HaPy

Call Haskell functions from Python! HaPy is set of Haskell bindings for Python. Initially written in 2011 as a final project for Stanford's CS240H Haskell class by Ashwin Siripurapu, William Rowan, and David Fisher. Now rewritten mostly from scratch by David Fisher with different tradeoffs (gaining far more stability at the expense of initial setup).

Usage:

• Install the Haskell module by running cabal install in the haskell directory. The dependencies are currently more restrictive than necessary, so you may need to relax them if you have compilation problems.
• Make a FFI export Haskell module. Due to Template Haskell restrictions, this must be in a different module from any functions you are exporting. See: example/Export.hs.
• Copy the c/HaPy_init.c file to your Haskell project directory. Build your project with: ghc --make -no-hs-main -optl '-shared' HaPy_init.c -o LIB_NAME.so HASKELL_FFI_FILE
• Install the Python module by running python setup.py install in the python directory.
• Put the resulting LIB_NAME.so file in the main directory of your Python project.
• Add from HaPy import LIB_NAME to the top of your Python file
• Call Haskell functions from Python, just like you would any other Python module.

Caveats:

• Only functions of certain types can be exported. Currently supported types:
• Bool
• Char
• Int
• Double
• String
• Lists (at any level of nesting) of all the above types (e.g. [Int], [[Int]], etc.)
• The FFI adds some overhead: all values are copied twice (e.g. from the Python representation to the C representation to the Haskell representation).

Future development:

• Near term:
• Allow the compiled Haskell binary to be in appropriate other directories
• Under consideration:
• Automatically compile Haskell library from Python
• Automatically generate Template Haskell export file in Python
• Add support for passing Python functions