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java-petitparser

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Dynamic parser combinators in Java.

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PetitParser for Java
====================

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Grammars for programming languages are traditionally specified statically. They
are hard to compose and reuse due to ambiguities that inevitably arise.
PetitParser combines ideas
from [scannnerless parsing](https://en.wikipedia.org/wiki/Scannerless_parsing)
, [parser combinators](https://en.wikipedia.org/wiki/Parser_combinator)
, [parsing expression grammars](https://en.wikipedia.org/wiki/Parsing_expression_grammar) (
PEG) and packrat parsers to model grammars and parsers as objects that can be
reconfigured dynamically.

This library is open source, stable and well tested. Development happens
on [GitHub](https://github.com/petitparser/java-petitparser). Feel free to
report issues or create a pull-request there. General questions are best asked
on [StackOverflow](http://stackoverflow.com/questions/tagged/petitparser+java).

Installation
------------

To include the latest release in your Java project follow the instructions
below.

### Maven JitPack

JitPack requires you to first append the repository to your `pom.xml` file:

```xml
<repository>
  <id>jitpack.io</id>
  <url>https://jitpack.io</url>
</repository>
```

To depend on the core library, add the following dependency:

```xml
<dependency>
  <groupId>com.github.petitparser.java-petitparser</groupId>
  <artifactId>petitparser-core</artifactId>
  <version>2.4.2</version>
</dependency>
```

To also include the example grammars, use the following dependency:

```xml
<dependency>
  <groupId>com.github.petitparser</groupId>
  <artifactId>java-petitparser</artifactId>
  <version>2.4.2</version>
</dependency>
```

Instructions for alternative build systems such as Gradle, SBT, or Leiningen you
can find on [JitPack](https://jitpack.io/#petitparser/java-petitparser).

### Bazel

Alternatively, if you'd like the latest code clone the repository:

```bash
git clone https://github.com/petitparser/java-petitparser.git
cd java-petitparser
git checkout master
```

Then build with [Bazel](http://bazel.io/):

```bash
bazel build ...:all
bazel test ...:all
```

### Maven Central (deprecated)

> [!WARNING]
> Version 2.4.0 is the last version of this package on Maven Central.
> Maintaining the Maven Central package is too much of a hassle, please
> migrate to one of the other options described above.

To depend on the core library, add the following dependency to your `pom.xml`
file:

```xml
<dependency>
  <groupId>com.github.petitparser</groupId>
  <artifactId>petitparser-core</artifactId>
  <version>2.4.0</version>
</dependency>
```

To also include the example grammars, use the following dependency:

```xml
<dependency>
  <groupId>com.github.petitparser</groupId>
  <artifactId>petitparser</artifactId>
  <version>2.4.0</version>
</dependency>
```

Instructions for alternative build systems you can find
on [Maven Central](https://search.maven.org/artifact/com.github.petitparser/petitparser-core)

Tutorial
--------

### Writing a Simple Grammar

Writing grammars with PetitParser is simple as writing Java code. For example,
to write a grammar that can parse identifiers that start with a letter followed
by zero or more letter or digits is defined as follows:

```java
import static org.petitparser.parser.primitive.CharacterParser.*;

class Example {
  public static void main(String[] arguments) {
    Parser id = letter().seq(letter().or(digit()).star());
    ...
  }
}
```

If you look at the object `id` in the debugger, you'll notice that the code
above builds a tree of parser objects:

- `SequenceParser`: This parser accepts a sequence of parsers.
    - `CharacterParser`: This parser accepts a single letter.
    - `PossessiveRepeatingParser`: This parser accepts zero or more times
      another parser.
        - `ChoiceParser`: This parser accepts a single word character.
            - `CharacterParser`: This parser accepts a single letter.
            - `CharacterParser`: This parser accepts a single digit.

### Parsing Some Input

To actually parse a `String` we can use the method `Parser#parse(String)`:

```java
Result id1 = id.parse("yeah");
Result id2 = id.parse("f12");
```

The method `parse` returns `Result`, which is either an instance of `Success`
or `Failure`. In both examples above we are successful and can retrieve the
parse result using `Success#get()`:

```java
System.out.println(id1.get());  // ['y', ['e', 'a', 'h']]
    System.out.

println(id2.get());  // ['f', ['1', '2']]
```

While it seems odd to get these nested arrays with characters as a return value,
this is the default decomposition of the input into a parse tree. We'll see in a
while how that can be customized.

