A small trouble with std.range.indexed

[Ali Çehreli]

On Thu, 08 Sep 2011 03:33:24 -0400, bearophile wrote:

> std.range.indexed of DMD 2.055 is very nice. But it signature shows
> constraints that I have had problems to work with, maybe the error is
> just mine, I don't know:
> 
> 
> import std.algorithm, std.range, std.array; void main() {
>     auto data = [7, 6, 5, 4, 3, 2, 1, 0]; auto indices = [6, 1, 7];
> 
>     //auto idxSet = array(uniq(indices.sort())); // OK

In that case, idxSet is a RandomAccessRange.

>     auto idxSet = uniq(indices.sort()); // not OK?

indices.sort() is a RandomAccessRange but uniq is not, as it needs to 
walk the elements lazily.

The documentation of indexed() says: "Source must be a random access 
range. The returned range will be bidirectional or random-access if 
Indices is bidirectional or random-access, respectively."

>     // std.range.indexed() constraints
>     static assert(isInputRange!(typeof(idxSet))); // OK static assert(
>     is(typeof((int[]).init[ElementType!(typeof(idxSet)).init])) ); // OK
> 
>     sort(indexed(data, idxSet));

Because idxSet is not RandomAccessRange, indexed() does not provide 
opIndex() and doesn't match sort()'s requirement.

Here is an excerpt of Indexed from range.d:

struct Indexed(Source, Indices)
if(isRandomAccessRange!Source && isInputRange!Indices &&
  is(typeof(Source.init[ElementType!(Indices).init])))
{
// ...

    static if(isRandomAccessRange!Indices)
    {
        /// Ditto
        auto ref opIndex(size_t index)
        {
            return _source[_indices[index]];
        }

// ...
}

>     assert(data == [7, 0, 5, 4, 3, 2, 1, 6]);
> }
> 
> 
> Bye and thank you,
> bearophile

It's unfortunate that template the error messages cannot be more 
helpful. :-/

Ali