More uses of operator "in"

[Baz via Digitalmars-d-learn]

On Tuesday, 28 October 2014 at 13:50:24 UTC, Nordlöw wrote:
> Has there been any proposals/plans to make operator "in" work 
> for elements in ranges such as
>
>     assert('x' in ['x']);
>
> I'm missing that Python feature when I work in D.

There is also something similar in Pascal, at the language level. 
Very handy when working with characters or enums.

I think in D it's possible to create some library types which 
allow an almost similar syntax. For example this one, briefly 
written after reading your post:

----
import std.stdio;

// global variable to get rid of 
https://issues.dlang.org/show_bug.cgi?id=11877
CharSet charSet;
struct CharSet
{
     private string str;
     public:
         typeof(this) opSlice(char lo, char hi)
         {
             CharSet result;
             foreach(c; lo .. hi)
                 result.str ~= c;
             return result;
         }
         typeof(this) opSlice(char lohi)
         {
             CharSet result;
             result.str ~= lohi;
             return result;
         }
         bool opIn_r(char elem)
         {
             if (str == "")
                 return false;
             else
                 return ((elem >= str[0]) & (elem <= str[$-1]));
         }
         string toString()
         {
             return str;
         }
}

void main(string args[])
{
     auto a2k = charSet['a' .. 'k'+1];
     auto A2K = charSet['A' .. 'K'+1];
     auto Z29 = charSet['0' .. '9'+1];

     assert( 'a' in a2k );
     assert( !('x' in a2k) );

     assert( 'A' in A2K );
     assert( !('X' in A2K) );

     import std.conv;
     assert( to!string(Z29) == "0123456789" );

     assert( 'x' in charSet['x'..'x'+1] );
}
----