nappe

A syntax-guided test case reducer implementing the Perses algorithm (Sun et al., ICSE 2018).

Reference implementation: nnunley/bonsai (Rust).

What It Does

Given a failing test case and a program file that triggers a bug, nappe reduces the file to the smallest possible program that still reproduces the failure — while guaranteeing syntactic validity at every step.

It also provides a check command for static analysis of source files, detecting reducible patterns like dead code, unused assignments, and style issues.

How It Works

1. Parse the input file into a concrete syntax tree using tree-sitter
2. Systematically try to remove or simplify subtrees via a priority queue (largest first)
3. Each candidate reduction is validated by reparsing — only syntactically valid reductions proceed
4. Test each valid candidate against an "interestingness test" (any shell command that exits 0 when the bug is still present)
5. Repeat until no further reductions are possible

Transforms

• Delete — remove a node entirely
• Unwrap — replace a node with one of its type-compatible children
• Unify identifiers — rename bindings to canonical short forms (planned: requires scope data)
• Dead definition removal — delete unreferenced definitions (planned: requires scope data)

Key Properties

• All intermediate results are syntactically valid
• Largest subtrees tried first (maximum reduction per test)
• Test results cached (duplicate calls avoided)
• Handles inputs with pre-existing parse errors

Supported Languages

Python, JavaScript, TypeScript, Rust, Go, C, C++ (via tree-sitter grammars).

Implementation

The project has two implementations:

• Rust (primary) — native implementation in src/, built with cargo. This is the default and recommended implementation.
• Python (reference) — pure Python implementation in src/nappe/, kept for comparison and as a reference for the algorithm. Accessible via the --legacy flag.

Why Rust?

The Rust implementation provides significantly better performance for large files and complex reduction scenarios, while maintaining full feature parity with the Python reference.

Installation

Rust (recommended)

cargo build --release
cp target/release/nappe ~/.local/bin/

Python (reference)

uv sync

Usage

nappe reduce — Syntax-guided reduction (default)

# Reduce using a pytest interestingness test (recommended)
nappe reduce --test test_interesting.py::test_still_fails input.py

# Reduce using a shell command
nappe reduce --test-cmd "grep -q 'error'" input.py

# Auto-reduce to smallest valid program (no test needed)
nappe reduce --auto input.py

# Limit reduction time
nappe reduce --test test_interesting.py --max-time 30m --max-tests 1000 input.py

# Use Python implementation (legacy)
nappe --legacy reduce --test test_interesting.py::test_still_fails input.py

nappe check — Static analysis and fixes

# Check files for reducible patterns
nappe check src/**/*.py

# Apply safe fixes automatically
nappe check --fix src/**/*.py

# Apply all fixes (including unsafe ones like dead code removal)
nappe check --unsafe-fixes src/**/*.py

# Output as JSON
nappe check --output-format json src/**/*.py

# Filter by rule
nappe check --select RED200,RED201 src/**/*.py

nappe shrink — Shrinkray-compatible interface

nappe shrink "./check.sh" input.py

Check Rules

Example: Reducing a Bug Trigger

Given a Python file that triggers a bug when fibonacci is called with print():

Before (input.py):

import sys

def fibonacci(n):
    if n <= 1:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)

def unused_helper(x):
    return x * 2

def also_unused():
    result = unused_helper(5)
    return result

class MyClass:
    def __init__(self):
        self.value = 42

    def method(self):
        return self.value

if __name__ == "__main__":
    for i in range(10):
        print(fibonacci(i))

Write a pytest interestingness test (test_interesting.py):

import sys

def test_still_fails():
    """Exit 0 = candidate is still interesting (bug still present)."""
    candidate = sys.argv[1]
    content = open(candidate).read()
    assert "def fibonacci" in content and "print(" in content

Run the reducer:

nappe reduce --test test_interesting.py::test_still_fails input.py

After (input.py):

def fibonacci(n):
    if n <= 1:
        return n
    return fibonacci(n - 1) + fibonacci(n - 2)

print(fibonacci(10))

Diff:

-import sys
-
 def fibonacci(n):
     if n <= 1:
         return n
     return fibonacci(n - 1) + fibonacci(n - 2)

-def unused_helper(x):
-    return x * 2
-
-def also_unused():
-    result = unused_helper(5)
-    return result
-
-class MyClass:
-    def __init__(self):
-        self.value = 42
-
-    def method(self):
-        return self.value
-
-if __name__ == "__main__":
-    for i in range(10):
-        print(fibonacci(i))
+print(fibonacci(10))

The reducer removed the unused import sys, the unused_helper and also_unused functions, the MyClass class, and simplified the if __name__ block — while preserving the core fibonacci function and the print() call that triggers the bug.

Example: Checking and Fixing Code

Input (demo.py):

import os

def greet(name):
    print(f"Hello, {name}")

def unused_helper(x):
    return x * 2

count = 0;;

class EmptyClass:
    pass

message = "hello"   

greet("world")

Run the checker:

nappe check demo.py

Output:

demo.py:14:17: RED203 Trailing whitespace
    14 | message = "hello"   
      ^^^^^^^^^^^^^^^^^ RED203

demo.py:9:10: RED202 Unnecessary semicolon
    9 | count = 0;;
     ^^^^^^^^^^^ RED202

demo.py:14:1: RED102 Unused variable assignment
    14 | message = "hello"   
      ^^^^^^^^^^^^^^^^^ RED102

demo.py:9:1: RED102 Unused variable assignment
    9 | count = 0;;
     ^^^^^^^^^^^ RED102

demo.py:6:1: RED100 Dead function (no callers)
    6 | def unused_helper(x):
     ^^^^^^^^^^^^^^^^^^^^^ RED100

demo.py:11:1: RED101 Dead class (no instantiations)
    11 | class EmptyClass:
      ^^^^^^^^^^^^^^^^^ RED101

Found 6 issues (2 safe, 4 unsafe).

Apply safe fixes:

nappe check --fix demo.py

After (demo.py):

import os

def greet(name):
    print(f"Hello, {name}")

def unused_helper(x):
    return x * 2

count = 0;

class EmptyClass:
    pass

message = "hello"

greet("world")

The --fix flag applies only safe fixes (RED200–RED204). Use --unsafe-fixes to also remove dead functions, dead classes, and unused assignments.

Development

Rust (primary)

cargo build --release      # build
cargo test                 # test
cargo clippy               # lint
cargo fmt                  # format

Python (reference)

uv run ruff check src/nappe/     # lint
uv run ruff format src/nappe/    # format
uv run ty check src/nappe/       # type check
uv run pytest tests/                    # test

License

MIT

References

• Syntax-Guided Program Reduction: A Survey and the nappe Implementation — Review paper covering the Perses algorithm, related work, and the nappe system.