Haskell FFI Tutorial

This is a demo repository to help out with Haskell FFI, namely, with nested structures. Everything I've found on the subject up till that point contained only partial information for what I've needed, so I decided to compose a complete tutorial together.

Covered subjects

You'll learn how to:

• (expressively) represent C struct in Haskell code
• call C code from Haskell
• call Haskell code from C
• operate on nested structs
• operate on struct arrays
• decode unions
• read and write C fixed-length strings and Pointer-type Strings
• how to import functions from, for example, stdlib

Motivation

There are many C bindings written for different Haskell projects, and every one seems to have it's own style. Also, because every C program is written in a very different style (some use fixed-length strings, some use *char, some have nested structs, some do not, some have unions, some do not), it's somewhat difficult to jump in and start writing your own C bindings in Haskell, even though all the functionality is available for you to use.

I've collected all I've learned about writing Haskell bindings into a single repository, and will document it all part by part to have more detailed descriptions, explanation and motivation about how to do things.

If you're a seasoned Haskell developer, and everything here is obvious for you, you can go through the concepts introduced here and give your feedback, since I am by no means an expert in this area, and may have misunderstood some things.

Calling Haskell from C

In order to call Haskell from C, you'd have to:

• create a C wrapper where you initialize Haskell Runtime
• write a callback function in Haskell, that will be called from C

Let's start with a callback function in Haskell, since it'll be used within the C code that we'll write later on. Open up a file, call it Example.hsc (hsc extension is used for the files that are interfacing C, it will be passed through hsc2hs preprocessor that will unwrap all the macros. We haven't used any for now, but we will later on).

{-# LANGUAGE CPP                         #-}
{-# LANGUAGE ForeignFunctionInterface    #-}

module Example where

foreign export ccall entrypoint :: IO ()

entrypoint :: IO ()
entrypoint = do
  print "Hello from Haskell"
  
  return ()

So far so good. Now, in order to convert hsc file to regular hs file, you have to run

hsc2hs Example.hsc

Preprocessor will create Example.hs file that unwraps all the macros.

In order to get a stub file that contains all the functions exported from Haskell to C, you have to run

ghc Example.hs

Example_stub.h will contain a valid signature for our entrypoint function. Namely, something like that (with some C boilerplate that's ommited for readability):

extern void entrypoint(void);

In order to create a C wrapper, just create a wrapper.c file:

// Include Haskell FFI file, which we will use to initialize a Haskell runtime
#include "HsFFI.h"

/* #ifdef __GLASGOW_HASKELL__ */
#include "Example_stub.h"
/* #endif */

int main( int argc, char *argv[] )
{
  // Initialize Haskell Runtime _before_ any calls to the Haskell code
  hs_init (&argc, &argv);
  
  // Make a call to Haskell code
  entrypoint();
}

Great, now you can actually compile the wrapper.c and run the resulting binary

ghc -no-hs-main wrapper.c Example.hs -o wrapper

You'll get a wrapper binary, that you can run and see the result:

➜  haskell-ffi-tutorial  ./wrapper
"Hello from Haskell"

Perfect, now you know how to call Haskell code from C.

License

Copyright (c) 2014 Alex Petrov

Licensed under MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.