foo(R)(R r) if (isForwardRange!R) {}
>>>>
>>>> void main() {
>>>> immutable arr = [1, 2];
>>>> foo(arr);
>>>> }
>>>>
>>>> Bye,
>>>> bearophile
>>>
>>> Strangely enough there is a unit test for this...
>>> https://github.com/D-Programming-Language/phobos/blob/phobos-2.053/std/al
>>> gorithm.d#L6742
>>>
>>>
>>> Does this code fail to compile for you too? I want to make sure I didn't
>>> mess up my config here.
>>
>> Ah, here we go:
>>
>> http://d.puremagic.com/issues/show_bug.cgi?id=6148
>>
>> Replacing immutable with auto works. I'm still confused how the phobos
>> unit tests pass though...
>
> That's because they're using immutable(int)[], not immutable int[]. The
> elements in the range are immutable, but the range is not. Whereas with
> immutable int[], the whole thing is immutable, so it doesn't work. The range
> has to be mutable.
>
> - Jonathan M Davis

Ah! How could I miss that? Some of the examples in std.algorithm should be updated to reflect this at least until that bug is fixed.

Johann MacDonagh wrote:
> Just the pointer is passed by value, referencing elements in the array mutate the source.

That's true, but when just moving through a range, the pointer is the only thing that changes. The stuff it points to can still be immutable.

On 7/2/2011 10:42 PM, Adam D. Ruppe wrote:
> Jonathan M Davis wrote:
>> The range has to be mutable.
>
> Is there any easy way around this, aside from casting away
> the outer immutable?
>
> It's a very annoying limitation that doesn't really have to be
> there - I believe the language itself would let you pass immutable
> int[] to a regular function that expects immutable(int)[], since the
> outermost reference is passed by value anyway.

Oh, I misread your comment. I agree, but I bet you run into issues when it comes to appending data to your newly casted immutable(int)[] reference (any appending would require a relocation). I should probably re-read the D-slices article.