VLERange: a range in between BidirectionalRange and RandomAccessRange

[spir]

On 01/18/2011 04:48 AM, Michel Fortin wrote:
> On 2011-01-17 17:54:04 -0500, Michel Fortin <michel.fortin at michelf.com>
> said:
>
>> More seriously, you have four choice:
>>
>> 1. code unit
>> 2. code point
>> 3. grapheme
>> 4. require the client to state explicitly which kind of 'character' he
>> wants; 'character' being an overloaded word, it's reasonable to ask
>> for disambiguation.
>
> This makes me think of what I did with my XML parser after you made code
> points the element type for strings. Basically, the parser now uses
> 'front' and 'popFront' whenever it needs to get the next code point, but
> most of the time it uses 'frontUnit' and 'popFrontUnit' instead (which I
> had to add) when testing for or skipping an ASCII character is
> sufficient. This way I avoid a lot of unnecessary decoding of code points.
>
> For this to work, the same range must let you skip either a unit or a
> code point. If I were using a separate range with a call to toDchar or
> toCodeUnit (or toGrapheme if I needed to check graphemes), it wouldn't
> have helped much because the new range would essentially become a new
> slice independent of the original, so you can't interleave "I want to
> advance by one unit" with "I want to advance by one code point".
>
> So perhaps the best interface for strings would be to provide multiple
> range-like interfaces that you can use at the level you want.
>
> I'm not sure if this is a good idea, but I thought I should at least
> share my experience.

This looks like a very interesting approach. And clear.
I guess range synchronisation would be based on an internal lowest-level 
(codeunit) index. Then, you also need internal validity-checking and/or 
offseting routines when a higher-level range is used after a lowel-level 
one has been used. (I mean eg to ensure start-of-codepoint after a 
codeunit popFront, or throw an error.)
Also, how to avoid duplicating many operational functions (eg find a 
given slice) for each level?

Denis
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