Why does enumerate over range return dchar, when ranging without returns char?
rikki cattermole
rikki at cattermole.co.nz
Thu May 3 10:00:04 UTC 2018
On 03/05/2018 9:50 PM, ag0aep6g wrote:
> On 05/03/2018 07:56 AM, rikki cattermole wrote:
>>> ```
>>> import std.stdio;
>>> import std.range : enumerate;
>>>
>>> void main()
>>> {
>>> char[] s = ['a','b','c'];
>>>
>>> char[3] x;
>>> auto i = 0;
>>> foreach(c; s) {
>>> x[i] = c;
>>> i++;
>>> }
>>>
>>> writeln(x);
>>> }
>>> ```
>>> Above works without cast.
>>>
>>> '''
>>> import std.stdio;
>>> import std.range : enumerate;
>>>
>>> void main()
>>> {
>>> char[] s = ['a','b','c'];
>>>
>>> char[3] x;
>>> foreach(i, c; enumerate(s)) {
>>> x[i] = c;
>>> i++;
>>> }
>>>
>>> writeln(x);
>>> }
>>> ```
> [...]
>> The first example uses auto-decoding (UTF-8 codepoints into a single
>> UTF-32 one). This is considered a bad thing. But the compiler can
>> disable it and leave it as UTF-8 code point upon request.
>
> The first example (foreach over a char[]) doesn't do any decoding. UTF-8
> stays UTF-8.
>
> Also, a `char` is a UTF-8 code *unit*, not a code *point*.
>
>> The second example returns a Voldemort type (means no-name) which
>> happens to be an input range. Where it can't disable anything and has
>> been told that it is returning a dchar. See[0] as to where this gets
>> decoded.
>
> This is auto decoding.
>
>> Writing two small functions to replace it (and popFront), will
>> override this behavior.
>
> This sounds like you can disable auto decoding by providing your own
> range primitives in your own module. That doesn't work, because Phobos
> would still use the ones from std.range.primitives.
Hmm, I swear this use to work.
Oh well, easy fix:
import std.algorithm;
struct Wrapper {
char[] input;
alias input this;
@property char front() { return input[0]; }
@property bool empty() {return input.length == 0;}
void popFront() { input = input[1 .. $]; }
}
void main() {
char[] text = ['1', '2', '3'];
foreach(c; Wrapper(text).filter!(a => a != '\0')) {
pragma(msg, typeof(c));
}
}
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