If we try to parse something invalid we get an instance of `Failure` as an
answer and we can retrieve a descriptive error message
using `Failure#getMessage()`:

```java
Result id3 = id.parse('123');
System.out.

println(id3.getMessage());  // "letter expected"
    System.out.

println(id3.getPosition());  // 0
```

Trying to retrieve the parse result by calling `Failure#get()` would throw the
exception `ParseError`. `Result#isSuccess()` and `Result#isFailure()` can be
used to decide if the parse was successful.

If you are only interested if a given string matches or not you can use the
helper method `Parser#accept(String)`:

```java
System.out.println(id.accept("foo"));  // true
    System.out.

println(id.accept("123"));  // false
```

### Different Kinds of Parsers

PetitParser provide a large set of ready-made parser that you can compose to
consume and transform arbitrarily complex languages. The terminal parsers are
the most simple ones. We've already seen a few of those:

- `CharacterParser.of('a')` parses the character _a_.
- `StringParser.of("abc")` parses the string _abc_.
- `CharacterParser.any()` parses any character.
- `CharacterParser.digit()` parses any digit from _0_ to _9_.
- `CharacterParser.letter()` parses any letter from _a_ to _z_ and _A_ to _Z_.
- `CharacterParser.word()` parses any letter or digit.

Many other parsers are available in `CharacterParser` and `StringParser`.

So instead of using the letter and digit predicate, we could have written our
identifier parser like this:

```java
Parser id = letter().seq(word().star());
```

The next set of parsers are used to combine other parsers together:

- `p1.seq(p2)` parses `p1` followed by `p2` (sequence).
- `p1.or(p2)` parses `p1`, if that doesn't work parses `p2` (ordered choice).
- `p.star()` parses `p` zero or more times.
- `p.plus()` parses `p` one or more times.
- `p.optional()` parses `p`, if possible.
- `p.and()` parses `p`, but does not consume its input.
- `p.not()` parses `p` and succeed when p fails, but does not consume its input.
- `p.end()` parses `p` and succeed at the end of the input.

To attach an action or transformation to a parser we can use the following
methods:

- `p.map(value -> ...)` performs the transformation given the function.
- `p.pick(n)` returns the `n`-th element of the list `p` returns.
- `p.flatten()` creates a string from the result of `p`.
- `p.token()` creates a token from the result of `p`.
- `p.trim()` trims whitespaces before and after `p`.

To return a string of the parsed identifier, we can modify our parser like this:

```java
Parser id = letter().seq(word().star()).flatten();
```

To conveniently find all matches in a given input string you can
use `Parser#matchesSkipping(String)`:

```java
List<Object> matches = id.matchesSkipping("foo 123 bar4");
    System.out.

println(matches);  // ["foo", "bar4"]
```

These are the basic elements to build parsers. There are a few more well
documented and tested factory methods in the `Parser` class. If you want, browse
their documentation and tests.

### Writing a More Complicated Grammar

Now we are able to write a more complicated grammar for evaluating simple
arithmetic expressions. Within a file we start with the grammar for a number (
actually an integer):

```java
Parser number = digit().plus().flatten().trim().map((String value) -> Integer.parseInt(value));
```

Then we define the productions for addition and multiplication in order of
precedence. Note that we instantiate the productions with undefined parsers
upfront, because they recursively refer to each other. Later on we can resolve
this recursion by setting their reference:

```java
SettableParser term = SettableParser.undefined();
SettableParser prod = SettableParser.undefined();
SettableParser prim = SettableParser.undefined();

term.

set(prod.seq(of('+').

trim()).

seq(term).

map((List<Integer> values) ->{
    return values.

get(0) +values.

get(2);
}).

or(prod));
    prod.

set(prim.seq(of('*').

trim()).

seq(prod).

map((List<Integer> values) ->{
    return values.

get(0) *values.

get(2);
}).

or(prim));
    prim.

set((of('(').

trim().

seq(term).

seq(of(')').

trim())).

map((List<Integer> values) ->{
    return values.

get(1);
}).

or(number));
```

To make sure that our parser consumes all input we wrap it with the `end()`
parser into the start production:

```java
Parser start = term.end();
```

That's it, now we can test our parser and evaluator:

```java
System.out.println(start.parse("1 + 2 * 3").

get());  // 7
    System.out.

println(start.parse("(1 + 2) * 3").

get());  // 9
```

As an exercise we could extend the parser to also accept negative numbers and
floating point numbers, not only integers. Furthermore it would be useful to
support subtraction and division as well. All these features
can be added with a few lines of PetitParser code.

