Representing Code 5

src/expr.d

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+import std.conv;
+import std.stdio;
+
+import taggedalgebraic;
+
+import token;
+import tokentype;
+
+abstract class Expr{
+	interface Visitor(R){
+		R visit(Binary expr);
+		R visit(Grouping expr);
+		R visit(Literal expr);
+		R visit(Unary expr);
+	}
+	abstract string accept(Visitor!string 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 class Binary : Expr{
+		Expr left;
+		Token operator;
+		Expr right;
+		mixin defCtorAndAccept;
+	}
+	static class Grouping : Expr{
+		Expr expression;
+		mixin defCtorAndAccept;
+	}
+	static class Literal : Expr{
+		TTokenValue value;
+		mixin defCtorAndAccept;
+	}
+	static class Unary : Expr{
+		Token operator;
+		Expr right;
+		mixin defCtorAndAccept;
+	}
+}
+
+class AstPrinter : Expr.Visitor!string{
+	string print(Expr expr){
+		return expr.accept(this);
+	}
+	string parenthesize(Args...)(string name, Args args){
+		string s = "(" ~ name;
+		static foreach(expr; args){
+			s ~= " ";
+			s ~= expr.accept(this);
+		}
+		return s ~ ")";
+	}
+	string visit(Expr.Binary expr){
+		return parenthesize(expr.operator.lexeme, expr.left, expr.right);
+	}
+	string visit(Expr.Grouping expr){
+		return parenthesize("group", expr.expression);
+	}
+	string visit(Expr.Literal expr){
+		if(expr.value.isNil)
+			return "nil";
+		return expr.value.to!string;
+	}
+	string visit(Expr.Unary expr){
+		return parenthesize(expr.operator.lexeme, expr.right);
+	}
+}
+
+unittest{
+	auto expression = new Expr.Binary(
+		new Expr.Unary(
+			new Token(TokenType.MINUS, "-", TTokenValue.nil(0), 1),
+			new Expr.Literal(123)),
+		new Token(TokenType.STAR, "*", TTokenValue.nil(0), 1),
+		new Expr.Grouping(
+			new Expr.Literal(45.67)));
+	assert(new AstPrinter().print(expression));
+}
+