infix-parser

A fast C++20 infix expression parser built on lexy. Parses mathematical expressions into a compact reverse-prefix AST suitable for stack-based evaluation and automatic differentiation.

Features

• Fast number parsing via fast-float
• Reverse-prefix encoding — binary [rhs, lhs, op], unary [child, op], ternary [else, then, cond, op]
• Rich operator set

Category	Symbols / names
Arithmetic	+-*/^%|
Comparison	<><=>===!=
Unary	absexpexp2loglog2log10sqrtcbrterfsincostanasinacosatansinhcoshtanhsigmoidsquarecube
Binary	aqpowminmaxatan2hypotfmod
Ternary	if (soft, differentiable)

• All functions are differentiable — designed to integrate with Ceres/operon

Prerequisites

Tool	Version
CMake	≥ 3.14
C++ compiler	C++20 (GCC 12+, Clang 15+, MSVC 2022)
vcpkg	any recent

Dependencies (managed by vcpkg): lexy, fast-float, fmt. Optional (enabled via VCPKG_MANIFEST_FEATURES=test): catch2, nanobench.

Building

Linux

# Bootstrap vcpkg (once)
git clone https://github.com/microsoft/vcpkg "$VCPKG_ROOT""$VCPKG_ROOT/bootstrap-vcpkg.sh"
cmake --preset=ci-ubuntu # release + clang-tidy + cppcheck
cmake --build build -j$(nproc)

Or without presets:

cmake -S . -B build \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_CXX_STANDARD=20 \
-DCMAKE_TOOLCHAIN_FILE="$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake"
cmake --build build -j$(nproc)

macOS

cmake --preset=release-darwin # Xcode generator, vcpkg
cmake --build build --config Release

Or without presets:

cmake -S . -B build \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_CXX_STANDARD=20 \
-DCMAKE_TOOLCHAIN_FILE="$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake"
cmake --build build -j$(sysctl -n hw.logicalcpu)

Windows (MSVC)

Run the commands in a Developer PowerShell for VS 2022 or any shell where cl.exe is on PATH.

cmake --preset=ci-windows # Visual Studio 17 2022, static vcpkg triplet
cmake --build build --config Release

Or without presets:

cmake -S .-B build `-G "Visual Studio 17 2022"-A x64 `-DCMAKE_CXX_STANDARD=20`-DCMAKE_TOOLCHAIN_FILE="$env:VCPKG_ROOT\scripts\buildsystems\vcpkg.cmake"`-DVCPKG_TARGET_TRIPLET=x64-windows-static-md
cmake --build build --config Release

Nix flake

nix build github:foolnotion/infix-parser

To use as a flake input:

inputs.infix-parser.url="github:foolnotion/infix-parser";# Share the foolnotion overlay to avoid duplicate lexy/fast-float in closure:inputs.infix-parser.inputs.foolnotion.follows="foolnotion";

Running tests

Tests require the test vcpkg feature and CMake's BUILD_TESTING:

cmake -S . -B build \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_TOOLCHAIN_FILE="$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake" \
-DVCPKG_MANIFEST_FEATURES=test \
-DBUILD_TESTING=ON
cmake --build build -j$(nproc)
ctest --test-dir build --output-on-failure

Benchmarks

cmake -S . -B build \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_TOOLCHAIN_FILE="$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake" \
-DVCPKG_MANIFEST_FEATURES=test \
-DBUILD_BENCHMARKS=ON
cmake --build build -j$(nproc)
./build/benchmark/infix-parser-bench

Consuming the library via CMake

After installation (cmake --install build --prefix /usr/local):

find_package(infix-parserREQUIRED)
target_link_libraries(my_targetPRIVATEinfix-parser::infix-parser)

All dependencies (lexy, fast-float, fmt) are private implementation details — consumers need no extra find_package calls.

Acknowledgements

The parser is built on lexy by Jonathan Müller — a header-only parser combinator library that made expressing the infix grammar straightforward and type-safe.

License

MIT — see LICENSE. Copyright © 2026 Bogdan Burlacu