A small MoonBit JSONPath parser and evaluator for RFC 9535 core query patterns.

jsonpath

json

parser

rfc9535

moonbit

moon add ppyj663/moon-jsonpath@0.2.0

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Author

ppyj663

Version

0.2.0

License

Apache-2.0

Repository

github.com/ppyj663/moon-jsonpath

Last updated

last month

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README

#MoonBit JSONPath

MoonBit JSONPath is a small JSONPath parser and evaluator written in MoonBit. It targets the core query patterns described by RFC 9535 and integrates directly with moonbitlang/core/json.

The project is maintained for the MoonBit Open Source Ecosystem Competition 2026. The public repositories are:

GitHub: https://github.com/ppyj663/moon-jsonpath
GitLink: https://gitlink.org.cn/ppyj663/moon-jsonpath

#Features

Parses JSONPath strings into a compact MoonBit AST.
Evaluates queries against Json values from moonbitlang/core/json.
Supports root, child, bracket child, recursive descendant, wildcard, array index, negative index, slicing, union, and filter selectors.
Supports escape sequences and unicode escapes in bracket name selectors.
Ships focused unit tests and a runnable CLI example.
Uses the Apache-2.0 license.

#Installation

The module name is:

ppyj663/moon-jsonpath

Install the published Mooncakes package from your project root:

moon add ppyj663/moon-jsonpath

Then import it from a package:

import {
  "ppyj663/moon-jsonpath" @jsonpath,
}

#Quick Start

let json = @json.parse(
  #|{
  #|  "store": {
  #|    "book": [
  #|      { "title": "Sayings of the Century", "price": 8.95 },
  #|      { "title": "Sword of Honour", "price": 12.99 }
  #|    ]
  #|  }
  #|}
)

match @jsonpath.query(json, "$.store.book[0].title") {
  Ok(values) => println(values[0].stringify())
  Err(message) => println("query failed: \{message}")
}

Run the included example:

moon run cmd/main

Expected output:

query: $.store.book[0].title
matches: 1
"Sayings of the Century"

#Supported Syntax

Syntax	Meaning	Example
$	Root node	$
.<name>	Object child selector	$.store.bicycle
['<name>'] or ["<name>"]	Bracket child selector (supports string escapes)	$['store']['book']
.. <name> without the space	Recursive descendant selector	$..price
.* / [*]	Object or array wildcard	$.store.book[*]
[<number>]	Array index (supports negative indices)	$.store.book[-1]
[<start>:<end>:<step>]	Array slicing (supports positive and negative steps)	$.store.book[1:3]
[<sel1>, <sel2>]	Union selector	$.store.book[0, 2]
[?(<expr>)]	Filter selector (supports comparisons, logical AND/OR/NOT)	$.store.book[?(@.price < 10)]

Not yet supported: function extensions.

#API

pub fn parse(input : String) -> Result[JSONPath, String]
pub fn evaluate(json : Json, path : JSONPath) -> Array[Json]
pub fn query(json : Json, path_str : String) -> Result[Array[Json], String]

query is the usual entry point. It returns Err(message) for unsupported or malformed JSONPath syntax and returns an empty array when a valid query simply finds no matching node.

#Development

moon update
moon fmt --check
moon check --deny-warn
moon test
moon run cmd/main

The repository also contains a GitHub Actions workflow that runs the same verification path on every push and pull request.

Mooncakes package ppyj663/moon-jsonpath version 0.2.0 is published. The release was validated with moon package, moon publish --dry-run, and moon publish.

#Competition Closeout

Closeout evidence is tracked in docs/competition/acceptance.md. The one-page project report is report.pdf.

#License

This project is released under the Apache License 2.0.

#
JSONPath

source

type JSONPath = Array[PathSegment]

An Array is a collection of values that supports random access and can grow in size.

#
ParseError

source

type ParseError derive(@moonbitlang/core/debug.Debug
Debug)

#
FilterExpr

source

pub enum FilterExpr {
  Exists(PathExpr)
  Eq(FilterVal, FilterVal)
  Ne(FilterVal, FilterVal)
  Lt(FilterVal, FilterVal)
  Le(FilterVal, FilterVal)
  Gt(FilterVal, FilterVal)
  Ge(FilterVal, FilterVal)
  And(FilterExpr, FilterExpr)
  Or(FilterExpr, FilterExpr)
  Not(FilterExpr)
  Value(FilterVal)
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

#
FilterVal

source

pub enum FilterVal {
  Path(PathExpr)
  Literal(Json)
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

#
PathExpr

source

pub enum PathExpr {
  Relative(Array[PathSegment])
  Absolute(Array[PathSegment])
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

#
PathSegment

source

pub enum PathSegment {
  Root
  Child(String)
  Descendant(String)
  Wildcard
  DescendantWildcard
  Bracket(Array[Selector])
  DescendantBracket(Array[Selector])
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

#
Selector

source

pub enum Selector {
  Name(String)
  Index(Int)
  Wildcard
  Slice(Int?, Int?, Int?)
  Filter(FilterExpr)
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

#
evaluate

source

fn evaluate(json : Json, path : Array[PathSegment]) -> Array[Json]

#
parse

source

fn parse(input : String) -> Result[Array[PathSegment], String]

#
query

source

fn query(json : Json, path_str : String) -> Result[Array[Json], String]

Source Files

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moon-jsonpath

#MoonBit JSONPath

#Features

#Installation

#Quick Start

#Supported Syntax

#API

#Development

#Competition Closeout

#License

#JSONPath

#ParseError

#FilterExpr

#FilterVal

#PathExpr

#PathSegment

#Selector

#evaluate

#parse

#query

Source Files

#
JSONPath

#
ParseError

#
FilterExpr

#
FilterVal

#
PathExpr

#
PathSegment

#
Selector

#
evaluate

#
parse

#
query