Result Types and void usage

[ag0aep6g via Digitalmars-d-learn]

On 07/15/2016 10:11 AM, nik wrote:
> One thing I cant figure out/don't know if is possible is to have a type
> that takes "void" (see last unittest)
[...]
> Result type:
>
> struct Result(T, E)
> {
>      this(inout T result) inout
>      {
>          _result = result;
>          _is_result = true;
>      }
>
>      this(inout E error) inout
>      {
>          _error = error;
>          _is_result = false;
>      }
>
>      bool is_result() const pure nothrow @safe @property
>      {
>          return _is_result;
>      }
>
>      T result() const pure nothrow @safe @property
>      {
>          return _result;
>      }
>
>       bool is_error() const pure nothrow @safe @property
>      {
>          return !_is_result;
>      }
>
>      E error() const pure nothrow @safe @property
>      {
>          return _error;
>      }
> private:
>      T _result;
>      bool _is_result;
>      E _error;
> }

void is somewhat special. It can't be used to declare variables or as a 
parameter type. So you'll have to approach this a bit differently. You 
also can't have a struct constructor with zero parameters.

You can detect void and make it a special case where slightly different 
code is generated:

----
struct Result(T, E)
{
     static if (!is(T == void)) this(inout T result) inout
     {
     ...
     }

     ...

     T result() const pure nothrow @safe @property
     {
         static if (!is(T == void)) return _result;
     }

     ...

     static if (!is(T == void)) T _result;
     bool _is_result = true; /* important when T is void */
}
----

[...]
> //unittest
> //{
> //    auto result_1 = Result!(void, string)(void);
> //    auto result_2 = Result!(void, string)(void);

`void` can't be an argument in D. Just leave the list empty: 
`Result!(void, string)()`.

> //    assert(result_1.is_result);
> //    assert(result_1 == result_2);
> //}