Writing a static cpp library for a haskell stack project

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06-10-2024

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Building a C++ Static Library for Your Haskell Stack Project

The Haskell ecosystem offers a robust and expressive environment for building applications, but sometimes you need to leverage the performance and low-level control of C++. A common solution is to create a C++ static library that can be integrated into your Haskell project. This approach allows you to write performance-critical code in C++ while still enjoying the benefits of Haskell's elegant syntax and powerful features.

The Problem

You're working on a Haskell project built with Stack, and you need to implement a specific algorithm or functionality that demands the performance of C++. You want to use a C++ static library to achieve this, but how do you integrate it seamlessly into your Stack project?

The Solution

Here's how you can build and integrate a C++ static library within your Haskell Stack project:

1. Setting up the C++ Project:

• Create a separate directory: For organizational clarity, create a new directory within your Stack project to hold the C++ code.
• Define a CMakeLists.txt file: CMake is a popular tool for cross-platform C++ build systems. Create a CMakeLists.txt file in your C++ directory to define your project structure.
• Write your C++ code: Write the C++ code for your library, ensuring you define functions and structures that you intend to expose to Haskell.
• Compile the static library: Use CMake to compile your C++ code into a static library (.a file).

Example CMakeLists.txt:

cmake_minimum_required(VERSION 3.10)
project(MyCppLib)

add_library(MyCppLib STATIC src/my_cpp_lib.cpp)

target_include_directories(MyCppLib PUBLIC include)

2. Integrating with Haskell:

• Create a Haskell interface: Write a Haskell module that provides a bridge between your C++ library and Haskell.
• Use the Foreign Function Interface (FFI): The FFI lets you call C functions from Haskell. You'll need to use the Foreign package in Haskell to define bindings for your C++ functions.
• Build the C++ library within Stack: Configure Stack to build the C++ library as a separate component. You can achieve this by adding a new build-tool section to your stack.yaml file.

Example Haskell Interface (my_cpp_lib.hs):

module MyCppLib where

import Foreign
import Foreign.C
import Foreign.Marshal.Array
import Foreign.Ptr
import Foreign.Storable

foreign import ccall "my_cpp_lib.h myCppLibFunction"
  myCppLibFunction :: Ptr CInt -> IO CInt

3. Modifying stack.yaml:

• Add build-tool section: Define a new build-tool section in your stack.yaml file to specify the build process for your C++ library.
• Configure C++ dependencies: Specify any C++ dependencies that your library needs.
• Link the library: Ensure that your Haskell project is linked with the compiled C++ library.

Example stack.yaml:

...
build-tools:
  - my-cpp-lib:
    ghc-options: -package foreign
    build-depends:
      - base
      - foreign
    source-dirs: src
    main-is: my_cpp_lib.hs
    build-depends:
      - my-cpp-lib
    build-tools:
      - my-cpp-lib

4. Linking and Compilation:

• Use the C++ library in your Haskell code: Import the Haskell interface module and call the functions exposed from your C++ library.
• Build your Haskell project: Run stack build to compile your Haskell project, which will automatically compile the C++ library as well.

Insights and Considerations:

• Shared vs. Static libraries: While static libraries are simple to integrate, shared libraries may be preferable for larger projects to avoid code duplication.
• Haskell-specific data structures: Consider how you will handle data conversion between Haskell and C++ data structures. You might use Haskell's Foreign.Marshal module for serialization and deserialization.
• Error handling: Develop a clear strategy for error handling. C++ exceptions won't directly propagate to Haskell; you'll need to use mechanisms like error codes or a specialized error handling library.
• Testing: Write comprehensive tests for both your C++ library and the Haskell interface module to ensure correct integration and prevent unexpected behavior.

Example Usage in Haskell:

import MyCppLib

main :: IO ()
main = do
  result <- myCppLibFunction (castPtr 0)
  print result 

Conclusion

Integrating a C++ static library into your Haskell Stack project allows you to tap into the performance of C++ while maintaining a high-level, expressive programming environment with Haskell. By following the outlined steps and understanding the key considerations, you can seamlessly combine the best of both worlds to build powerful and efficient applications.