Ranges, constantly frustrating

[Regan Heath]

On Tue, 11 Feb 2014 19:48:40 -0000, Jesse Phillips  
<Jesse.K.Phillips+D at gmail.com> wrote:

> On Tuesday, 11 February 2014 at 10:10:27 UTC, Regan Heath wrote:
>> Things like this should "just work"..
>>
>> File input ...
>>
>> auto range = input.byLine();
>> while(!range.empty)
>> {
>>   range.popFront();
>>   foreach (i, line; range.take(4))  //Error: cannot infer argument types
>>   {
>>     ..etc..
>>   }
>>   range.popFront();
>> }
>>
>> Tried adding 'int' and 'char[]' or 'auto' .. no dice.
>>
>> Can someone explain why this fails, and if this is a permanent or  
>> temporary limitation of D/MD.
>>
>> R
>
> In case the other replies weren't clear enough. A range does not have an  
> index.

It isn't *required* to (input/forward), but it could (random access).  I  
think we even have a template to test if it's indexable as we can optimise  
some algorithms based on this.

> What do you expect 'i' to be? Is it the line number? Is it the index  
> within the line where 'take' begins? Where 'take' stops?

If I say take(5) I expect 0,1,2,3,4.  The index into the take range itself.

The reason I wanted it was I was parsing blocks of data over 6 lines - I  
wanted to ignore the first and last and process the middle 4.  In fact I  
wanted to skip the 2nd of those 4 as well, but there was not single  
function (I could find) which would do all that so I coded the while above.

> There is a feature of foreach and tuple() which results in the tuple  
> getting expanded automatically.

And also the opApply overload taking a delegate with both parameters.

> byLine has its own issues with reuse of the buffer, it isn't inherent to  
> ranges. I haven't really used it (needed it from std.process), when I  
> wanted to read a large file I went with wrapping std.mmap:
>
> https://github.com/JesseKPhillips/libosm/blob/master/source/util/filerange.d

Cool, thanks.

R

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