expr.d: metafy a bit more

src/expr.d

@@ -1,5 +1,6 @@
 import std.conv;
 import std.stdio;
+import std.meta : AliasSeq;
 
 import taggedalgebraic;
 
@@ -13,13 +14,16 @@ abstract class Expr{
 		R visit(Literal expr);
 		R visit(Unary expr);
 	}
-	abstract string accept(Visitor!string visitor);
+	private alias rTypes = AliasSeq!(string);
+	static foreach(T; rTypes)
+		abstract T accept(Visitor!T visitor);
 	private template defCtorAndAccept(){
 		this(Args...)(auto ref Args args){
 			static foreach(i, a; args)
 				this.tupleof[i] = a;
 		}
-		override string accept(Visitor!string visitor) => visitor.visit(this);
+		static foreach(T; rTypes)
+			override T accept(Visitor!T visitor) => visitor.visit(this);
 	}
 
 	static class Binary : Expr{