VLERange: a range in between BidirectionalRange and RandomAccessRange

[Andrei Alexandrescu]

On 1/13/11 8:52 AM, Steven Schveighoffer wrote:
> I see it as having two vast improvements:
>
> 1. If we replace char[] with a specific type for string, then char[] can
> be considered a true array by phobos, and phobos can now deal with a
> char[] array without the need to cast.
> 2. It protects the casual user from incorrectly using a string by making
> the default the correct API.
>
> Those to me are very important.

Let's take a look:

// Incorrect string code
void fun(string s) {
   foreach (i; 0 .. s.length) {
     writeln("The character in position ", i, " is ", s[i]);
   }
}

// Incorrect string_t code
void fun(string_t!char s) {
   foreach (i; 0 .. s.codeUnits) {
     writeln("The character in position ", i, " is ", s[i]);
   }
}

Both functions are incorrect, albeit in different ways. The only 
improvement I'm seeing is that the user needs to write codeUnits instead 
of length, which may make her think twice. Clearly, however, copiously 
incorrect code can be written with the proposed interface because it 
tries to hide the reality that underneath a variable-length encoding is 
being used, but doesn't hide it completely (albeit for good 
efficiency-related reasons).

But wait, there's less. Functions for random-access range throughout 
Phobos routinely assume fixed-length encoding, i.e. s[i + 1] lies next 
to s[i]. From a cursory look at string_t, std.range will qualify it as a 
RandomAccessRange without length. That's an odd beast but does not 
change the fixed-length encoding assumption. So you'd need to 
special-case algorithms for string_t, just like right now certain 
algorithms are specialized for string.

Where's the progress?


Andrei