diff --git a/readme.md b/readme.md
index 53ba4f1..da77cb7 100644
--- a/readme.md
+++ b/readme.md
@@ -12,32 +12,52 @@ Just like the usage of Parser Combinator in Haskell, jparser does not generate A
 </dependency>
 ```
 
-## hello world
+## hello world: Simple Calculator
 ```java
+public class Calculator {
 
     @Test
-    public void testHelloWorld() {
-        String time = "2023-05-01 12:59:59";
-        Parser timeParser = NumberParsers.anyIntStr() //year
-                .chain(() -> TextParsers.one('-').ignore()) //-
-                .chain(() -> NumberParsers.anyIntStr()) //mon
-                .chain(() -> TextParsers.one('-').ignore()) //-
-                .chain(() -> NumberParsers.anyIntStr()) //day
-                .chain(() -> TextParsers.one(' ').ignore())// ' '
-                .chain(() -> NumberParsers.anyIntStr()) //hour
-                .chain(() -> TextParsers.one(':').ignore()) // ':'
-                .chain(() -> NumberParsers.anyIntStr()) //minute
-                .chain(() -> TextParsers.one(':').ignore()) // ':'
-                .chain(() -> NumberParsers.anyIntStr()) //second
-                .chain(() -> TextParsers.eof());
-        Result result = timeParser.runParser(Buffer.builder().data(time.getBytes()).build());
-        assert result.<Integer>get(0) == 2023;
-        assert result.<Integer>get(1) == 5;
-        assert result.<Integer>get(2) == 1;
-        assert result.<Integer>get(3) == 12;
-        assert result.<Integer>get(4) == 59;
-        assert result.<Integer>get(5) == 59;
+    public void testCalc() {
+        Result result = expr().parse(Buffer.builder().data("(1+2)*3-(4*2)".getBytes()).build());
+        assert result.<Double>get(0).compareTo(1.0) == 0;
+        result = expr().parse(Buffer.builder().data("1+2*3-(4*2)".getBytes()).build());
+        assert result.<Double>get(0).compareTo(-1.0) == 0;
     }
+
+    public Parser expr() {
+        return Parser.choose(
+                () -> term().chain(TextParsers.one('+').ignore())
+                        .chain(() -> expr()).map(s -> (double)s.get(0) + (double)s.get(1)),
+                () -> term().chain(TextParsers.one('-').ignore())
+                        .chain(() -> expr()).map(s -> (double)s.get(0) - (double)s.get(1)),
+                () -> term()
+        );
+    }
+
+    public Parser term() {
+        return Parser.choose(
+                () -> factor().chain(TextParsers.one('*').trim(false).ignore())
+                        .chain(() -> term()).map(s -> (double)s.get(0) * (double)s.get(1)),
+                () -> factor().chain(TextParsers.one('/').trim(false).ignore())
+                        .chain(() -> term()).map(s -> (double)s.get(0) / (double)s.get(1)),
+                () -> factor()
+        );
+    }
+
+    public Parser factor() {
+        return Parser.choose(
+                TextParsers.one('(').ignore()
+                        .chain(() -> expr())
+                        .chain(TextParsers.one(')').ignore()),
+                number()
+        );
+    }
+
+    public Parser number() {
+        return NumberParsers.anyDoubleStr();
+    }
+}
+
 ```
 
 ## basic parsers
diff --git a/src/main/java/io/github/janlely/jparser/Parser.java b/src/main/java/io/github/janlely/jparser/Parser.java
index 51bfc39..2a60d0e 100644
--- a/src/main/java/io/github/janlely/jparser/Parser.java
+++ b/src/main/java/io/github/janlely/jparser/Parser.java
@@ -481,6 +481,31 @@ public Result parse(IBuffer buffer) {
         };
     }
 
+    /**
+     * Same as Combinator Choose
+     * @param parser A Parser generator
+     * @return A new Parser that is composed of the specific Parser generator
+     */
+    public Parser or(Parser parser) {
+        return new Parser() {
+            @Override
+            public Result parse(IBuffer buffer) {
+                Result result = Parser.this.runParser(buffer);
+                if (result.isSuccess()) {
+                    return result;
+                }
+                Result result2 = parser.runParser(buffer);
+                if (result2.isSuccess()) {
