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This repository contains a Cabal 2.0 project showing how to build a haskell library to use in c/c++ using the new foreign-library feature. The foreign-library feature greatly simplifies the process of creating haskell libraries for use by other languages.

The lens and deepseq packages are included mainly as a test case for dependencies. The example contains code that is intended to test features I need for my own projects, in particular AnimalClub. I would love to merge a PR including more relevant examples 😘.


make run

It was tested on Mac and Linux (Ubuntu 19.04 with cabal 2.4 and ghc 8.6.5).


potato.cabal defines our module using foreign-library

foreign-library potato
  type:                native-shared

  if os(Windows)
    options: standalone
    mod-def-file: PotatoLib.def

  other-modules:       Potato
    base ^>=
    , lens == 4.*
    , deepseq == 1.4.*
  hs-source-dirs:      src
  c-sources:           csrc/potato.cpp
  default-language:    Haskell2010

Note, to support stack pipeline, this is done using the verbatim field in package.yaml.

Please see cabal docs for a more thorough description explaining the meaning of each field. The important detail here is

c-sources:           csrc/potato.cpp

which points to a file that cabal build will build for us (and handle all linker/include issues for us). In this case, we wrap all exported methods from our 🥔 Haskell module and from ghc's HsFFI.h inside of helper functions. Only potato.h needs to be included by the user. In this manner, we are building a 🥔 c++ library that calls our 🥔 haskell library. If you don't wish to wrap the functionality, you can simply leave potato.cpp empty and call the exported methods from our 🥔 haskell library directly. In this case, you'll need to include Potato_stub.h which is generated by cabal build and likely HsFFI.h when you use the library.

void potatoInit(void); and void potatoExit(void); simply wrap hs_init and hs_exit which start and stop the haskell runtime respectively. void test() calls all the functions in our 🥔 haskell module.

The capp/ folder contains our cpp source that will call code from our haskell potato module. The makefile builds the cpp app using g++ with the needed flags. Note that it expects the hs library files to be in this directory to work.

g++ -g -Wall potatomain.cpp -o $@ \
-I../csrc \
-lpotato \

As mentioned earlier, if you want your capp/potatomain.cpp to use methods from the Haskell library directly instead of calling through csrc/potato.h, then you will need to add the flags -I../dist/build/potato/potato-tmp for Potato_stub.h and something like -I/usr/local/lib/ghc-8.4.4/include/ for HsFFI.h. I don't recommend this since it's unclear to me where to fetch these dependencies from in a build pipeline.

Finally, the makefile in the root directory runs stack build and copies the compiled library into the capp folder. Then it calls make inside of capp. make run runs the app it compiled in capp.


make usingcabal will do the same thing with cabal instead of stack. It uses a different .cabal file but you could probably make it work with the stack generated one too.


I used this guide as a starting point which includes links to many other resources I found helpful so I won't list them here. The guide contains a script for gathering the scattered libraries but I didn't seem to need it here. As far as I can tell, Cabal 2.0 will package everything that's needed into a single shared library.



Example project of using cabal 2.0`s foreign-library feature to build a haskell library that lets you call haskell code from C/C++.






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