syncraft 0.8.1

Bidirectional parser/generator combinators for Python with recursive regex-style grammar fragments.

Syncraft

Syncraft is a bidirectional parser/generator combinator library for Python.

Define a grammar once.

• Parse text into structured data.
• Generate text back from that same structure.
• Keep both directions consistent by construction.

It provides Packrat-style performance and supports direct left recursion.

Status

Pre-1.0 (Release Candidate) — APIs may change before 1.0.

Philosophy

Syncraft is built on three beliefs.

1. One source of truth

Parsing, generation, and validation should come from a single grammar model.

If you define a language once, you should not need:

• a parser,
• a transformer,
• a serializer,
• and a separate validator.

A single definition should unify all directions.
Roundtripping should fall out of the model, not be manually maintained.

---

2. Shape matters

A parsing library should deliver values in the shape the user actually wants.

Not:

• raw parse trees,
• intermediate ASTs that require post-processing,
• or mandatory visitor passes.

Transformation is not an afterthought — it is part of the grammar definition.
Grammars describe both structure and meaning.

---

3. CFGs should feel like regex

Context-free grammars shouldn’t feel heavier than regular expressions.

If you can sketch a regex, you should be able to sketch a recursive grammar just as quickly.

Syntax.rp exists to make recursive grammar fragments feel as lightweight and composable as regex — without giving up context-free power.

Core capabilities

Syncraft provides two core capabilities:

1. Bidirectional grammar + transformation

   • Define grammar and data transformation together.
   • Parse text into structured values.
   • Generate text back from structured values from the same grammar model.

2. Regex++

   • Embed named recursive grammar fragments inside a regex-like syntax, effectively turning regular expressions into composable context-free grammar fragments.
   • Compose those fragments with grammar combinators.

Scope

Syncraft is a strong fit when you need:

If you need...	Syncraft provides...
A small DSL or config language	Regex++ grammar sketching + structured AST generation
Strict roundtrip guarantees	One grammar model for parse() and generate()
Evolving grammars during prototyping	Combinator + fragment composition model

Syncraft is also good for parsing-only/extraction workflows. Generation and roundtrip constraints become additional advantages when you need them, not requirements for adoption.

Quick example: regex++ parsing for a common mini-language

This style is useful when you want to sketch and evolve a small language quickly. Here, we parse a recursive expression grammar:

• expr := number | '(' expr op expr ')'
• op := + | - | * | /

from syncraft.syntax import Syntax as S

num = S.rp(r"[0-9]+").bimap(int, str)
op = S.rp(r"[+\-*/]")

expr = S.lazy(lambda: S.rp(
	r"(?&num)|(\((?&expr)\s*(?&op)\s*(?&expr)\))",
	num=num, op=op, expr=expr
))

print(expr.parse("7"))
print(expr.parse("(2+3)"))
print(expr.parse("((1+2)*3)"))

Expected output:

7
(2, '+', 3)
((1, '+', 2), '*', 3)

Adding structured data transformations

Transform parsed tuples into dataclasses and generate text back from those dataclasses: The case() combinator defines bidirectional structural mappings. Each case provides a pair of functions: one to extract values from parsed tuples, and one to construct domain objects."

from dataclasses import dataclass

@dataclass
class Number:
    value: int

@dataclass
class BinaryOp:
    left: Number | BinaryOp
    op: str
    right: Number | BinaryOp


expr_ast = expr.case(
    (lambda env: env.number, lambda env: Number(env.number)),
    (lambda env: (env.left, env.op, env.right), lambda env: BinaryOp(env.left, env.op, env.right))
)

# Parse into dataclasses
result = expr_ast.parse("((1+2)*3)")
print(result)
# Output: BinaryOp(left=BinaryOp(left=Number(value=1), op='+', right=Number(value=2)), op='*', right=Number(value=3))

# Generate text back from dataclasses
text = expr_ast.generate(result)
print(text)
# Output: ((1+2)*3)

Installation

Python 3.10+ is required.

With pip

pip install syncraft

With uv

uv add syncraft

Documentation

Full documentation is available at: https://afmkt.github.io/syncraft/