### Using the Expression Builder

Writing such expression parsers is pretty common and can be quite tricky to get
right. To simplify things, PetitParser comes with a builder that can help you to
define such grammars easily. It supports the definition of operator precedence;
and prefix, postfix, left- and right-associative operators.

The following code creates the empty expression builder:

```java
ExpressionBuilder builder = new ExpressionBuilder();
```

Then we define the operator-groups in descending precedence. The highest
precedence are the literal numbers themselves. This time we accept floating
point numbers, not just integers. In the same group we add support for
parenthesis:

```java
builder.group()
  .

primitive(digit().

plus().

seq(of('.')
      .

seq(digit().

plus()).

optional())
    .

flatten().

trim().

map(Double::parseDouble))
    .

wrapper(of('(').

trim(),of(')').

trim(),
      (
List<Double> values)->values.

get(1));
```

Then come the normal arithmetic operators. Note, that the action blocks receive
both, the terms and the parsed operator in the order they appear in the parsed
input:

```java
// negation is a prefix operator
builder.group()
  .

prefix(of('-').

trim(), (
List<Double> values)->-values.

get(1));

// power is right-associative
    builder.

group()
  .

right(of('^').

trim(), (
List<Double> values)->Math.

pow(values.get(0),values.

get(2)));

// multiplication and addition are left-associative
    builder.

group()
  .

left(of('*').

trim(), (
List<Double> values)->values.

get(0) *values.

get(2))
    .

left(of('/').

trim(), (
List<Double> values)->values.

get(0) /values.

get(2));
    builder.

group()
  .

left(of('+').

trim(), (
List<Double> values)->values.

get(0) +values.

get(2))
    .

left(of('-').

trim(), (
List<Double> values)->values.

get(0) -values.

get(2));
```

Finally we can build the parser:

```java
Parser parser = builder.build().end();
```

After executing the above code we get an efficient parser that correctly
evaluates expressions like:

```java
parser.parse("-8");      // -8
parser.

parse("1+2*3");   // 7
parser.

parse("1*2+3");   // 5
parser.

parse("8/4/2");   // 1
parser.

parse("2^2^3");   // 256
```

You can find this example as test case
here: [ExamplesTest.java](petitparser-core/src/test/java/org/petitparser/ExamplesTest.java)

Misc
----

### Examples

The package comes with a large collections of grammars and language experiments
ready to explore:

- `petitparser-json` contains a complete JSON grammar and parser.
- `petitparser-xml` contains a complete XML grammar and parser.
- `petitparser-smalltalk` contains a complete Smalltalk parser.

### History

PetitParser was originally implemented
in [Smalltalk](http://scg.unibe.ch/research/helvetia/petitparser). Later on, as
a mean to learn these languages, I reimplemented PetitParser
in [Java](https://github.com/petitparser/java-petitparser)
and [Dart](https://github.com/petitparser/dart-petitparser). The implementations
are very similar in their API and the supported features. If possible, the
implementations adopt best practises of the target language.

### Implementations

- [Dart](https://github.com/petitparser/dart-petitparser)
- [Java](https://github.com/petitparser/java-petitparser)
- [PHP](https://github.com/mindplay-dk/petitparserphp)
- [Smalltalk](http://scg.unibe.ch/research/helvetia/petitparser)
- [Swift](https://github.com/philipparndt/swift-petitparser)
- [TypeScript](https://github.com/mindplay-dk/petitparser-ts)

### License

The MIT License,
see [LICENSE](https://raw.githubusercontent.com/petitparser/java-petitparser/master/LICENSE)
.

java-petitparser

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