Howdy,

The following program fails to compile if the second line
is uncommented:

import std;

void main() {
    writeln([1, 2, 3].choice);
    //writeln(['a', 'b', 'c'].choice);
}

Error: template std.random.choice cannot deduce function from argument types !()(char[], MersenneTwisterEngine!(uint, 32LU, 624LU, 397LU, 31LU, 2567483615u, 11LU, 4294967295u, 7LU, 2636928640u, 15LU, 4022730752u, 18LU, 1812433253u)), candidates are:
/usr/include/dmd/phobos/std/random.d(2559):        std.random.choice(Range, RandomGen = Random)(auto ref Range range, ref RandomGen urng) if (isRandomAccessRange!Range && hasLength!Range && isUniformRNG!RandomGen)
/usr/include/dmd/phobos/std/random.d(2569):        std.random.choice(Range)(auto ref Range range)

What is going on here? I get it that choice() isn't simply an algorithm
over T[], that there are some additional constraints, but surely a
char[] is just as random acc...

...

Oh. It's because of emojicode.

This works:

import std;

void main() {
    writeln([1, 2, 3].choice);
    writeln(cast(char)(cast(uint8_t[])['a', 'b', 'c']).choice);
}

and this outputs false: writeln(isRandomAccessRange!(char[]));

I no longer have any questions but I wish the dlang.org docs were
much more generous with examples.

On 11/18/19 12:32 PM, mipri wrote:
> Howdy,
> 
> The following program fails to compile if the second line
> is uncommented:
> 
> import std;
> 
> void main() {
>      writeln([1, 2, 3].choice);
>      //writeln(['a', 'b', 'c'].choice);
> }
> 
> Error: template std.random.choice cannot deduce function from argument types !()(char[], MersenneTwisterEngine!(uint, 32LU, 624LU, 397LU, 31LU, 2567483615u, 11LU, 4294967295u, 7LU, 2636928640u, 15LU, 4022730752u, 18LU, 1812433253u)), candidates are:
> /usr/include/dmd/phobos/std/random.d(2559): std.random.choice(Range, RandomGen = Random)(auto ref Range range, ref RandomGen urng) if (isRandomAccessRange!Range && hasLength!Range && isUniformRNG!RandomGen)
> /usr/include/dmd/phobos/std/random.d(2569): std.random.choice(Range)(auto ref Range range)
> 
> What is going on here? I get it that choice() isn't simply an algorithm
> over T[], that there are some additional constraints, but surely a
> char[] is just as random acc...

Nope, phobos treats a narrow character array (such as char[] or wchar[]) as a bidirectional range of dchar. It's called autodecoding, and it's continually causing problems for about 10 years now.

> ....
> 
> Oh. It's because of emojicode.

unicode. I hope that was a joke ;)

> This works:
> 
> import std;
> 
> void main() {
>      writeln([1, 2, 3].choice);
>      writeln(cast(char)(cast(uint8_t[])['a', 'b', 'c']).choice);

You could also use cast(dchar[]), and avoid the cast back to char.

-Steve

On Mon, Nov 18, 2019 at 7:25 PM Steven Schveighoffer via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:
>
>
> You could also use cast(dchar[]), and avoid the cast back to char.
>
> -Steve

or use byCodeUnit

writeln(['a', 'b', 'c'].byCodeUnit.choice);