On Thu, 13 Jan 2011 14:08:36 -0500, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
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).
You might be looking at my previous version. The new version (recently
posted) will throw an exception for that code if a multi-code-unit
code-point is found.
It also supports this:
foreach(i, d; s)
{
writeln("The character in position ", i, " is ", d);
}
where i is the index (might not be sequential)
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.
isRandomAccessRange requires hasLength (see here:
http://www.dsource.org/projects/phobos/browser/trunk/phobos/std/range.d#L532).
This is not a random access range per that definition. But a string isn't
a random access range anyways (it's specifically disallowed by std.range
per that same reference).
The plan is you would *not* have to special case algorithms for string_t
as you do currently for char[]. If that's not the case, then we haven't
achieved much. Simply put, we are separating out the strange nature of
strings from arrays, so the exceptional treatment of them is handled by
the type itself, not the functions using it.
-Steve
