Loki

Loki is a C++20 library for syntactic and semantic parsing and translation of PDDL files. Loki separates the parsing and translation of domain and problem files, allowing users to effectively work with collection of problems.

The parser is based on the canonical parser structure proposed in the Boost Spirit X3 library.

The translator is based on the method presented in section four of the paper "Concise finite-domain representations for PDDL planning tasks by Malte Helmert (AIJ 2009)".

Supported PDDL Requirements

• :strips
• :typing
• :negative-preconditions
• :disjunctive-preconditions
• :equality
• :existential-preconditions
• :universal-preconditions
• :quantified-preconditions
• :conditional-effects
• :numeric-fluents
• :adl
• :derived-predicates
• :action-costs
• :non-deterministic (unsupported in the translator)
• :probabilistic-effects (unsupported in the translator)

Dependencies

Loki depends on a fraction of Boost's header-only libraries (Fusion, Spirit x3, Container), its performance benchmarking framework depends on GoogleBenchmark, and its testing framework depends on GoogleTest.

Loki consumes native dependencies from Python packages:

• pyyggdrasil >= 0.0.9 for shared third-party native dependencies.

The shared workspace layout and general Python/CMake integration pattern are documented in the Planning and Learning build instructions.

For offline/local development, install pyyggdrasil from the sibling source checkout instead:

cd ../yggdrasil
uv pip install --python ../loki/.venv/bin/python .

Build C++

Install Loki's native dependency providers into the active Python environment, then configure CMake with their native prefixes:

python -m pip install 'pyyggdrasil>=0.0.9'

cmake -S . -B build \
  -DCMAKE_PREFIX_PATH="$(python -c 'import pyyggdrasil; print(pyyggdrasil.native_prefix())')"

cmake --build build -j4

CMake options:

Option	Default	Description
LOKI_BUILD_TESTS	OFF	Build Loki tests.
LOKI_BUILD_EXECUTABLES	OFF	Build Loki executables.
LOKI_BUILD_PROFILING	OFF	Build Loki profiling targets.
LOKI_BUILD_PYPDDL	OFF	Build Loki for the pypddl Python wheel.

Run tests from a build configured with -DLOKI_BUILD_TESTS=ON:

ctest --test-dir build --output-on-failure

Install Loki from a configured build directory with:

cmake --install build --prefix=<path/to/installation-directory>

Build Python

python -m pip install .

CMake Integration

The Python package pypddl installs Loki's native headers, shared library, and CMake package config under pypddl.native_prefix(). It depends on pyyggdrasil>=0.0.9 for third-party native dependencies:

import pypddl
import pyyggdrasil

print(pypddl.native_prefix())
print(pyyggdrasil.native_prefix())

Downstream CMake projects should include the native prefixes of pypddl and its native package dependencies in CMAKE_PREFIX_PATH:

cmake -S . -B build \
  -DCMAKE_PREFIX_PATH="$(python -c 'import os, pyyggdrasil, pypddl; print(os.pathsep.join(map(str, [pyyggdrasil.native_prefix(), pypddl.native_prefix()])))')"

Loki exports the loki::parsers target.

Running the Executables

The executable illustrates how to use Loki. It is disabled by default and can be enabled with -DLOKI_BUILD_EXECUTABLES=ON.

./build/exe/loki data/gripper/domain.pddl data/gripper/p-2-0.pddl

Citing Loki

If you use Loki in your research, please cite it as follows:

@misc{drexler-zenodo2026,
  author =       "Dominik Drexler",
  title =        "{Loki}: A {PDDL} Parser and Normalizer",
  publisher =    "Zenodo",
  year =         "2026",
  doi =          "10.5281/zenodo.20081136",
  url =          "https://doi.org/10.5281/zenodo.20081136",
}

Acknowledgements

This work was partially supported by the Wallenberg AI, Autonomous Systems and Software Program (WASP) funded by the Knut and Alice Wallenberg Foundation.