Loki: PDDL Parsing and Translation in C++ and Python

Loki is a C++20 library with Python bindings for 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 >= 0.0.26, < 0.1 for shared third-party native dependencies.
• pypddl-datasets >= 0.0.9, < 0.1 for the PDDL benchmark data used by the C++ test suite and the example executables (resolved from its cache at CMake configure time).

The shared workspace layout, layered install order, and the common build-from-source and CMake-integration patterns are documented in the Planning and Learning build instructions; the sections below cover loki/pypddl-specific details.

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>=0.0.26,<0.1' 'pypddl-datasets>=0.0.9,<0.1'

cmake -S . -B build

CMake discovers the installed pyyggdrasil automatically through cmake/bootstrap_pyyggdrasil.cmake (which locates the package and adds its native prefix to CMAKE_PREFIX_PATH, after which find_package(yggdrasil) provides the shared helper functions) and links against the yggdrasil::yggdrasil target. To point at a different prefix explicitly:

cmake -S . -B build \
  -DCMAKE_PREFIX_PATH="$(python -m pyyggdrasil --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.
LOKI_USE_LLD	ON	Use LLVM lld with Clang when available.
LOKI_ENABLE_LTO	ON	Enable link-time optimization for Release builds.

Single-config CMake builds default to Release. On GCC and Clang, Debug builds use -Og with debug symbols, RelWithDebInfo keeps frame pointers and disables LTO, and Release LTO uses GCC LTO or Clang ThinLTO. Editable installs and wheels disable LOKI_USE_LLD and LOKI_ENABLE_LTO by default for build reliability.

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 .

Python API

The Python package exposes the semantic parser, translator, and reparseable PDDL formatter through pypddl.formalism:

from pypddl import formalism as pddl

parser = pddl.Parser("""
(define (domain ready-domain)
  (:predicates (ready))
)
""")

translation = pddl.translate_domain(parser.domain())
domain_text = pddl.format_domain(translation.translated_domain)
reparsed = pddl.Parser(domain_text)
assert reparsed.domain().get_name() == "ready-domain"

With default options the parser completes :action-costs artifacts and the translator compiles typing and materializes equality; see Parser and Translator Options to override.

C++ API

The umbrella header exposes the semantic parser, translator, and reparseable PDDL formatter through the top-level loki namespace:

#include <loki/loki.hpp>

#include <string>

int main()
{
    auto parser = loki::Parser(std::string { "(define (domain ready-domain) (:predicates (ready)))" });
    const auto translation = loki::translate_domain(parser.get_domain());
    const auto domain_text = loki::format_domain(translation.get_translated_domain());
    auto reparsed = loki::Parser(domain_text);
    return reparsed.get_domain().get_name() == "ready-domain" ? 0 : 1;
}

Parser and Translator Options

Both APIs accept options at construction/translation time. The library defaults are normalization-friendly for downstream consumers; every option can be turned off individually.

ParserOptions (second argument of Parser):

Option	Default	Description
strict	false	Strict semantic validation for requirements, arity, and type compatibility. Numeric-fluents violations error even in permissive mode: reads require :fluents/:numeric-fluents/:action-costs, and writes other than (increase (total-cost) ...) require :fluents/:numeric-fluents.
add_action_costs	true	Complete missing :action-costs artifacts (total-cost function, initial value, minimize metric) instead of erroring. If the domain declares neither :action-costs nor :fluents/:numeric-fluents, additionally injects the requirement and a unit-cost effect (increase (total-cost) 1) into every action that does not already write total-cost. Genuine numeric domains are left untouched: an absent metric means unit costs.

TranslatorOptions (second argument of translate_domain/translate_task):

Option	Default	Description
compile_typing	true	Compile typing away into type predicates and remove type annotations.
compile_conditional_effects	false	Multiply conditional effects out into unconditional actions; the normalization phases re-run afterwards so preconditions stay conjunctive (quantified when-conditions become derived predicates). Worst case exponential in the number of conditional effects per action.
materialize_equality	true	Add the = predicate and (= o o) initial literals. Turn off for consumers with native equality handling.

Pass the same TranslatorOptions to the domain and the task translation; mismatched options between the two can fail (e.g. equality materialization requires the = predicate added during domain translation).

options = pddl.TranslatorOptions()
options.materialize_equality = False
translation = pddl.translate_domain(parser.domain(), options)

The loki executable exposes the same options as opt-in flags (all off by default): --strict, --add-action-costs, --compile-typing, --compile-conditional-effects, and --materialize-equality.

CMake Integration

This section covers pypddl-specific paths and targets; the general pattern for consuming the native prefixes from CMake is in the common CMake integration instructions.

The Python package pypddl installs Loki's native headers, shared library, and CMake package config under pypddl.native_prefix(). It depends on pyyggdrasil>=0.0.26,<0.1 for third-party native dependencies:

import pypddl
import pyyggdrasil

print(pypddl.cmake_prefix())       # prefix to put on CMAKE_PREFIX_PATH
print(pypddl.cmake_dir())          # directory containing lokiConfig.cmake
print(pyyggdrasil.cmake_prefix())

The same paths are available from the shell via python -m pypddl --prefix, --cmake-dir, --include-dir, and --version.

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="$(python -m pypddl --prefix);$(python -m pyyggdrasil --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. Example PDDL inputs come straight from the pypddl-datasets cache:

./build/exe/loki $(python -c "import pypddl_datasets as pb; t = pb.fetch_task('classical/tests/gripper/test-1.pddl'); print(t.domain_path, t.task_path)")

Use --out-domain/--out-problem to write the translated PDDL to files, and see ./build/exe/loki --help for the parser and translator flags (--strict, --add-action-costs, --compile-typing, --compile-conditional-effects, --materialize-equality).

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.