Can't add a const ubyte to a dynamic array of ubyte?

[Jonathan M Davis]

On Tuesday, August 20, 2019 3:27:36 AM MDT ads via Digitalmars-d-learn 
wrote:
> import std.stdio;
>
> ubyte[] extend(in uint[] arr)
> {
>   ubyte[] result;
>   foreach (n; arr)
>   {
>       if (n < 10)
>       {
>           result ~= n;
>                          // source/app.d(10,11): Error: cannot
> append type const(uint) to type ubyte[]
>       }
>       else
>       {
>           import std.conv : to;
>
>           foreach (digit; n.to!string)
>           {
>               result ~= digit.to!ubyte;
>           }
>       }
>   }
>   return result;
> }
>
> unittest
> {
>   import std.algorithm : equal;
>
>   assert(extend([1, 25, 70, 0]).equal([1, 2, 5, 7, 0, 0]));
> }
>
>
> How can I get around this? I want to ensure that the array is not
> mutated in the function in the signature too.

arr contains uints, not ubytes, so n is a uint, and you're trying to append
a uint to result, which is an array of ubytes. If you want to append n to
result, then you need to convert it to a ubyte - either by casting or by
using to!uint (the difference being that to will throw a ConvException if
the value of n doesn't fit in a ubyte). D does not allow implicit narrowing
conversions (because there's no guarantee that the value will fit in the
target type), so you can't assign a uint to a ubyte without an explicit
conversion - and that includes appending to an array of ubytes.

- Jonathan M Davis