peel

Dynamic parsing within trees 🌲 🌳 🌴

Target of this library is to provide a flexible approach in parsing data. This will mainly be done within arena based parser trees which can be modified during runtime. Every parser is using the nom framework for the actual parsing work. A complete source code example can be found within the src/example directory of the crate.

Architecture and usage

Every Peel instance can be seen as a parsing graph structure which has different states and transitions. In the example within the crate the structure looks like this:

Example parser diagram

Independently of what these parser do, the creation of this structure is done within the peel_example function:

``rust /// Return aPeel` instance for the example parsers pub fn peel_example() -> Peel { // Create a tree let mut p = Peel::new();

// Create and link the parsers
let parser_1 = p.new_parser(Parser1);

// Append Parser2 to Parser1
p.link_new_parser(parser_1, Parser2);

// Append Parser3 to Parser1
let parser_3 = p.link_new_parser(parser_1, Parser3);

// Append Parser4 to Parser3
p.link_new_parser(parser_3, Parser4);

p

} ```

The first created parser will automatically be the root parser and the entry point for the tree traversal. Every parser returns an actual result, which will be pushed into a vector. This means for our example that the result is an enum of different types (in this case only bool values for simplicity):

```rust /// Return values of the parsers pub enum ParserResult { /// The result of the first example parser Result1(bool),

/// The result of the second example parser
Result2(bool),

/// The result of the third example parser
Result3(bool),

/// The result of the fourth example parser
Result4(bool),

} ```

Beside this result a ParserState is needed to make a decision about the next parsing step:

```rust /// Possible actions to be done if a parser succeed pub enum ParserState { /// Default behavior, continue traversing the Parser tree with the next child ContinueWithFirstChild,

/// Continue traversing with the next sibling of the current parser
ContinueWithNextSibling,

/// Continue traversing with the current parser
ContinueWithCurrent,

/// Immediately stop the parsing
Stop,

} ```

So in our example image above we have the following available stages: - Parser 1: - Succeed: ContinueWithFirstChild - Failed: Return an error - Parser 2: - Succeed/Failed: ContinueWithNextSibling - Parser 3: - Succeed: - Internal pattern matched: ContinueWithCurrent - Internal Pattern not matched: ContinueWithFirstChild - Failed: Overall parsing done, because no siblings left - Parser 4: - Failed/Succeed: Overall parsing done, because no child parsers left

This means that the traversal method of Peel will try to find the deepest possible path within the tree structure, whereas the parsers itself can tell Peel how to continue beside the default ContinueWithFirstChild behavior.

After the creation of the structure the traversal can begin:

```rust let mut peel = peelexample(); peel.setlog_level(LogLevel::Trace); let result = peel.traverse(b"1234", vec![]).unwrap();

assert_eq!(result.len(), 5); println!("{:?}", result); ```

With the help of the log crate it will output: [peel] [INFO ] Log level set to: Trace [peel] [DEBUG] Parser 1 parsing succeed, left input length: 3 [peel] [DEBUG] Continue traversal to first child of the parser [peel] [DEBUG] Parser 2 parsing succeed, left input length: 2 [peel] [DEBUG] Continue traversal to next sibling of the parser [peel] [DEBUG] Parser 3 parsing succeed, left input length: 1 [peel] [DEBUG] Trying the current parser again [peel] [DEBUG] Parser 3 parsing succeed, left input length: 1 [peel] [DEBUG] Continue traversal to first child of the parser [peel] [DEBUG] Parser 4 parsing succeed, left input length: 0 [Result1(true), Result2(true), Result3(true), Result3(true), Result4(true)]

A minimal parser has to implement the Parser trait which could look like this: ```rust pub struct Parser1;

impl Parser for Parser1 { /// The result of the parser type Result = ParserResult;

/// The variant of the parser
type Variant = ParserVariant;

/// The actual parsing entry point
fn parse<'a>(&self,
             input: &'a [u8],                    // The input for the parser
             node: Option<&ExampleNode>,         // The current node within the tree
             arena: Option<&ExampleArena>,       // Access to possible other nodes via the arena
             result: Option<&Vec<Self::Result>>) // The current parsing result
             -> IResult<&'a [u8], (Self::Result, ParserState)> {
    do_parse!(input,
        tag!("1") >>
        (ParserResult::Result1(true), ParserState::ContinueWithFirstChild)
    )
}

// Returns the actual parser variant
fn variant(&self) -> Self::Variant {
    ParserVariant::Variant1(self.clone())
}

} ```

For event more advanced behavior the node and arena can be used to find out where the parser is located within the current structure. Access to the current parsing result is possible as well.

Current limitations

Result values referencing to the actual input is currently not implemented
Going back during traversal is not possible

Contributing

You want to contribute to this project? Wow, thanks! So please just fork it and send me a pull request.