Cannot deduce function from argument type when single candidate

[Preetpal]

On Monday, 29 March 2021 at 01:39:16 UTC, Adam D. Ruppe wrote:
> On Monday, 29 March 2021 at 01:24:13 UTC, Preetpal wrote:
>>     writeln(isRandomAccessRange(arr));
>
> Template arguments are passed by !(), not just ().
>
> I believe you must also pass `typeof(arr)` since 
> isRandomAccessRange is only interested in types.

Thanks. This answer led me in the right direction. What I was 
really trying to figure out was why I was not able to use the 
std.random library to take a random sampling from an instance of 
char[].


void main() {
     import std.random;
     import std.range;
     import std.stdio;
     import std.string;

     char[] arr = ['a', 'b', 'c'];
     int[] arr2 = [1, 2, 3];

     writeln(isRandomAccessRange!(typeof(arr)));
     writeln(typeid(10.iota));
     writeln(typeid(arr[0..$]));

     writeln(hasLength!(typeof(arr)));
     writeln(hasLength!(typeof(arr2)));

     auto rnd = Random(unpredictableSeed);

     auto randomnumbers = 3.iota.randomSample(2, rnd);

     writeln(randomnumbers);

     // auto randomchars = arr.randomSample(2, rnd).array; // does 
not work
     auto randomchars = arr.representation.randomSample(2, 
rnd).array;

     writeln(cast(char[])randomchars);
}


I could not understand the error messages that the compiler was 
giving me. The fact that the compiler was providing candidates 
was leading me to believe that there was a type deduction issue 
(like as if it was not able to choose between candidates because 
of an ambiguity). This led me to not pay attention to the most 
important part of the the error message related to the type 
constraints.

mini_example.d(19): Error: template std.random.randomSample 
cannot deduce function from argument types !()(char[], int, 
MersenneTwisterEngine!(uint, 32u, 624u, 397u, 31u, 2567483615u, 
11u, 4294967295u, 7u, 2636928640u, 15u, 4022730752u, 18u, 
1812433253u)), candidates are:
b:\D\DMD\D\dmd2\windows\bin\..\..\src\phobos\std\random.d(3858):  
       randomSample(Range)(Range r, size_t n, size_t total)
b:\D\DMD\D\dmd2\windows\bin\..\..\src\phobos\std\random.d(3865):  
       randomSample(Range)(Range r, size_t n)
b:\D\DMD\D\dmd2\windows\bin\..\..\src\phobos\std\random.d(3872):  
       randomSample(Range, UniformRNG)(Range r, size_t n, size_t 
total, auto ref UniformRNG rng)
b:\D\DMD\D\dmd2\windows\bin\..\..\src\phobos\std\random.d(3879):  
       randomSample(Range, UniformRNG)(Range r, size_t n, auto ref 
UniformRNG rng)
   with Range = char[],
        UniformRNG = MersenneTwisterEngine!(uint, 32u, 624u, 397u, 
31u, 2567483615u, 11u, 4294967295u, 7u, 2636928640u, 15u, 
4022730752u, 18u, 1812433253u)
   must satisfy the following constraint:
        hasLength!Range

I did not realize that the compiler was telling me that the range 
that I was providing did not satisfy that constraint. I only 
started to try testing the type constraints as a sanity check 
(which was when I ran into this error).