← All showcases

json — a JSON parser & formatter

Source on GitHub ↗

Reads JSON into native Raku values with a grammar, and writes them back out — pretty-printed or minified — with an optional jq-style path to pull one value. Where lisp and forth implement whole languages, this is the everyday job a grammar is for: read a data format, do something, write it back.

(There's a smaller examples/json.raku that only parses to a data structure; this one round-trips and queries.)

Run it

build/rakupp showcase/json/json.raku showcase/json/sample.json          # pretty-print
build/rakupp showcase/json/json.raku --compact showcase/json/sample.json # minify
build/rakupp showcase/json/json.raku --query='.users[0].name' showcase/json/sample.json
cat data.json | build/rakupp showcase/json/json.raku                     # from stdin
# or compile a standalone binary:
build/rakupp --exe -o json showcase/json/json.raku

Examples

Pretty-printing sorts object keys, so the output is stable (nice for diffs):

$ echo '{"b":2,"a":[1,2],"c":{"x":true}}' | build/rakupp showcase/json/json.raku
{
  "a": [
    1,
    2
  ],
  "b": 2,
  "c": {
    "x": true
  }
}

Minify and query:

$ build/rakupp showcase/json/json.raku --compact showcase/json/sample.json
{"escapes":"a \"quote\", ...","maintainer":null,"name":"rakupp", ... }

$ build/rakupp showcase/json/json.raku --query='.users[0].name' showcase/json/sample.json
"Ada"

What it handles

• Objects, arrays, strings, numbers, true/false/null — objects become Hashes, arrays Arrays, scalars Str/Int/Num/Bool, null a sentinel.
• String escapes both ways: \" \\ \/ \n \t \r \b \f and \uXXXX (so "café ❤" decodes to café ❤ and re-encodes cleanly).
• Numbers: integers stay Int, decimals and exponents become Num.
• --query: a path of .key and [index] steps, e.g. .users[0].name.

How it works

• Grammar — a proto rule value {*} with one :sym<…> alternative per JSON value kind (object / array / string / number / true / false / null); the actions class reduces each match into a native Raku value as it parses.
• Escape decoding is a single left-to-right pass (sequential global substs would corrupt an escaped backslash followed by a letter).
• Serializer — a recursive to-json walks the value: objects and arrays indent by depth (or collapse under --compact), strings are re-escaped, and every scalar is formatted for valid JSON.