FSONL

Function-Styled Object Notation Lines

A line-based serialization format where each record's type is immediately visible at the start of the line.

@schema rm(target: string, --force?: bool = false)
rm("tmp.log", force=true)
rm("/var/cache", force=false)
log("info", "server started")

Install

pip install fsonl

Quick Start

Parse with inline schema

import fsonl

text = """
@schema rm(target: string, --force?: bool = false)
rm("tmp.log")
rm("/var/cache", force=true)
"""

result = fsonl.loads(text)
for entry in result.entries:
    print(entry)
# {'type': 'rm', 'target': 'tmp.log', 'force': False}
# {'type': 'rm', 'target': '/var/cache', 'force': True}

Parse with code schema

import fsonl

schema = fsonl.Schema.from_string(
    "@schema log(level: string, msg: string)"
)

result = fsonl.loads('log("info", "started")\n', schema=schema)
print(result.entries[0])
# {'type': 'log', 'level': 'info', 'msg': 'started'}

Define schema from Python functions (3.10+)

import fsonl

schema = fsonl.Schema()

@schema.define
def rm(target: str, *, force: bool = False): ...

result = fsonl.loads('rm("tmp.log", force=true)\n', schema=schema)
print(result.entries[0])
# {'type': 'rm', 'target': 'tmp.log', 'force': True}

Serialize

import fsonl

schema = fsonl.Schema.from_string(
    "@schema log(level: string, msg: string)"
)

print(fsonl.dumps([{"type": "log", "level": "info", "msg": "hello"}], schema=schema))
# @schema log(level: string, msg: string)
# log("info", "hello")

Writer (append-friendly)

import fsonl

schema = fsonl.Schema.from_string("@schema event(level: string, msg: string)")

# First run: creates file with schema header + entries
with fsonl.Writer("events.fsonl", schema=schema) as w:
    w.write({"type": "event", "level": "info", "msg": "started"})

# Later: appends entries without duplicating the header
with fsonl.Writer("events.fsonl", schema=schema) as w:
    w.write({"type": "event", "level": "warn", "msg": "timeout"})

Stream from file

import fsonl

with open("events.fsonl", newline="") as f:
    for entry in fsonl.iter_entries(f):
        print(entry)

Raw mode (no schema binding)

import fsonl

result = fsonl.loads_raw('x(1, "hello", flag=true)\n')
entry = result[0]
print(entry["type"])        # 'x'
print(entry["positional"])  # [1, 'hello']
print(entry["named"])       # {'flag': True}

API

Parsing

Function	Description
loads(text, *, schema, ignore_inline_schema, extra_fields)	Parse FSONL text with schema binding
load(fp, **kwargs)	Parse from file object with schema binding
loads_raw(text)	Parse FSONL text without binding (Stage 1 only)
load_raw(fp)	Parse from file object without binding (Stage 1 only)
iter_entries(source, *, schema, ignore_inline_schema, extra_fields)	Lazy iterator over bound entries
iter_raw(source)	Lazy iterator over raw entries
bind(entry, schema, *, line, extra_fields)	Bind a single raw dict to a Schema

Serialization

Function	Description
dumps(entries, *, schema, allow_extra, exclude_schema)	Serialize to FSONL text
dump(entries, fp, *, schema, allow_extra, exclude_schema)	Serialize to file object
Writer(path, *, schema)	Append-friendly file writer (auto header management)

Schema

Method	Description
Schema.from_string(text)	Create from @schema lines
Schema.from_fsonl(text)	Extract @schema from FSONL text (non-schema lines ignored)
Schema.from_file(path)	Load @schema from a .fsonl file
@schema.define	Decorator: define schema from function signature (Python 3.10+)
schema.add(text)	Add more @schema definitions
schema.get(type_name)	Look up a type definition
schema.has(type_name)	Check if a type is defined
schema.type_names()	List all defined type names

Options

Option	Default	Description
ignore_inline_schema	False	Skip @schema directives in the content
allow_extra	False	(dumps only) Ignore extra keys not in schema
exclude_schema	False	(dumps only) Omit @schema lines from output
extra_fields	ExtraFieldPolicy.ERROR	Policy for undeclared named arguments

Errors

All errors include line numbers: str(error) produces "line 42: message".

Exception	Kind	Stage
ParseError	syntax_error	Stage 1 (syntax parse)
SchemaError	schema_error	Schema definition / cross-validation
BindError	bind_error	Stage 2 (data vs schema mismatch)

All inherit from FsonlError, which inherits from Exception.

Schema Types

string          -- JSON string
number          -- JSON number (int or float)
bool            -- true / false
null            -- null
any             -- any JSON value
string[]        -- array of strings
(string | number)[]  -- array of union
{ cmd: string, id?: number }  -- fixed-shape object
string | null   -- nullable string

CLI

# Parse with schema binding (default)
echo '@schema x(a: number)\nx(1)' | python -m fsonl parse

# Raw parse (no binding)
echo 'x(1)' | python -m fsonl parse --raw

# Extract @schema directives only
echo '@schema x(a: number)\nx(1)' | python -m fsonl parse --schema

Format Overview

• One entry per line: type(args)
• Values are JSON literals: strings, numbers, booleans, null, arrays, objects
• Positional args come before named args: log("info", tag="v2")
• Comments: // ... (outside argument lists only)
• File extension: .fsonl, MIME: text/fsonl, encoding: UTF-8

Specification

• SPEC.ko.md -- Language specification (Korean)
• GRAMMAR.ko.peg -- PEG formal grammar (Korean)

License

MIT