/+dub.sdl:

dependency "sumtype" version="~>0.9.5"

+/

import std.stdio;

import sumtype;

​

struct Payload(T...){

  alias Types = T;

  T contents;

}

​

struct ParseError{

    string s;

}

​

template ParseResult(P, PE){

  static if(is(P : Payload!Args, Args...) && is(PE == ParseError)){

    alias ParseResult = SumType!(P, PE);

  }

}

​

bool isParseError(PR: ParseResult!(P, PE), P, PE)(PR pr){

  return pr.match!(

    (P p) => false,

    (PE pe) => true);

}

​

bool isParseError2(PR: SumType!(P, PE), P, PE)(PR pr){

    return pr.match!(

    (P p)=> false,

    (PE pe) => true);

}

​

void main(){

   auto pi = Payload!int(5);

   auto pe = ParseError("error");

    alias PRType = ParseResult!(Payload!int, ParseError);

    auto pr = PRType(pi);

    writeln(pr.isParseError2);

    writeln(pr.isParseError);

​

​

}