I want to use some Haskell libraries (e.g. Darcs, Pandoc) from Python, but it seems there’s no direct foreign function interface to Haskell in Python. Is there any way to do that?
Provided you can get your Python code to call C, you can call Haskell functions that have been exported via the FFI
Another approach would be to write a standard IPC interface, in the case of darcs and pandoc just calling them as vanilla executables and parsing their output might be the way to go.
As to automating the generation of boring, repetitive, FFI and marshalling code on the Haskell side, I'd recommend c2hs, which allows you to auto-generate a lot based on an existing C interface. There's probably similar things for python.
SWIG, alas, has, to the best of my knowledge, never been implemented for Haskell, presumably because it caters to less strictly-typed languages.
One additional idea: Something less efficient than a direct C binding, but more efficient than shelling out to Haskell is an rpc system such as Apache Thrift: http://incubator.apache.org/thrift/
I've found thrift easy to use, well supported, and reasonably performant. Once you have your Haskell server running, the cost of local communication is fairly cheap, although you pay a bit more in marshalling/unmarshalling than using c types directly.
There are also at least two packages for calling Python from Haskell, missingpy (http://hackage.haskell.org/package/MissingPy) and cpython (http://hackage.haskell.org/package/cpython). The latter claims that support in the other direction is planned -- although you'd have to ask the author if this is still the case, and if so when.