How to use a DLL in a Haskell project?

You’ll need to use extra-lib-dirs and extra-libraries in the executable section of your .cabal file like so:

name:                 MyApp
version:              0.1.0.0
synopsis:
homepage:
author:               simon.bourne
category:
build-type:           Simple
cabal-version:        >=1.10

library
  exposed-modules:      HelloWorld
  build-depends:        base >= 4.7 && < 5
  hs-source-dirs:       src
  default-language:     Haskell2010

executable MyApp
  main-is:              Main.hs
  extra-lib-dirs:       lib
  extra-libraries:      helloWorld
  build-depends:        base >= 4.7 && < 5,
                        MyApp
  hs-source-dirs:       app

default-language: Haskell2010

Put your dll and .so in lib. Be warned, you’ll run into link order problems if you use a static library (.a instead of .so) on linux.

See this for an example. Don’t be fooled by the name as it works fine with .so files.

stack ghci should just work provided it can find your library (LD_LIBRARY_PATH on linux).

The C API (mentioned in the comments on your question) is already there. You just need to write the Haskell FFI signatures, for example:

foreign import ccall safe "helloWorld" c_helloWorld :: IO ()

I’d very strongly advise using safe ccalls, and don’t wrap the functions in unsafePerformIO.

If you need to pass non opaque structs, you might want to investigate c2hs or hsc2hs, but I don’t think you’ll need to. See this question for more details.