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