How to import a haskell module that uses FFI without refering to the c object? - c

Question

I'm trying to write a haskell module that wraps a bunch of c functions.
I want to be able to import this module like any other haskell module without referring to the c object files.
I can't find any examples about how to do this.
This is what I've tried. I have a c file "dumbCfunctions.c":
double addThree(double x) {
return x+3;
}
and a haskell file with a module defined in it "Callfunctions.hs"
module Callfunctions (
addThree
) where
import Foreign.C
foreign import ccall "addThree" addThree :: Double -> Double
main = print $ addThree 4
I can make an executable doing:
ghc --make -o cf_ex Callfunctions.hs dumbCfunctions.o
Which correctly gives me 7.
I can also import it into ghic by calling ghci with
shane> ghci dumbCfunctions.o
Prelude> :l Callfunctions.hs
[1 of 1] Compiling Callfunctions ( Callfunctions.hs, interpreted )
Ok, modules loaded: Callfunctions.
*Callfunctions> addThree 3
6.0
But I want to be able to treat it like any other module with out referring to "dumbCfunctions.o":
shane> ghci
Prelude> :l Callfunctions.hs
[1 of 1] Compiling Callfunctions ( Callfunctions.hs, interpreted )
Ok, modules loaded: Callfunctions.
*Callfunctions> addThree 3
But now I get the error
ByteCodeLink: can't find label
During interactive linking, GHCi couldn't find the following symbol:
addThree
This may be due to you not asking GHCi to load extra object files,
archives or DLLs needed by your current session. Restart GHCi, specifying
the missing library using the -L/path/to/object/dir and -lmissinglibname
flags, or simply by naming the relevant files on the GHCi command line.
Alternatively, this link failure might indicate a bug in GHCi.
If you suspect the latter, please send a bug report to:
glasgow-haskell-bugs#haskell.org
This makes sense because I haven't refereed to the object anywhere. So I must be able to do something better by first compiling the module, but I couldn't find out how to do this. I must be looking in the wrong places.

Answer 1

You can create a library through Cabal, and cabal install it.
This would link the C code inside your Haskell library. Later on, when you load the module, you will not need to manually load the C parts.