So I have an XLSX (MS Excel 2007+ file format) library that I wrote  (https://github.com/TransientResponse/dlang-xlsx) that I recently converted from std.xml to dxml. That went well and it still works (much faster too).

Now I had the idea that a lazy forward range API (like dxml itself or std.csv) would be more convenient than my current API, which reads an XLSX sheet in its entirety as a giant array of arrays (string[][]).

So I set about creating a wrapper class that implements front (returns a string[], the current row), popFront (runs through the dxml range until it reaches the end of the next row and populates the current row array from that), and empty.

I've figured out that my wrapper Sheet class has to be a templated class (don't know why) in order for my empty method to not fail on matching std.range.primitive's definition of empty.

Since I'm trying to create a range of string[]s, I templated my class as
```
class XLSheet(R) if(isRandomAccessRange!R && isSomeChar!(ElementType!R))
```

And now when I go to use it, if I declare it as say:
```
auto sheetRange = new XLSheet!(string[])();
```

DMD (2.080.1 Win10 x64) gives me the error:
```
source\xlsx.d(203,27): Error: template instance `xlsx.XLSheet!(string[])` does not match template declaration XLSheet(R) if (isRandomAccessRange!R && isSomeChar!(ElementType!R))
```

I'm guessing that `string[]` doesn't satisfy the requirements of `isRandomAccessRange` and such.

Can anyone smarter than me (or at least better at D than me) educate me as to why this doesn't compile, and how I might go about defining my wrapper class?

Full source code is on GitHub as I mentioned, in the `rangeAPI` branch.

On Sunday, 24 June 2018 at 20:33:32 UTC, Rudy Raab wrote:
> So I have an XLSX (MS Excel 2007+ file format) library that I wrote  (https://github.com/TransientResponse/dlang-xlsx) that I recently converted from std.xml to dxml. That went well and it still works (much faster too).
>
> [...]

I think it's the isSomeChar!(ElementType!R), not the isRandomAccessRange (because string isSomeString and !isSomeChar)?

Cheers,
- Ali

On Sunday, 24 June 2018 at 21:28:06 UTC, aliak wrote:
> On Sunday, 24 June 2018 at 20:33:32 UTC, Rudy Raab wrote:
>> So I have an XLSX (MS Excel 2007+ file format) library that I wrote  (https://github.com/TransientResponse/dlang-xlsx) that I recently converted from std.xml to dxml. That went well and it still works (much faster too).
>>
>> [...]
>
> I think it's the isSomeChar!(ElementType!R), not the isRandomAccessRange (because string isSomeString and !isSomeChar)?
>
> Cheers,
> - Ali

Changing it to isSomeString!(ElementType!R) moves the error to my empty() function:
```
source\xlsx.d(205,22): Error: template std.range.primitives.empty cannot deduce function from argument types !()(XLSheet!(string[])), candidates are:
C:\D\dmd2\windows\bin\..\..\src\phobos\std\range\primitives.d(2090,16):        std.range.primitives.empty(T)(auto ref scope const(T) a) if (is(typeof(a.length) : size_t) || isNarrowString!T)
```

I tried implementing a length() function (the number of rows remaining in the range, which is known at runtime), but the error remains.