On 02/24/2011 08:39 AM, Jonathan M Davis wrote:
On Wednesday 23 February 2011 22:41:53 Christopher Bergqvist wrote:
Hi!
I've run into an issue which I don't understand.
Boiled down code:
import std.regex;
void main()
{
//string str = "sdf"; // works
//const string str = "sdf"; // doesn't work
immutable str = "sdf"; // doesn't work
auto pat = regex(", *");
auto split = splitter(str, pat);
}
Error:
/Library/Compilers/dmd2/osx/bin/../../src/phobos/std/regex.d(3022):
Error: this is not mutable
Should splitter() be able to cope with const/immutable ranges?
(That's with the latest official v2.052 dmd/phobos distribution for
mac. I got the same error before upgrading from the v2.051
also).
Pretty much _nothing_ copes with const or immutable ranges. And if you think
about it, it generally makes sense. You can't pop the front off of a const or
immutable range. So, how could you possibly process it? The are some cases where
having tail const with ranges would work (assuming that we could have tail const
with ranges - which we currently can't), but on the whole, const and immutable
ranges don't really make sense. They can hold const or immutable data, but a
const or immutable range is pretty useless on the whole.
That's one question I'm wondering about for months (but always forget to ask):
Why should /collection/ traversal shrink them? Why does the regular range
stepping func (popFront) read, for arrays:
this.data = thid.data[1..$];
instead of:
++ this.cursor;
??? Should then be caled eg "stepFront", which imo is much better to express
the semantics of traversal / iteration.
I guess the issue expressed in this thread is /invented/ by the regular process
of range, precisely by popFront. There is no reason to mutate a collection just
to traverse it!
And then, how do you traverse the collection again?
unittest {
auto a = [1,2,3];
while (! a.empty()) {
write(a.front() ,' ');
a.popFront();
}
writeln();
// below nothing written onto terminal
while (! a.empty()) {
write(a.front() ,' ');
a.popFront();
}
writeln();
}
Denis
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