Awesome

call-haskell-from-anything

Call Haskell functions from any programming language via serialization and dynamic libraries.

Skip the philosophy, jump right to the code!

I just want to call that function

Want to call Haskell from Python?
Want to call Haskell from Ruby?
Want to call Haskell from C?
Want to call Haskell from Node.js?
Want to call Haskell from C#?
Want to call Haskell from Java?
Want to call Haskell from browsers?

Yes, Haskell can do that.

Using the Foreign Function Interface (FFI) you can expose Haskell functions at the C level.

But damn, it's so hard!

You have two high-level languages here (Haskell and X), but even though you "just want to call that function", you have to think about and write low-level system code on both sides. Going via C is painful: An interface that does not even support the idea of many of something is not very supportive (no, C doesn't even have arrays, it only has pointers to the start of something).

What we really want for most cases:

a slightly higher level, intuitive interface
as invisible as possible
just calling that function.

Want to call Haskell from ... anything?

The simplest FFI: Serialization

In the end, the C calling convention is just another wire format: Data is to be shipped from one function to another.

So we could just as well use a wire format that is not as uncomfortable as the C FFI.

Any serialization library does that for us, and most of them (e.g. JSON) are simpler to reason about and manage than raw memory in C.

call-haskell-from-anything implements FFI function calls where function arguments and return value are serialized using MessagePack. Any function is then exported via the standard FFI as a raw bytes (CString -> IO CString) interface.

Usage

call-haskell-from-anything allows you to write a function, say:

chooseMax :: [Int] -> Int
chooseMax xs = ...

Add this:

foreign export ccall chooseMax_export :: CString -> IO CString
chooseMax_export = export chooseMax

and compile it into a shared library (.so or .dll). You can now call it from any language that supports MessagePack, e.g. Python:

chooseMax = wrap_into_msgpack(cdll.LoadLibrary('mylib.so').chooseMax_export)

print chooseMax([1,5,3])

In detail, it will transform your functions of type

f :: a -> b -> ... -> r

to an equivalent (it is actually a type-level list) of

f' :: (a, b, ...) -> r

so that the function input (arguments) can be easily de-serialized.

The wrap_into_msgpack function used above sets the return type of the foreign function to raw bytes and wraps arguments and return value into MessagePack, prepended by a 64-bit length:

def wrap_into_msgpack(foreign_fun):
    foreign_fun.restype = c_char_p

    def wrapped_fun(*args):
        packed = msgpack.packb(args)
        length_64bits = struct.pack("q", len(packed)) # native-endian
        ptr = fun(length_64bits + packed)
        data_length = cast(ptr[:8], POINTER(c_longlong))[0]
        res = msgpack.unpackb(ptr[8:8+data_length])
        free(ptr)
        return res

    return wrapped_fun

A full example

You can run the stock example in this repository:

sudo apt-get install python-msgpack ruby-msgpack  # or equivalent for your system
stack build

# If any of these work, you're all fine!
python test.py
ruby test.rb

FAQ

Is call-haskell-from-anything an RPC framework?

No. RPC means Remote Procedure Call, and nothing in call-haskell-from-anything assumes to be remote.

Calls are blocking as you would expect from standard C libraries.

Are there restrictions on what arguments functions can take?

Yes: all arguments and the return value must be serializable.

This means you cannot pass around pointers or callback functions; you have to use the C style FFI or an RPC mechanism for that.

Why is MsgPack used for serialization?

Because it is simple, available (there are implementations for most programming languages, and writing new ones is easy due to its simplicity), supports dumb binary (passing around arbitrary custom data does not require to think about encoding), and fast (in many implementations).

However, call-haskell-from-anything is not fixed to use only MsgPack as wire-format; anything that can conveniently encode lists/arrays is suitable (FFI.Anything.TypeUncurry.Msgpack is the only implementation so far, though).

How fast are serialized FFI calls? What is the trade-off compared to a C style FFI?

Calls from one programming language into another are usually slower than calls inside the programming language, so it does make sense to check if a foreign call is worth it.

In some preliminary cPython 2.7 benchmark using functions that take a single Int and return a single Int (e.g. the +1 function), a foreign call using MsgPack serialization takes around 15 times longer than an in-Python function call (on the tested Core i5 machine, 1M calls took 15s, in pure Python they took 1s). However, as soon as you perform a somewhat expensive computation, the call into native Haskell code becomes worth it (take for example a naive recursive fibonacci implementation for 100000 calls of fib(15); in-Python: 90s, with call-haskell-from-anything: 4.5s).

In comparison to a C style FFI to an immediately returning Int -> Int function, the overhead of a serializing function call is around 6 times higher, and, as usual, becomes insignificant as soon as the function does something.

More detailed benchmarks are planned, and contributions are welcome.