Making sense of ranges

[Dmitry Olshansky]

On 24.03.2012 22:19, Stewart Gordon wrote:
> The documentation for std.range states
>
> http://dlang.org/phobos/std_range.html
> "This module defines the notion of range (by the membership tests
> isInputRange, isForwardRange, isBidirectionalRange,
> isRandomAccessRange), range capability tests (such as hasLength or
> hasSlicing), and a few useful range incarnations."
>
> But that intro doesn't describe what a range actually is.
>
> Here's what I've made out so far:
>
> - Ranges are basically a generalised API for accessing a container or
> stream that is linear in logical structure and can have any element type.
>
> - What is a range and what isn't is determined by compile-time duck
> typing - testing whether it implements certain methods.
>
> - A range over a finite data structure is essentially equivalent to a
> start/end iterator pair as used by various C++ STL functions.
>
> - Where a function in std.range is described as iterating through ranges
> in a particular way, what this function does is to return a range that
> delivers the resulting sequence.
>
> Have I got all this right?
>

More or less fine.

>
> Things I'm confused by:
>
> - One of the expansions of put is
> r.front = e; r.popFront();

Strange looking this one, but it allows, for instance, to output things 
to a container that defines range over it's elements.
>
> What has putting something at the front of a range and then popping it
> to do with outputting to the range?
>
> - Why is a range that can save checkpoints called a "forward" range?
>

It's not truly checkpoints, but rather the whole state can copied.
I believe, in some use cases checkpoints can be much smaller then the 
whole state, and we haven't got a checkpoint range yet. Could be a nice 
addition if proposal is sound.


-- 
Dmitry Olshansky