+                    return result2;
+                }
+                return Result.builder()
+                        .pos(buffer.getPos())
+                        .errorMsg("No suitable Parser to choose")
+                        .build();
+            }
+        };
+    }
+
     /**
      * make this Parser optional
      * @return A new Parser
@@ -594,6 +619,19 @@ public static Parser choose(Supplier<Parser> ...parsers) {
         return parser;
     }
 
+    /**
+     * choose a Parser from array of Parser
+     * @param parsers Parsers candidates
+     * @return A new Parser that is composed of the parsers
+     */
+    public static Parser choose(Parser ...parsers) {
+        Parser parser = Parser.broken();
+        for (Parser p : parsers) {
+            parser = parser.or(p);
+        }
+        return parser;
+    }
+
     /**
      * choose a Parser from array of Parser
      * @param parsers Parsers candidates
diff --git a/src/main/java/io/github/janlely/jparser/parsers/NumberParsers.java b/src/main/java/io/github/janlely/jparser/parsers/NumberParsers.java
index 38de65b..1be8af9 100644
--- a/src/main/java/io/github/janlely/jparser/parsers/NumberParsers.java
+++ b/src/main/java/io/github/janlely/jparser/parsers/NumberParsers.java
@@ -35,6 +35,17 @@ public static Parser anyIntStr() {
                 .map(s -> Integer.parseInt((String) s.get(0)));
     }
 
+    /**
+     * Parse any double encoded as a string.
+     * @return A new Parser
+     */
+    public static Parser anyDoubleStr() {
+        return TextParsers.one('-').optional().chain(NumberParsers.anyIntStr())
+                .chain(TextParsers.one('.').chain(NumberParsers::anyDoubleStr).optional())
+                .map(Mapper.toStr())
+                .map(s -> Double.parseDouble((String) s.get(0)));
+    }
+
     /**
      * Parse an arbitrary long integer encoded in big-endian format.
      * @return A new Parser
diff --git a/src/test/java/io/github/janlely/jparser/Calculator.java b/src/test/java/io/github/janlely/jparser/Calculator.java
new file mode 100644
index 0000000..d6bab0c
--- /dev/null
+++ b/src/test/java/io/github/janlely/jparser/Calculator.java
@@ -0,0 +1,51 @@
+package io.github.janlely.jparser;
+
+import io.github.janlely.jparser.parsers.NumberParsers;
+import io.github.janlely.jparser.parsers.TextParsers;
+import io.github.janlely.jparser.util.Buffer;
+import org.junit.Test;
+
+public class Calculator {
+
+    @Test
+    public void testCalc() {
+        Result result = expr().parse(Buffer.builder().data("(1+2)*3-(4*2)".getBytes()).build());
+        assert result.<Double>get(0).compareTo(1.0) == 0;
+        result = expr().parse(Buffer.builder().data("1+2*3-(4*2)".getBytes()).build());
+        assert result.<Double>get(0).compareTo(-1.0) == 0;
+    }
+
+    public Parser expr() {
+        return Parser.choose(
+                () -> term().chain(TextParsers.one('+').ignore())
+                        .chain(() -> expr()).map(s -> (double)s.get(0) + (double)s.get(1)),
+                () -> term().chain(TextParsers.one('-').ignore())
+                        .chain(() -> expr()).map(s -> (double)s.get(0) - (double)s.get(1)),
+                () -> term()
+        );
+    }
+
+    public Parser term() {
+        return Parser.choose(
+                () -> factor().chain(TextParsers.one('*').trim(false).ignore())
+                        .chain(() -> term()).map(s -> (double)s.get(0) * (double)s.get(1)),
+                () -> factor().chain(TextParsers.one('/').trim(false).ignore())
+                        .chain(() -> term()).map(s -> (double)s.get(0) / (double)s.get(1)),
+                () -> factor()
+        );
+    }
+
+    public Parser factor() {
+        return Parser.choose(
+                TextParsers.one('(').ignore()
+                        .chain(() -> expr())
+                        .chain(TextParsers.one(')').ignore()),
+                number()
+        );
+    }
+
+    public Parser number() {
+        return NumberParsers.anyDoubleStr();
+    }
+}
+