@@ -108,67 +108,66 @@
 
 /// Entry point of this module
 /// If splitCtx = true, then inner variable declaration do not overwrite ctx.
 /// lay is the layer ID for evaluation (standard value semantics uses "@v").
-
-Value eval(AST _e, Table ctx, bool splitCtx, Layer lay)
-{
-	if( auto e = cast(StrLiteral)_e )
-	{
-		return new StrValue(e.data);
-	}
-	else
-	if( auto e = cast(IntLiteral)_e )
-	{
-		return new IntValue(e.data);
-	}
-	else
-	if( auto e = cast(VarExpression)_e )
-	{
-		return ctx.get(e.var, lay, e.pos);
-	}
-	else
-	if( auto e = cast(LayeredExpression)_e )
-	{
-		return eval(e.expr, ctx, false, e.lay);
-	}
-	else
-	if( auto e = cast(LetExpression)_e )
-	{
-		// for letrec, we need this, but should avoid overwriting????
-		// ctx.set(e.var, "@v", new UndefinedValue, e.pos);
-		Value v = eval(e.init, ctx, true, lay);
-		if(splitCtx)
-			ctx = new Table(ctx, Table.Kind.NotPropagateSet);
-		ctx.set(e.var, (e.layer.length ? e.layer : lay), v, e.pos);
-		return eval(e.expr, ctx, false, lay);
-	}
-	else
-	if( auto e = cast(FuncallExpression)_e )
-	{
-		Value _f = eval(e.fun, ctx, true, lay);
-		if( auto f = cast(FunValue)_f ) {
-			Value[] args;
-			foreach(a; e.args)
-				args ~= eval(a, ctx, true, lay);
-			return f.call(e.pos, lay, args);
-		} else
+import std.typetuple;
+Value eval(AST e, Table ctx, bool splitCtx, Layer lay)
+{
+	return e.match(
+		(StrLiteral e)
+		{
+			return new StrValue(e.data);
+		},
+		(IntLiteral e)
+		{
+			return new IntValue(e.data);
+		},
+		(VarExpression e)
+		{
+			return ctx.get(e.var, lay, e.pos);
+		},
+		(LayeredExpression e)
+		{
+			return eval(e.expr, ctx, false, e.lay);
+		},
+		(LetExpression e)
+		{
+			// for letrec, we need this, but should avoid overwriting????
+			// ctx.set(e.var, "@v", new UndefinedValue, e.pos);
+			Value v = eval(e.init, ctx, true, lay);
+			if(splitCtx)
+				ctx = new Table(ctx, Table.Kind.NotPropagateSet);
+			ctx.set(e.var, (e.layer.length ? e.layer : lay), v, e.pos);
+			return eval(e.expr, ctx, false, lay);
+		},
+		(FuncallExpression e)
+		{
+			Value _f = eval(e.fun, ctx, true, lay);
+			if( auto f = cast(FunValue)_f ) {
+				Value[] args;
+				foreach(a; e.args)
+					args ~= eval(a, ctx, true, lay);
+				return f.call(e.pos, lay, args);
+			}
 			throw genex!RuntimeException(e.pos, "Non-funcion is applied");
-	}
-	else
-	if( auto e = cast(FunLiteral)_e )
-	{
-		return new FunValue(delegate Value(immutable LexPosition pos, string lay, Value[] args){
-			if( e.params.length != args.length )
-				throw genex!RuntimeException(e.pos, sprintf!"Argument Number Mismatch (%d required but %d given)"
-					(e.params.length, args.length));
-			Table ctxNeo = new Table(ctx, Table.Kind.NotPropagateSet);
-			foreach(i,p; e.params)
-				ctxNeo.set(p.name, lay, args[i]);
-			return eval(e.funbody, ctxNeo, true, lay);
-		});
-	}
-	throw genex!RuntimeException(_e.pos, sprintf!"Unknown Kind of Expression %s"(typeid(_e)));
+		},
+		(FunLiteral e)
+		{
+			return new FunValue(delegate Value(immutable LexPosition pos, string lay, Value[] args){
+				if( e.params.length != args.length )
+					throw genex!RuntimeException(e.pos, sprintf!"Argument Number Mismatch (%d required but %d given)"
+						(e.params.length, args.length));
+				Table ctxNeo = new Table(ctx, Table.Kind.NotPropagateSet);
+				foreach(i,p; e.params)
+					ctxNeo.set(p.name, lay, args[i]);
+				return eval(e.funbody, ctxNeo, true, lay);
+			});
+		},
+		delegate Value (AST e)
+		{
+			throw genex!RuntimeException(e.pos, sprintf!"Unknown Kind of Expression %s"(typeid(e)));
+		}
+	);
 }
 
 unittest
 {