Hello, I want to use a foreign C++ Function in a Haskell Library, and the Library in a Haskell Program. The C++ function is separate in a file and I have a header file.  I made a minimal reproducing repository. The closes I have found is this repository which wraps the c++ in c and makes a full library, needing a makefile etc. So while I think I'd get it working like that, it looks to be quite a lot to do for my simple task. the repository structure looks as follows: Projekt - cpp-src - some.cpp - some.h - lib-src - CPPLib.hs - program-src - program.hs - project.cabal And my (currently failing) project.cabal the following: cabal-version: 3.0 name: cpp_lib [...] library exposed-modules: CPPLib other-extensions: ForeignFunctionInterface build-depends: base hs-source-dirs: lib-src default-language: Haskell2010 include-dirs: ./cpp-src cxx-sources: ./cpp-src/some.cpp install-includes: ./cpp-src/some.h executable CPPApp main-is: program.hs default-language: Haskell2010 build-depends: HaskellForeignCPP, base hs-source-dirs: program-src The some.cpp is trivial for this case lets assume its a function multiply :: Int -> Int -> Int, and both .cpp and .h are well-formed and don't have further dependencies. The library compiles fine, but when building the executable it fails telling me collect2: error: ld returned 1 exit status `gcc' failed in phase `Linker'. (Exit code: 1) Can someone help me out? I find very little documentation on this topic. I know there is Inline-CPP which would help as well, but I just very much like the idea of having a nice separate file and interface for modularity. best Leonhard